- Constraints are used to specify rules for the data in a table.
- If there is any violation between the constraint and the data action, the action is aborted by the constraint.
- Constraints can be specified when the table is created (inside the CREATE TABLE statement) or after the table is created (inside the ALTER TABLE statement).
In SQLite, we have the following Constraints:
- NOT NULL – Indicates that a column cannot store NULL value.
1column_name1 data_type(size) NOT NULL
- UNIQUE – Ensures that each row for a column must have a unique value.
1column_name1 data_type(size) UNIQUE
- PRIMARY KEY – A combination of a NOT NULL and UNIQUE. Ensures that a column (or combination of two or more columns) have an unique identity which helps to find a particular record in a table more easily and quickly.
1column_name1 data_type(size) PRIMARY KEY
- FOREIGN KEY – Ensure the referential integrity of the data in one table to match values in another table.
1FOREIGN KEY(Current_Table_Column) REFERENCES Reference_Table(Column)
- CHECK – Ensures that the value in a column meets a specific condition.
1column_name1 data_type(size) CHECK(Condition)
- DEFAULT – Specifies a default value when specified none for this column.
1column_name1 data_type(size) DEFAULT 'Value'