Database Terms

Alias :

The name of a work area; an alternate name given to a database file. Aliases are often used to give database files descriptive names and are assigned when the database file is opened. If no alias is specified when the database file is USEd, the name of the database file becomes the alias.

An alias can be used to reference both fields and expressions (including user-defined functions). In order to alias an expression, the expression must be enclosed in parentheses.

See also : Work Area

Attribute :

As a formal DBMS term, refers to a column or field in a table or database file.

See Also: Column, Field

Beginning of File :

The top of the database file. In Clipper language there is no beginning of file area or record. Instead, it is indicated by BOF() returning true (.T.) if an attempt is made to move the record pointer above the first record in the database file or the database file is empty.

Cell :

In a table, a cell is the intersection of a Row and a Column.

Column :

A database term used to describe a field in a table or database file.

See Also: Field

Concurrency :

The degree to which data can be accessed by more than one user at the same time.

Condition :

A logical expression that determines whether an operation will take place. With database commands, a logical expression that determines what records are included in an operation. Conditions are specified as arguments of the FOR or WHILE clause.

See Also: Scope

Controlling/Master Index :

The index currently being used to refer to records by key value or sequential record movement commands.

See Also: Index, Natural Order

Database :

An aggregation of related operational data used by an application system. A database can contain one or more data files or tables.

See Also: Field, Record, Tuple, View

DBMS :

An acronym for the term database management system. A DBMS is a software system that mediates access to a database through a data manipulation language.

Delimited File :

A text file that contains variable-length database records with each record separated by a carriage return/linefeed pair (CHR(13) + CHR(10)) and terminated with an end of file mark (CHR(26)). Each field within a delimited file is variable length, not padded with either leading or trailing spaces, and separated by a comma. Character strings are optionally delimited to allow for embedded commas.

End of File :

The bottom of a database file. In Clipper language, this is LASTREC() + 1 and is indicated by EOF() returning true (.T.).

Field :

The basic column unit of a database file. A field has four attributes: name, type, length, and decimals if the type is numeric.

See Also: Database, Record, Tuple, Vector, View

Field Variable :

A variable that refers to data in a database field, as opposed to data in memory.

See Also: Local Variable, Memory Variable, Variable

Index :

An ordered set of key values that provides a logical ordering of the records in an associated database file. Each key in an index is associated with a particular record in the 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.

See Also: Controlling/Master Index, Key Value, Natural Order

Join :

An operation that takes two tables as operands and produces one table as a result. It is, in fact, a combination of other operations including selection and projection.

See Also: Projection, Selection

Key Expression :

An expression, typically based on one or more database fields, that when evaluated, yields a key value for a database record. Key expressions are most often used to create indexes or for summarization operations.

See Also: Index, Key Value

Key Value :

The value produced by evaluating a key expression. When placed in an index, a key value identifies the logical position of the associated record in its database file.

See Also: Index, Key Expression

Master Index :

The index currently being used to refer to records by key value or sequential record movement commands.

See Also : Controlling/Master Index

Memo Type :

A special database field type consisting of one or more characters in the extended character set. The maximum size of a memo field In Clipper language is 65,534 bytes. A memo field differs only from a character string by the fact it is stored in a separate memo (.DBT file) and the field length is variable-length.

See Also: Character String

Natural Order :

For a database file, the order determined by the sequence in which records were originally entered into the file. Also called unindexed order.

See Also: Index

Normalization :

The process of elimination and consolidation of redundant data elements in a database system.

Projection :

A DBMS term specifying a subset of fields. In Clipper, the analogy is the FIELDS clause.

See Also: Join Selection

Query :

A request for information to be retrieved from a database. Alternately, a data structure in which such a request is encoded.

Record :

The basic row unit of a database file consisting of one or more field elements.

See Also: Database, Field, Table, Tuple

Relation :

A link between database files that allows the record pointer to move in more than one database file based on the value of a common field or expression. This allows information to be accessed from more than one database file at a time.

Relational Database System :

A system that stores data in rows and columns, without system dependencies within the data. In other words, relationships between different databases are not stored in the actual database itself, as is the case in a system that uses record pointers.

Row :

A group of related column or field values that are treated as a single entity. It is the same as a Clipper language record.

See Also: Column, Field, Record

Search Condition :

See : Condition, Scope

Scope :

In a database command, a clause that specifies a range of database records to be addressed by the command. The scope clause uses the qualifiers ALL, NEXT, RECORD, and REST to define the record scope.

See Also: Condition

SDF File :

A text file that contains fixed-length database records with each record separated by a carriage return/linefeed pair (CHR(13) + CHR(10)) and terminated with an end of file mark (CHR(26)). Each field within an SDF file is fixed-length with character strings padded with trailing spaces and numeric values padded with leading spaces. There are no field separators.

See Also: Database, Delimited File, Text File

Selection :

A DBMS term that specifies a subset of records meeting a condition. The selection itself is obtained with a selection operator. In Clipper language, the analogy is the FOR clause.

Separator :

The character or set of characters that differentiate fields or records from one another. In Clipper language, the DELIMITED and SDF file types have separators. The DELIMITED file uses a comma as the field separator and a carriage return/linefeed pair as the record separator. The SDF file type has no field separator, but also uses a carriage return/linefeed pair as the record separator.

See Also: Delimiter

Sort Order :

Describes the various ways database files and arrays are ordered.

. Ascending

Causes the order of data in a sort to be from lowest value to highest value.

. Descending

Causes the order of data in a sort to be from highest value to lowest value.

. Chronological

Causes data in a sort to be ordered based on a date value, from earliest to most recent.

. ASCII

Causes data in a sort to be ordered according to the ASCII Code values of the data to be sorted.

