USE

USE

Open an existing database (.dbf) and its associated files

Syntax

      USE [<xcDatabase>
            [INDEX <xcIndex list>]
            [ALIAS <xcAlias>] [EXCLUSIVE | SHARED]
            [NEW] [READONLY]
            [VIA <cDriver>]]

Arguments

<xcDatabase> is the name of the database file to be opened and may be specified either as a literal file name or as a character expression enclosed in parentheses.

INDEX <xcIndex list> specifies the names of 1 to 15 index files to be opened in the current work area. Specify each index as a literal file name or as a character expression enclosed in parentheses. The first index in the list becomes the controlling index. If you specify an <xcIndex> as an expression and the value returned is spaces or NIL, it is ignored.

ALIAS <xcAlias> specifies the name to associate with the work area when the database file is opened. You may specify the alias name as a literal name or as a character expression enclosed in parentheses. A valid <xcAlias> may be any legal identifier (i.e., it must begin with an alphabetic character and may contain numeric or alphabetic characters and the underscore). Within a single application, Harbour will not accept duplicate aliases. If this clause is omitted, the alias defaults to the database file name.

EXCLUSIVE opens the database file for nonshared use in a network environment. All other users are denied access until the database file is CLOSEd.

SHARED opens the database file for shared use in a network environment. Specifying this clause overrides the current EXCLUSIVE setting.

NEW opens <xcDatabase> in the next available work area making it the current work area. If this clause is not specified, <xcDatabase> is opened in the current work area.

READONLY opens <xcDatabase> with a read-only attribute. This lets you open database files marked read-only. If you cannot open the <xcDatabase> this way, a runtime error is generated. If this clause is not specified, <xcDatabase> is opened as read-write.

VIA <cDriver> specifies the replaceable database driver (RDD) with which to process the current work area. <cDriver> is the name of the RDD specified as a character expression. If <cDriver> is specified as a literal value, it must be enclosed in quotes.

If the VIA clause is omitted, the DBFNTX driver is used by default. Note that if the specified driver is not linked, an unrecoverable error occurs.

In no arguments are specified, the database file open in the current work area is closed.

Description

USE opens an existing database (.dbf) file, its associated memo (.dbt) file, and optionally associated index (.ntx or .ndx) file(s) in the current or the next available work area. In Harbour, there are 250 work areas with a maximum of 255 total files open in DOS 3.3 and above. Before USE opens a database file and its associated files, it closes any active files already open in the work area. When a database file is first opened, the record pointer is positioned at the first logical record in the file (record one, if there is no index file specified).

In a network environment, you may open database files as EXCLUSIVE or SHARED. EXCLUSIVE precludes the USE of the database file by other users until the file is closed. SHARED allows other users to USE the database file for concurrent access. If the database file is SHARED, responsibility for data integrity falls upon the application program. In Harbour, FLOCK() and RLOCK() are the two basic means of denying other users access to a particular work area or record. If a USE is specified and neither EXCLUSIVE nor SHARED is specified, the database file is opened according to the current EXCLUSIVE setting. In Harbour, all USE commands should explicitly specify how the database file is to be opened, EXCLUSIVE or SHARED. The implicit open mode specified by SET EXCLUSIVE is supplied for compatibility purposes only and not recommended.

Opening a database file in a network environment requires some special handling to be successful. First, attempt to USE the database file without specifying the INDEX list. Then, test for the success of the operation using NETERR(). If NETERR() returns false (.F.), the open operation succeeded and you can SET INDEX TO the index list. A USE will fail in a network environment if another user has EXCLUSIVE USE of the database file. Refer to the “Network Programming” chapter in the Programming and Utilities Guide for more information on opening files in a network environment.

You can open index files with USE or SET INDEX. The first index in the list of indexes defines the current ordering of records when they are accessed. This index is referred to as the controlling index. You can change the current controlling index without closing any files by using the SET ORDER command.

To close a database and its associated files in the current work area, specify USE or CLOSE with no arguments. To close database files in all work areas, use CLOSE DATABASEs. To close index files in the current work area without closing the database file, use CLOSE INDEX or SET INDEX TO with no arguments.

Refer to the “Basic Concepts” chapterfor more information about the Harbour database paradigm.

Notes

. Setting the maximum open files: Control of the number of file handles available to a Harbour application is controlled by a combination of the CONFIG.SYS FILES command, and the F parameter of the CLIPPER environment variable. The F parameter specifies the maximum number of files that can be opened at any one time within the current Harbour program. Harbour determines the number of files that can be opened using the smaller of the two parameters. For example, if the FILES command is set to 120 and the F parameter is set to 50, the maximum number of files that can be opened is 50. In a network environment, file handles also need to be set in the network configuration file.

The file limit is controlled by the operating system. Under DOS versions less than 3.3, the maximum number of files that can be opened at one time is 20 files. In DOS versions 3.3 and greater, the maximum limit is 255 files.

. Opening the same database file in more than one work area: Although opening a database file in more than one work area is possible in a network environment, this practice is strongly discouraged. If done, each file must be opened with a different alias, otherwise a runtime error will occur.

