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The SQL constraints are the rules enforced on the data in a table. A constraint can be specified with the CREATE TABLE statement at the time table creation or we can specify it with the ALTER TABLE statement after the table creation.


CREATE TABLE <em>tableName</em>
<em>columnName1 datatype</em>(<em>size</em>) <em>constraintName</em>,
<em>columnName2 data_type</em>(<em>size</em>) <em>constraintName</em>,
<em>columnName2 data_type</em>(<em>size</em>) <em>constraintName</em>,

Commonly used constraints in SQL:

1. SQL PRIMARY KEY Constraint.


3. SQL NOT NULL Constraint.

4. SQL UNIQUE Constraint.

5. SQL CHECK Constraint

6. SQL DEFAULT Constraint

7. SQL INDEX Constraint.

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