5. Using Constraints to Improve Performance
Simply defined, constraints are rules that a database designer specifies when setting up a table. MySQL enforces these rules when changes are made to information stored in the database. These changes usually occur via INSERT
, UPDATE
, or DELETE
statements, although they can also be triggered by structural alterations to the tables themselves.
MySQL offers the following constraints:
• UNIQUE
—Guarantees that there will be no duplicate values in a column
• PRIMARY KEY
—Identifies the primary unique identifier of a row
• FOREIGN KEY
—Codifies and enforces the relationships among two or more tables with regard to appropriate behavior when data changes
• DEFAULT
—Provides an automatic value for ...
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