. Opening two database files with the same names, in different directories: Although opening two database files with the same names in different directories is possible, the database files MUST have unique alias names; otherwise, a runtime error will occur.

Examples

      .  This example opens a shared database file with associated
         index files in a network environment.  If NETERR() returns
         false (.F.), indicating the USE was successful, the indexes
         are opened:

      USE Accounts SHARED NEW
      IF !NETERR()
         SET INDEX TO AcctNames, AcctZip
      ELSE
         ? "File open failed"
         BREAK
      ENDIF

      .  This example opens a database file with several indexes
         specified as extended expressions.  Note how the array
         of index names is created as a constant array:

      xcDatabase = "MyDbf"
      xcIndex = {"MyIndex1", "MyIndex2", "MyIndex3"}
      USE (xcDatabase) INDEX (xcIndex[1]), ;
            (xcIndex[2]), (xcIndex[3])

Seealso

CLOSE, DBRSELECT(), DBSETINDEX(), DBUSEAREA(), NETERR()

SEEK

Search an order for a specified key value

Syntax

      SEEK <expSearch> [SOFTSEEK]

Arguments

<expSearch> is an expression to match with an order key value.

SOFTSEEK causes the record pointer to be moved to the next record with a higher key value after a failed order search. Default behavior moves the record pointer to EOF() after a failed order search.

Description

SEEK is a database command that searches the controlling order from the first or last key value (depending on whether the LAST keyword is specified) and proceeds until a match is found or there is a key value greater than <expSearch>. If there is a match, the record pointer is positioned to the identity found in the order. If SOFTSEEK is OFF (the default) and SEEK does not find a match, the record pointer is positioned to LASTREC() + 1, EOF() returns true (.T.), and FOUND() returns false (.F.).

SOFTSEEK enables a method of searching an order and returning a record even if there is no match for a specified key.

When SOFTSEEK is ON and a match for a SEEK is not found, the record pointer is set to the next record in the order with a higher key value than the SEEK argument. Records are not visible because SET FILTER and/or SET DELETED are skipped when searching for the next higher key value. If there is no record with a higher key value, the record pointer is positioned at LASTREC() + 1, EOF() returns true (.T.), and FOUND() returns false (.F.). FOUND() returns true (.T.) only if the record is actually found. FOUND() never returns true (.T.) for a relative find.

When SOFTSEEK is OFF and a SEEK is unsuccessful, the record pointer is positioned at LASTREC() + 1, EOF() returns true (.T.), and FOUND() returns false (.F.).

SEEK with the SOFTSEEK clause is, effectively, the same as performing SET SOFTSEEK and then SEEK in earlier versions of Clipper except that it does not change the global setting of SOFTSEEK.

Examples

      .  The following example searches for "Doe" using the SEEK
         command:

      USE Customer NEW
      SET ORDER TO Customer
      ? SET( _SET_SOFTSEEK )      // (.F.)
      SEEK "Doe"
      ? SET( _SET_SOFTSEEK )      // Still (.F.)
      IF FOUND()
         .
         . < statements >
         .
      ENDIF

      .  The following example performs a soft seek for "Doe" using
         SOFTSEEK clause of the SEEK command:

      USE Customer NEW
      SET ORDER TO Customer
      ? SET( _SET_SOFTSEEK )      // (.F.)
      SEEK "Doe" SOFTSEEK
      ? SET( _SET_SOFTSEEK )      // Still (.F.)
      IF !FOUND()
         ? Customer->Name         // Returns next logical name after "Doe"
      ENDIF

Seealso

DBSEEK(), DBSETINDEX(), DBSETORDER(), EOF(), SET INDEX

C5DG-3 RDD Reference

Clipper 5.x – Drivers Guide

Chapter 3

RDD Reference

 

APPEND FROM     Import records from a (.dbf) or ASCII file                  
COPY TO         Export records to a new (.dbf) or ASCII file                
DBAPPEND()      Append a new record to the database in the current work area
DBGOTO()        Position record pointer to a specific identity              
DBRLOCK()       Lock the record at the current or specified identity        
DBRLOCKLIST()   Return an array of the current Lock List                    
DBRUNLOCK()     Release all or specified record locks                       
DBSETINDEX()*   Empty Orders from an Order Bag into the Order List          
DELETE TAG      Delete a Tag                                                
GO              Move the pointer to the specified identity                  
INDEX           Create an index file                                        
ORDBAGEXT()     Return the default Order Bag RDD extension                  
ORDBAGNAME()    Return the Order Bag name of a specific Order               
ORDCREATE()     Create an Order in an Order Bag                             
ORDDESTROY()    Remove a specified Order from an Order Bag                  
ORDFOR()        Return the FOR expression of an Order                       
ORDKEY()        Return the key expression of an Order                       
ORDLISTADD()    Add Orders to the Order List                                
ORDLISTCLEAR()  Clear the current Order List                                
ORDLISTREBUI()  Rebuild all Orders in Order List of the current work area   
ORDNAME()       Return the name of an Order in the Order List               
ORDNUMBER()     Return the position of an Order in the current Order List   
ORDSETFOCUS()   Set focus to an Order in an Order List                      
RDDLIST()       Return an array of available Replaceable Database Drivers   
RDDNAME()       Return name of RDD active in current or specified work area 
RDDSETDEFAULT() Set or return the default RDD for the application           
RECNO()         Return the identity at the position of the record pointer   
SEEK            Search an Order for a specified key value                   
SET INDEX       Open one or more Order Bags in the current work area        
SET ORDER       Select the controlling Order

