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SQL constraints are used to specify rules for the data in a table.
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The following constraints are commonly used in SQL
Not Null | Ensures that a column cannot have a NULL value |
UNIQUE | Ensures that all values in a column are different |
PRIMARY KEY | A combination of a NOT NULL and UNIQUE. Uniquely identifies each row in a table |
FOREIGN KEY | Uniquely identifies a row/record in another table |
CHECK | Ensures that all values in a column satisfies a specific condition |
DEFAULT | Sets a default value for a column when no value is specified |
INDEX | Use to create and retrieve data from the database very quickly |
Constraints and Indexes
A foreign key checks that a referenced value exists; several rows can reference the same parent. It does not make the referencing column unique.
An index is an access structure, rather than a general SQL integrity constraint. PRIMARY KEY and UNIQUE constraints can use indexes to enforce their rules, but an ordinary index need not enforce uniqueness. DEFAULT supplies a value when one is omitted; it does not by itself validate every value a caller provides.