. Dictionary

The data in a sort is ordered in the way it would appear if the items sorted were entries in a dictionary of the English language.

. Collating Sequence

Data in a sort will be placed in sequence following the order of characters in the Extended Character Set.

. Natural

The order in which data was entered into the database.

Table :

A DBMS term defining a collection of column definitions and row values. In Clipper, it is represented and referred to as a database file.

Tuple :

A formal DBMS term that refers to a row in a table or a record in a database file. In DIF files, tuple also refers to the equivalent of a table record.

See Also: Database, Field, Record

Update :

The process of changing the value of fields in one or more records. Database fields are updated by various commands and the assignment operator.

Vector :

In a DIF file, vector refers to the equivalent of a table field.

See Also: Database, Field, Record, Tuple

View :

A DBMS term that defines a virtual table. A virtual table does not actually exist but is derived from existing tables and maintained as a definition. The definition in turn is maintained in a separate file or as an entry in a system dictionary file. In Clipper, views are supported only by DBU.EXE and are maintained in (.vew) files.

See Also: Database, Field, Record

Work Area :

The basic containment area of a database file and its associated indexes. Work areas can be referred to by alias name, number, or a letter designator.

See Also: Alias

Variable Terms

Argument :

Generally, a value or variable supplied in a function or procedure call, or an operand supplied to an operator. In function and procedure calls, arguments are often referred to as actual parameters.

See also : Parameter

Constant :

The representation of an actual value. For example, .T. is a logical constant, string is a character constant, 21 is a numeric constant. There are no date and memo constants.

Declaration :

A statement used by the compiler to define a variable, procedure, or function identifier. The scope of the declaration is determined by the position of the declaration statement in the source file.

See Also: Identifier, Scope

Dynamic Scoping :

A method of determining an item’s existence or visibility based on the state of a program during execution. Example: A Clipper public variable may or may not be visible within a particular function, depending on whether the variable has been created and whether a previously called function has obscured it by creating a private variable with the same name.

See Also: Lexical Scoping, Scope

Field Variable :

A variable that refers to data in a database field, as opposed to data in memory.

See Also: Local Variable, Memory Variable, Variable

Filewide Declaration :

A variable declaration statement that has the scope of the entire source file. Filewide declarations are specified before the first procedure or function declaration in a program file and the program file must be compiled with the /N option.

See Also: Scope, Storage Class

Identifier :

A name that identifies a function, procedure, variable, constant or other named entity in a source program. In Clipper language, identifiers must begin with an alphabetic character and may contain alphabetic characters, numeric characters, and the underscore character.

Initialize :

To assign a starting value to a variable. If initialization is specified as part of a declaration or variable creation statement, the value to be assigned is called an initializer.

See Also: Assignment

Lexical Scoping :

A method of determining an item’s existence, visibility, or applicability (i.e., the item’s scope) by it’s position within the text of a program.

See Also: Local Variable, Scope, Static Variable

Lexically Scoped Variable :

A variable that is only accessible in a particular section of a program, where that section is defined using simple textual rules. For example, a local variable is only accessible within the procedure that declares it.

See Also: Dynamic Scoping, Local Variable, Static Variable

Lifetime of a Variable :

The period of time during which a variable retains its assigned value. The lifetime of a variable depends on its storage class.

See Also: Scope, Visibility

Local Variable :

A variable that exists and retains its value only as long as the procedure in which it is declared is active (i.e., until the procedure returns control to a higher-level procedure). Local variables are lexically scoped; they are accessible by name only within the procedure where they are declared.

See Also: Dynamic Scoping, Lexical Scoping, Static Variable

Memory Variable :

In general, a variable that resides in memory, as opposed to a database field variable. Sometimes used specifically to refer to variables of the MEMVAR storage class (private and public variables), as opposed to static or local variables.

See Also: Field Variable, Private Variable, Public Variable, Variable

Parameter :

A identifier that receives a value or reference passed to a procedure or user-defined function. A parameter is sometimes referred to as a formal parameter.

See Also: Activation, Argument, Function, Procedure, Reference

Private Variable :

A variable of the MEMVAR storage class. Private variables are created dynamically at runtime using the PRIVATE statement, and accessible within the creating procedure and any lower-level procedures unless obscured by another private variable with the same name.

See Also: Activation, Dynamic Scoping, Function, Public Variable

Public Variable :

A variable of the MEMVAR storage class. Public variables are created dynamically at runtime using the PUBLIC statement, and are accessible from any procedure at any level unless obscured by a private variable with the same name.

See Also: Activation, Dynamic Scoping, Function, Private Variable

Reference :

A special value that refers indirectly to a variable or array. If one variable contains a reference to a second variable (achieved by passing the second variable by reference in a function or procedure call), operations on the first variable (including assignment) are passed through to the second variable. If a variable contains a reference to an array, the elements of the array can be accessed by applying a subscript to the variable.

See Also: Array Reference, Parameter

Static Variable :

A variable that exists and retains its value for the duration of execution. Static variables are lexically scoped; they are only accessible within the procedure that declares them, unless they are declared as filewide, in which case they are accessible to any procedure in the source file that contains the declaration.

See Also: Dynamic Scoping, Lexical Scoping, Local Variable

Storage Class :

Defines the two characteristics of variables: lifetime and visibility.

See Also: Lifetime, Scope, Visibility

Variable :

An area of memory that contains a stored value. Also, the source code identifier that names a variable.

See Also: Local Variable, Private Variable, Static Variable

Visibility :

The set of conditions under which a variable is accessible by name. A variable’s visibility depends on its storage class.

See Also: Dynamic Scoping, Lexical Scoping