C5DG-2 RDD Architecture

Clipper 5.x – Drivers Guide

Chapter 2

Replaceable Database Driver Architecture

Clipper supports a driver architecture that allows Clipper applications to use Replaceable Database Drivers (RDDs). The RDD system makes Clipper applications data-format independent. Such applications can, therefore, access the data formats of other database systems, including the dBASE IV (.mdx), FoxPro (.cdx), and Paradox (.db) formats on a variety of equipment. This driver architecture can even support database drivers that are not file-based, although all of the drivers supplied with Clipper 5.x are file-based.

The concept of replaceable drivers is not new to this version of Clipper. In previous versions, the use of the default database driver (DBFNTX.LIB) was hidden by the fact that it was automatically linked into your application. In fact, this is still the case. The DBFNTX driver has been replaceable since it was first introduced in version 5.0. Before this version, the DBFNTX driver was the only RDD supplied as part of the system.

In This Chapter

With the introduction of the new RDDs, Clipper provides many new and enhanced commands and functions that access and manipulate databases. These language elements can enable your applications to access data regardless of the RDD under which it is ordered. There are also commands and functions that give you specific information about the RDDs in use.

The Language Implementation section of this chapter includes tables that summarize these new and enhanced language elements. This chapter also covers basic terminology, implementation principals, and general concepts of the Order Management System.

The following major topics are discussed:

. RDD Basics

. Basic Terminology

. The Language Implementation

. Order Management System

RDD Basics

The cornerstone of the replaceable database driver system is the Clipper work area. All Clipper database commands and functions operate in a work area through a database driver that actually performs the access to the stored database information. The layering of the system looks like this:

                      +———————————+

                      | Database Commands and Functions |
                      ----------------------------------|
                      |          RDD Interface          |
                      |---------------------------------|
                      |         Database driver         |
                      |---------------------------------|
                      |           Stored Data           |
                      +---------------------------------+

 In this system, each work area is associated with a single database driver. Each database driver, in turn, is supplied as a separate library file (.LIB) you link into your application programs. Within an application, you specify the name of the database driver when you open or access a database file or table with the USE command or DBUSEAREA() function. If you specify no database driver at the time a file is opened, the default driver is used. You may select which driver will be used as the default driver.

Once you open a database in a work area, the RDD used for that work area is automatically used for all operations on that database (except commands and functions that create a new table). Any command or function that creates a new table (i.e., SORT, CREATE FROM, DBCREATE(), etc.) uses the default RDD. Most of the new commands and functions let you specify a driver other than the default driver.

The normal default database driver, DBFNTX (which supports the traditional (.dbf), (.ntx), and (.dbt) files) is installed into your \CLIPPER5\LIB directory. This driver is linked into each program automatically to provide backwards compatibility.

To use any of the other supplied drivers, either as an additional driver or an alternate driver, you must use the REQUEST command to assure that the driver will be linked in. You must also include the appropriate library on the link line.

All Clipper applications will automatically include code generated by RDDSYS.PRG from the \CLIPPER5\SOURCE\SYS subdirectory. If you wish to automatically load another RDD, you must modify and compile RDDSYS.PRG and link the resulting object file into your application. The content of the default RDDSYS.PRG is shown below. Only the portion in bold should be modified

 
     //  Current RDDSYS.PRG
     #include "rddsys.ch"

     ANNOUNCE RDDSYS                     // This line must not change
     INIT PROCEDURE RddInit
        REQUEST DBFNTX                   // Force link for DBFNTX RDD
        RDDSETDEFAULT( "DBFNTX" )        // Set up DBFNTX as default
                                         // driver

        RETURN

     // eof: rddsys.prg

To change the default to a new automatically-loading driver, modify the bold lines in RDDSYS.PRG to include the name of the new driver. For example:

     //  Revised RDDSYS.PRG
     #include "rddsys.ch"

     ANNOUNCE RDDSYS                     // This line must not change
     INIT PROCEDURE RddInit
        REQUEST DBFCDX                   // Force link for DBFCDX RDD
        RDDSETDEFAULT( "DBFCDX" )        // Set up DBFCDX as default
                                         // driver

        RETURN

     // eof: rddsys.prg

If you change this file, all Clipper applications in which it is linked will automatically include the new RDD.

To use any RDD other than the default, you must explicitly identify it through use of the VIA clause of the USE command.

You need not disable the automatic DBFNTX loading to use other RDDs in your applications, but if your application will not use any DBFNTX functionality, you can save its code overhead by disabling it.

