What are Constraints:- Constraints are an ANSI-standard method of enforcing data integrity. Constraints ensure valid data values are entered in columns and that the relationships between tables should be maintained.
| Integrity Type | Constraint type | Target | Description |
| Domain | DEFAULT | Column | Specifies the default value for the column when INSERT statement does not provide a value. |
| CHECK | Column | Specifies data values that are acceptable in a column. The CHECK constraint is recommended alternative to a rule object. | |
| FOREIGN KEY | Table | Specifies the data values that are acceptable to update, based on values in a column in another table. | |
| NULL | Column | Specifies whether the value of a column can be NULL. | |
| Entity | PRIMARY KEY | Table | Identifies each row uniquely. Null values are not allowed. |
| UNIQUE | Column | Prevents duplication of alternative (nonprimary) keys and ensures that an index is created to enhance performance. A single null value is allowed. | |
| Referential | FOREIGN KEY | Table | Defines a column or combination of columns with values that match the primary key of the same or another table. |
| CHECK | Column | Specifies the data values that are acceptable in a column based on values in other columns in the same table. |
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