June 2018
Intermediate to advanced
478 pages
10h 52m
English
Content preview from IBM Db2 11.1 Certification Guide
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UNIQUE constraints
The UNIQUE constraint ensures that the values assigned to one or more columns in a table are always unique.
Unlike the NOT NULL, DEFAULT, and CHECK constraints, which can be associated with only a single column in a table, the UNIQUE constraint can be associated with either a single column or a set of columns in the table. The columns specified in a UNIQUE constraint must also have a NOT NULL constraint.
When a UNIQUE constraint is created, the Db2 database manager checks this to determine whether an index for the columns that the UNIQUE constraint refers to already exists. If so, that index will be marked as unique and system required. If not, an appropriate index is created with a SYSIBM schema and marked as unique and ...
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