To completely disable the automatic loading of a default RDD, remove the two lines shown above in bold. For example:

     //  New Revised RDDSYS.PRG
     //  disables auto-loading
     #include "rddsys.ch"

     ANNOUNCE RDDSYS                     // This line must not change
     INIT PROCEDURE RddInit

        RETURN
     // eof: rddsys.prg

Basic Terminology

The RDD architecture introduces several new terms and concepts that are key to the design and usage of RDDs. You should familiarize yourself with these concepts and terms as you begin to use the RDD functionality. The meaning of some earlier terminology is also further defined. The following RDD functional glossary defines the terminology for all RDDs.

. Key Expression : A valid Clipper expression that creates a key value from a single record.

. Key Value : A value that is based on value(s) contained within database fields, associated with a particular record in a database.

. Identity : A unique value guaranteed by the structure of the data file to reference a specific record in a database even if the record is empty. In the Xbase file (.dbf), the identity is the record number; but it could be the value of a unique primary key or even the offset of an array in memory.

. Keyed-Pair : A pair consisting of a key value and an identity.

. Identity Order : Describes a database arranged by identity. In Xbase, this refers to the physical arrangement of the records in the database in the order in which they were entered (natural order).

. Tag : A set of keyed-pairs that provides ordered access to the table based on a key value. Usually, an Order in a multiple-Order index (Order). An Order.

. Order : A named mechanism (index) that provides logical access to a database according to the keyed-pairs. This term encompasses both single indexes and the Tags in multiple-Tag indexes.

Orders are not, themselves, data files. They provide access to data that gives the appearance of an ordering of the data in a specific way. This ordering is defined by the relationships between keyed- pairs. An Order does not change the physical (the natural or entry) order of data in a database.

. Controlling Order : The active Order (index) for a particular work area. Only one Order may control a work area at any time, and it controls the order in which the database is accessed during paging and searching.

. Order List : A list of all the Orders available to the database in the specified work area.

. Order Bag : A container that holds zero or more Orders. Normally a disk or memory file. A traditional index like (.ntx) is an Order Bag that holds only one Order. A multiple-Tag index (.mdx or .cdx) is an Order Bag that holds zero or more Orders. Though Order Bags may be a memory or disk file, Clipper 5.x only supports Order Bags as disk files.

. Record : A record in the traditional database paradigm is a row of one or more related columns (fields) of data. In the expanded architecture of Clipper, a record could be data that does not exactly fit this definition.

A record is, in this expanded context, data associated with a single identity. In an Xbase data structure, this corresponds to a row (fields associated with a record number); in other data structures, this may not be the case.

In this document we use “record” in the traditional sense, but you should be aware that Clipper permits expansion of the meaning of record.

. single-Order Bag : An Order Bag that can contain only one Order. The (.ntx) and (.ndx) files are examples of single-Order Bags.

. multiple-Order Bag : An Order Bag that can contain any number of Orders; a multiple-Tag index. The (.cdx) and (.mdx) files are examples of multiple-Order Bags.

. maintainable scoped Orders : Scoped (filtered) Orders created using the FOR clause. The FOR condition is stored in the index header. Orders of this type are correctly updated using the expression to reflect record updates, deletions and additions.

. non-maintainable/temporary Orders : Orders created using the WHILE or NEXT clauses. These Orders are useful because they can be created quickly. However, the conditions in these clauses are not stored in the index header. Therefore, Orders of this type are not correctly updated to reflect record updates, deletions and additions. They are only for temporary use.

. Lock List : A list of the records that are currently locked in the work area.

The Language Implementation

To support the RDD architecture and let you design applications that are independent of the data format you are using, many existing Clipper commands and functions have been enhanced, and several new language elements have been added. The following tables summarize these changes and additions. See the Reference chapter of this guide for more detailed information on a particular item.

     Enhanced Commands and Functions
     ------------------------------------------------------------------------
     Command/Function  Changes
     ------------------------------------------------------------------------
     APPEND FROM       VIA clause
     COPY TO           VIA clause
     DBAPPEND()        Terminology
     GO                Terminology
     DBAPPEND()        Terminology
     INDEX             ALL, EVAL, EVERY, NEXT, RECORD, REST, TAG, and
                       UNIQUE clauses
     SEEK              SOFTSEEK option
     SET INDEX         ADDITIVE clause
     SET ORDER         IN, TAG clauses
     DBSETINDEX()      Terminology
     RECNO()           Terminology
     ------------------------------------------------------------------------

