Toggle inclusion of non-unique keys into an index
SET UNIQUE on | OFF | <xlToggle>
ON causes index files to be created with a uniqueness attribute.
OFF causes index files to be created without a uniqueness attribute.
<xlToggle> is a logical expression that must be enclosed in parentheses. A value of true (.T.) is the same as ON, and a value of false (.F.) is the same as OFF.
SET UNIQUE is a database command that controls whether indexes are created with uniqueness as an attribute. With UNIQUE ON, new indexes are created including only unique keys. This is the same as creating an index with the INDEX…UNIQUE command.
If, during the creation or update of an unique index, two or more records are encountered with the same key value, only the first record is included in the index. When the unique index is updated, REINDEXed, or PACKed, only unique records are maintained, without regard to the current SET UNIQUE value.
Changing key values in a unique index has important implications. First, if a unique key is changed to the value of a key already in the index, the changed record is lost from the index. Second, if there is more than one instance of a key value in a database file, changing the visible key value does not bring forward another record with the same key until the index is rebuilt with REINDEX, PACK, or INDEX…UNIQUE.
With UNIQUE OFF, indexes are created with all records in the index. Subsequent updates to the database files add all key values to the index independent of the current UNIQUE SETting.
SET UNIQUE is a compatibility command not recommended. It is superseded by the UNIQUE clause of the INDEX command.
DBCREATEIND(), INDEX, PACK, REINDEX, SEEK