     New Commands and Functions
     ------------------------------------------------------------------------
     Command/Function    Description
     ------------------------------------------------------------------------
     DELETE TAG          Delete a Tag (Order)
     DBGOTO()            Position record pointer to a specific identity
     DBRLOCK()           Lock the record at the current or specified identity
     DBRLOCKLIST()       Return an array of the currently locked records
     DBRUNLOCK           Release all or specified record locks
     ORDBAGEXT()         Return the Order Bag file extension
     ORDBAGNAME()        Return the Order Bag name of a specific Order
     ORDCREATE()         Create an Order in an Order Bag
     ORDDESTROY()        Remove a specified Order from an Order Bag
     ORDFOR()            Return the FOR expression of an Order
     ORDKEY()            Return the Key expression of an Order
     ORDLISTADD()        Add Order Bag contents or single Order to the Order
                         List
     ORDLISTCLEAR()      Clear the current Order List
     ORDLISTREBUILD()    Rebuild all Orders in the Order List of the current
                         work area
     ORDNAME()           Return the name of an Order in the work area
     ORDNUMBER()         Return the position of an Order in the current Order
                         List
     ORDSETFOCUS()       Set focus to an Order in an Order List
     RDDLIST()           Return an array of the available Replaceable
                         Database Drivers
     RDDNAME()           Return the name of the RDD active in the current or
                         specified work area
     RDDSETDEFAULT()     Set or return the default RDD for the application
     ------------------------------------------------------------------------

User Interface Levels

We want to make it easy for you to quickly take advantage of the added functionality provided in Clipper 5.x. In order to effectively use the RDDs, you should read the following discussions. They are provided as a means of identifying the degree of programming knowledge or Clipper experience that will let you effectively use the RDD features.

For this purpose the RDD feature set is arbitrarily divided into levels A and B. Tables listing the commands or functions that comprise these access levels are also supplied. In addition, an RDD Features Summary is provided in table form which outlines the features available in each driver. The commands and functions in both of these levels of access are described in the Reference chapter of this guide.

Level A – Command-Level Interface

Level A. a simple command-level interface very similar to those found in other languages (e.g., dBASE IV, FoxPro). This is the primary access for new Clipper users who may or may not be familiar with other languages.

The following table lists the commands and functions accessible by the Clipper programmer with background in languages such as dBASE or FoxPro. The commands and functions in this table provide access to the additional features without requiring an advanced knowledge of Clipper or other programming concepts.

     Basic Commands and Functions
     ------------------------------------------------------------------------
     Command/Function  Changes
     ------------------------------------------------------------------------
     DELETE TAG        Delete a Tag
     GOTO              Move the pointer to the specified identity
     INDEX             Create an index file
     SEEK              Search an Order for a specified key value
     SET INDEX         Open one or more Order Bags in the current work area
     SET ORDER         Select the controlling Order
     DBAPPEND()        Append a new record to the current Lock List
     DBRLOCK()         Lock the record at the current or specified identity
     DBRLOCKLIST()     Return an array of the current Lock List
     DBRUNLOCK         Release all or specified record locks
     ------------------------------------------------------------------------

Level B – Function-Level Interface

Level B. Clipper also adds a function level interface that not only allows access to the enhanced functionality of the drivers, but permits the building of higher-level functions using these composing behaviors. This level is meant for more experienced Clipper users who need to take advantage of the full power of the driver and Order Management System.

The following table lists the DML and Order Management functions recommended to the intermediate to advanced Clipper programmer. These functions provide the greatest flexibility in accessing the extended features of these drivers

     Advanced Functions (including Order Management)
     ------------------------------------------------------------------------
     Command/Function    Description
     ------------------------------------------------------------------------
     DBAPPEND()          Append a new record to the current Lock List
     DBRLOCK()           Lock the record at the current or specified identity
     DBRLOCKLIST()       Return an array of the current Lock List
     DBRUNLOCK()         Release all or specified record locks
     ORDBAGEXT()         Return the default Order Bag RDD extension
     ORDBAGNAME()        Return the Order Bag name of a specific Order
     ORDCREATE()         Create an Order in an Order Bag
     ORDDESTROY()        Remove a specified Order from an Order Bag
     ORDFOR()            Return the FOR expression of an Order
     ORDKEY()            Return the Key expression of an Order
     ORDLISTADD()        Add Order Bag contents or single Order to the Order
                         List
     ORDLISTCLEAR()      Clear the current Order List
     ORDLISTREBUILD()    Rebuild all Orders in the Order List of the current
                         work area
     ORDNAME()           Return the name of an Order in the work area
     ORDNUMBER()         Return the position of an Order in the current Order
                         List
     ORDSETFOCUS()       Set focus to an Order in an Order List
     RDDLIST()           Return an array of the available Replaceable
                         Database Drivers
     RDDNAME()           Return the name of the RDD active in the current or
                         specified work area
     RDDSETDEFAULT()     Set or return the default RDD for the application
     ------------------------------------------------------------------------

RDD Features

The following decision table summarizes the availability of key features across RDDs. It lists the features available in each RDD so you can use it as an aid in correct RDD implementation and data access.

     RDD Features Summary
     ------------------------------------------------------------------------
     Item                                NTX   NDX   MDX   CDX  DBPX
     ------------------------------------------------------------------------
     Implicit record unlocking in        Yes   Yes   Yes   Yes  Yes
     single lock mode
     Multiple Record Locks               Yes   Yes   Yes   Yes  No
     Number of Concurrent Record Locks   *1    *1    *1    *1   1
     Order Management (Tag support)      Yes   Yes   Yes   Yes  No
     Orders (Tags) per Order Bag (File)  1     1     47    50   N/A
     Number of Order Bags (Files)        15    15    15    15   N/A
     per work area
     Conditional Indexes (FOR clause)    Yes   No    Yes   Yes  No
     Temporary (Partial) Indexes         Yes   No    No    Yes  No
     (WHILE, ... )
     Descending via DESCENDING clause    Yes   No    Yes   Yes  No
     Unique via the UNIQUE clause        Yes   Yes   Yes   Yes  No
     EVAL and EVERY clause support       Yes   No    No    Yes  No
     Production/Structural Indexes       No    No    Yes   Yes  No
     Maximum Key Expression length       256   256   220   255  N/A
     (bytes)
     Maximum FOR Condition length        256   N/A   261   255  N/A
     (bytes)
     ------------------------------------------------------------------------

     *1 determined by available memory.

Clipper 5.x Order Management

Clipper includes a new Order Management System which provides a more effective and flexible way of indexing data. The main objective of the new Order Management implementation is to raise the Xbase indexing paradigm from a low level of abstraction (Xbase database specific) to a higher, more robust, level. This higher level of abstraction allows the user to build new commands and functions.

Low level abstraction refers to manipulation of discrete elements in the database architecture (i.e., field names and sizes, methods of handling controlling indexes, etc.).

High level abstraction refers to manipulation of general elements in a data source. It lets us, for example, set a controlling Order without explicitly addressing the character of the data file structure. This higher level of abstraction was achieved by reviewing all the processes that indexes have in common.

The Order Management function set was generically named (i.e. non-dbf specific) to provide a semantic that could encompass future RDD implementations that may not be file-bound. For example, an RDD could easily be created that orders (indexes) on a memory array, or other data structure, instead of a database. Therefore, all Order Management functions simply begin with ORD (for Order). You will find the function names to be self-explanatory (e.g., ORDCREATE() creates an Order, and ORDDESTROY() destroys an Order).

Concept

An Order is a set of keyed-pairs that provides a logical ordering of the records in an associated database file. Each key in an Order (index) is associated with a particular identity (record number) in the data set (database file). The records can be processed sequentially in key order, and any record can be located by performing a SEEK operation with the associated key value. An Order never physically changes the data that it’s applied against, but creates a different view of that data.

There are at least four basic types of processes that you can perform with an Order:

1. Ordering: Changes the sequence in which you view the data records.

2. Scoping: Constrains the visibility of data to specified upper and lower bounds. Determines the range of data items included, through a scoping rule, like the WHILE clause.

3. Filtration: Visibility of data is subject to conditional evaluation. Filtration determines which items of data are included, through a filter rule, like the FOR clause.

4. Translation: Values in underlying data source are translated (or converted) in some form based on a selection criteria. For example:

INDEX ON IIF(CUSTID > 1000, "NEW", "OLD")

The difference between scope and condition as it applies to FOR and WHILE is that the WHILE clause provides scope, but not filtering, but a FOR clause can provide both.

There are three primary elements in Order Management:

. Order: An Order is a set that has two elements in it: an Order Name, which is a logical name that can be referenced, and an Order Expression which supplies the view of the data. The Order Name provides logical access to the expression and the Order Expression provides a way of viewing the underlying data source. Data ordering can also be modified to ascending or descending sequence.

– Order Name: An Order Name is a symbolic name, that you use to manipulate an Order, like a file’s alias. The difference between an Order Name and the Order Number with which you would normally access indexes (Orders), is that the Order Name is stored in the index file. It is available each time you run the program, and is maintained by the system. The Order Number is generated each time the Order is added to an Order List and may change from one program execution to another. This makes Order Name the preferred means of referencing Orders.

– Order Expression: Is any valid Clipper expression. This is an index expression such as:

CUSTLIST->CUSTID

This expression produces the ordered view of the data. The values derived from this expression are sorted, and it is the relationship of these values to one another that provides the actual ordering.

. Order Number: An Order Number is provided by the Order List. An Order Number is only valid as long as the work area to which it belongs is open.

– Order Numbers provide one of the services performed by Order Names, allowing you to access a specific Order. In general, you should avoid accessing Orders by number.

– The ORDNUMBER() function returns the ordinal position of the specified <orderName> within the specified <orderList>.

. Order Bag: Unsorted collection of Orders. Each Order contains two elements (Order Name and Order Expression). Each Order Bag may have zero to n Orders. The maximum is determined by the RDD driver being used. Order Bags are similar to multiple-index files in that there’s no guarantee of any specific order within the container or Bag. Within an Order Bag you can access specific Orders by referencing a particular Order Name. Order Bags have persistence between activations of the program.

. Order List: An Order List orders the collection of Orders that are associated with and active in the current work area. It provides an access to the Orders active within a given work area. Each work area has an Order List, and there is only one Order List per work area. An Order List is created when a new work area is opened, and exists only as long as that work area is active. Once you close a work area, the Order List ceases to exist.

When you SET INDEX TO, the contents of the Order Bag are emptied into the Order List. At this point, the Orders in the Order List are active in the work area, where they will be updated as the data associated with the work area is modified. You may access an Order in the list by its Order Number or by its Order Name. You should access an Order by its name rather than a hard-coded ordinal position. You can make any Order in the Order List the controlling Order by giving it focus, as explained below.

. Order List Focus: Order List Focus is, essentially, a pointer to the Order that is used to change the view of the data. It is synonymous with controlling Order or controlling index, and defines the active index order. The SET ORDER TO command does not modify the Order List in any way. It does not clear the active indexes. It only changes the Order List Focus (the controlling order in the Order List).

Notes

The following list contains specific information regarding Order Bag usage and limitations with DBFNDX and DBFNTX index files:

. Single-Order Bags: With DBFNDX and DBFNTX you can explicitly assign the Order Name within the Order creation syntax. You can then use the Order Name in any command or function that accepts an Order Name (Tag) as a parameter.

. Single-Order Bag with INDEX ON: Single-Order Bags may retain the Order Name between activations. During creation, DBFNTX stores an optionally supplied Order Name in the file’s header for subsequent use. Therefore, the Order Name is not necessarily the same as that of the file. By contrast, DBFNDX cannot store an Order Name since this would prevent dBASE from accessing the file. By default DBFNDX Orders inherit the name of their index file.

Summary

This chapter has introduced you to the RDD concept, giving you specific information on the architecture that implements RDDs in Clipper. The basic terminology of RDDs has also been defined.

Finally, you have seen an overview of the language enhancements designed to make using RDDs straightforward and to let you build applications that do not depend on the RDD in use. The next chapter elaborates on these language enhancements, discussing syntax and usage in detail.

C5_DBSETINDEX

 DBSETINDEX()
 Empty orders from an order bag into the order list
------------------------------------------------------------------------------
 Syntax

     DBSETINDEX(<cOrderBagName>) --> NIL

 Arguments

     <cOrderBagName> is the name of a disk file containing one or more
     orders.  You may specify <cOrderBagName> as the file name with or
     without the path name or extension.  If you do not include the extension
     as part of <cOrderBagName>, Clipper uses the default extension of the
     current RDD.

 Returns

     DBSETINDEX() always returns NIL.

 Description

     DBSETINDEX() is a database function that adds the contents of an order
     bag into the order list of the current work area.  Any orders already
     associated with the work area continue to be active.  If the newly
     opened order bag is the only order associated with the work area, it
     becomes the controlling order; otherwise, the controlling order remains
     unchanged.  If the order bag contains more than one order, and there are
     no other orders associated with the work area, the first order in the
     new order bag becomes the controlling order.

     Note: DBSETINDEX() does not close all currently open index files.

     DBSETINDEX() is a compatibility command and therefore is not
     recommended.  It is superseded by the ORDLISTADD() function.

 Examples

     USE Customer NEW
     DBSETINDEX( "Cust01" )         // Open the index Cust01
                                   // in the current work area
     DBSETINDEX( "Cust02" )         // Open the index Cust02
                                   // leaving Cust01 open

See Also: ORDLISTADD()



C5 Index Commands and Functions

Index Commands and Functions

Commands :

DELETE TAG :

Delete a Tag

DELETE TAG <cOrderName> [IN <xcOrderBagName>]
    [, <cOrderName> [IN xcOrderBagName] list>]

INDEX ON … :

Create an index file

INDEX ON <expKey>
    [TAG <cOrderName>]
    TO <xcOrderBagName>
    [FOR <lCondition>] [ALL]
    [WHILE <lCondition>]
    [NEXT <nNumber>]
    [RECORD <nRecord>]
    [REST]
    [EVAL <bBlock>
    [EVERY <nInterval>]
    [UNIQUE]
    [ASCENDING|DESCENDING]

REINDEX :

Rebuild open indexes in the current workarea

REINDEX
    [EVAL <lCondition>]
    [EVERY <nRecords>]]

SET INDEX

Open index file(s) in the current work area

SET INDEX TO [<xcIndex list>]

SET ORDER

Set a new controlling index

SET ORDER TO [<nOrder> | [TAG <cOrderName>]
    [IN <xcOrderBagName>]]>

SET UNIQUE* : 

Toggle the inclusion of nonunique keys into an index

SET UNIQUE on | OFF | <xlToggle>

Functions :

DBCLEARINDEX() :

Close all indexes for the current work area

DBCLEARINDEX() --> NIL

DBCREATEINDEX() :

Create an index file

DBCREATEINDEX( <cIndexName>, <cKeyExpr>,
    <bKeyExpr>, 
    [<lUnique>] ) --> NIL

DBREINDEX() : 

Recreate all active indexes for the current work area

DBREINDEX() --> NIL

DBSEEK() : 

Move to the record having the specified key value

DBSEEK( <expKey>, [<lSoftSeek>] ) --> lFound

DBSETINDEX() : 

Open an index for the current work area

 DBSETINDEX( <cIndexName> ) --> NIL
 DBSETINDEX( <cOrderBagName> ) --> NIL

DBSETORDER() : 

Set the controlling order for the current work area

DBSETORDER( <nOrderNum> ) --> NIL

DESCEND() : 

Return a descending index key value

DESCEND( <exp> ) --> ValueInverted

FOUND() : 

Determine if the previous search operation succeeded

FOUND() --> lSuccess

INDEXEXT() : 

Return the default index extension

INDEXEXT() --> cExtension

INDEXKEY() : 

Return the key expression of a specified index

INDEXKEY( <nOrder> ) --> cKeyExp

INDEXORD() : 

Return the order position of the controlling index

INDEXORD() --> nOrder

ORDBAGEXT() :

Return the default Order Bag RDD extension

ORDBAGEXT() --> cBagExt

ORDBAGNAME() :

Return the Order Bag name of a specific Order

ORDBAGNAME(<nOrder> | <cOrderName>) --> cOrderBagName

ORDCOND()

Specify conditions for ordering

ORDCOND([FOR <lCondition>]
             [ALL] [WHILE <;lCondition>]
             [EVAL <bBlock> [EVERY <nInterval>]]
             [RECORD <nRecord>] [NEXT <nNumber>]
             [REST] [DESCENDING])

ORDCONDSET()

Set the condition and scope for an order

     ORDCONDSET([<cForCondition>],
        [<bForCondition>],
        [<lAll>],
        [<bWhileCondition>],
        [<bEval>],
        [<nInterval>],
        [<nStart>],
        [<nNext>],
        [<nRecord>],
        [<lRest>],
        [<lDescend>],
        [<lAdditive>],
        [<lCurrent>],
        [<lCustom>],
        [<lNoOptimize>]) --> lSuccess

ORDCREATE():

Create an Order in an Order Bag

ORDCREATE(<cOrderBagName>,[<cOrderName>], <cExpKey>,

    [<bExpKey>], [<lUnique>]) --> NIL

ORDDESCEND()

Return and optionally change the descending flag of an order

ORDDESCEND([<cOrder> | <nPosition>],[<cIndexFile>],
            [<lNewDescend>]) --> lCurrentDescend

ORDDESTROY() :

Remove a specified Order from an Order Bag

ORDDESTROY(<cOrderName> [, <cOrderBagName> ]) --> NIL

ORDFOR() :

Return the FOR expression of an Order

ORDFOR(<cOrderName> | <nOrder>
    [, <cOrderBagName>]) --> cForExp

ORDISUNIQUE()

          Return the status of the unique flag for a given order

    ORDISUNIQUE([<cOrder> | <nPosition>],
        [<cIndexFile>]) --> lUnique

ORDKEY() :

Return the Key expression of an Order

ORDKEY(<cOrderName> | <nOrder>
 [, <cOrderBagName>]) --> cExpKey

ORDKEYADD()

Add a key to a custom built order

         ORDKEYADD([<cOrder> | <nPosition>],
            [<cIndexFile>],[<expKeyValue>]) --> lSuccess

 

ORDKEYCOUNT()

Return the number of keys in an order

         ORDKEYCOUNT([<cOrder> | <nPosition>],
              [<cIndexFile>]) --> nKeys

ORDKEYDEL()

Delete a key from a custom built order

        ORDKEYDEL([<cOrder> | <nPosition>],
            [<cIndexFile>],
            [<expKeyValue>]) --> lSuccess

ORDKEYGOTO()

Move to a record specified by its logical record number

ORDKEYGOTO(<nKeyNo>) --> lSuccess

ORDKEYNO()

 Get the logical record number of the current record

         ORDKEYNO([<cOrder> | <nPosition>],
             [<cIndexFile>]) --> nKeyNo

 ORDKEYVAL()

Get key value of the current record from controlling order

ORDKEYVAL() --> xKeyValue

 

ORDLISTADD() :

Add Order Bag contents or single Order to the Order List

ORDLISTADD(<cOrderBagName>
    [, <cOrderName>]) --> NIL

ORDLISTCLEAR() :

Clear the current Order List

ORDLISTCLEAR() --> NIL

ORDLISTREBUILD() :

Rebuild all Orders in the Order List of the current work area

ORDLISTREBUILD() --> NIL

ORDNAME() :

Return the name of an Order in the work area

ORDNAME(<nOrder>[,<cOrderBagName>])
    --> cOrderName

ORDNUMBER() :

Return the position of an Order in the current Order List

ORDNUMBER(<cOrderName>[, <cOrderBagName>]) --> nOrderNo

 ORDSCOPE()

Set or clear the boundaries for scoping key values

ORDSCOPE(<nScope>, [<expNewValue>]) --> uCurrentValue

ORDSETFOCUS() :

Set focus to an Order in an Order List

ORDSETFOCUS([<cOrderName> | <nOrder>]
    [,<cOrderBagName>]) --> cPrevOrderNameInFocus

ORDSETRELAT()

Relate a specified work area to the current work area

    ORDSETRELATION(<nArea> | <cAlias>,<bKey>, [<cKey>])

             --> NIL

ORDSKIPUNIQUE()

Move record pointer to the next or previous unique key

ORDSKIPUNIQUE([<nDirection>]) –> lSuccess