Constraints

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Constraints offer additional data integrity by enforcing conditions on the data within a column. Whenever values are manipulated (inserted, deleted, or updated), constraints are checked and modifications that violate constraints are rejected.

For example, the UNIQUE constraint requires that all values in a column be unique from one another (except NULL values). If you attempt to write a duplicate value, the constraint rejects the entire statement.

Supported constraints

Constraint	Description
`CHECK`	Values must return `TRUE` or `NULL` for a Boolean expression.
`DEFAULT` value	If a value is not defined for the constrained column in an `INSERT` statement, the `DEFAULT` value is written to the column.
`FOREIGN KEY`	Values must exactly match existing values from the column it references.
`NOT NULL`	Values may not be `NULL`.
`PRIMARY KEY`	Values must uniquely identify each row (one per table). This behaves as if the `NOT NULL` and `UNIQUE` constraints are applied, as well as automatically creates an index for the table using the constrained columns.
`UNIQUE`	Each non-`NULL` value must be unique. This also automatically creates an index for the table using the constrained columns.

Using constraints

Add constraints

How you add constraints depends on the number of columns you want to constrain, as well as whether or not the table is new.

One column of a new table has its constraints defined after the column's data type. For example, this statement applies the PRIMARY KEY constraint to foo.a:
```
> CREATE TABLE foo (a INT PRIMARY KEY);
```
Multiple columns of a new table have their constraints defined after the table's columns. For example, this statement applies the PRIMARY KEY constraint to foo's columns a and b:
```
> CREATE TABLE bar (a INT, b INT, PRIMARY KEY (a,b));
```

Note:

The DEFAULT and NOT NULL constraints cannot be applied to multiple columns.

Existing tables can have the following constraints added:
- CHECK, FOREIGN KEY, and UNIQUE constraints can be added through ALTER TABLE...ADD CONSTRAINT. For example, this statement adds the UNIQUE constraint to baz.id:
```
> ALTER TABLE baz ADD CONSTRAINT id_unique UNIQUE (id);
```
- DEFAULT values and NOT NULL constraints can be added through ALTER TABLE...ALTER COLUMN. For example, this statement adds the DEFAULT value constraint to baz.bool:
```
> ALTER TABLE baz ALTER COLUMN bool SET DEFAULT true;
```
- PRIMARY KEY constraints can be added with ADD CONSTRAINT/ADD PRIMARY KEY in the following circumstances:
  - A DROP CONSTRAINT statement precedes the ADD CONSTRAINT/ADD PRIMARY KEY statement in the same transaction. For examples, see Add constraints and Drop constraints.
  - The current primary key is on rowid, the default primary key created if none is explicitly defined at table creation.
  - The ADD CONSTRAINT/ADD PRIMARY KEY is in the same transaction as a CREATE TABLE statement with no primary key defined.

Order of constraints

The order in which you list constraints is not important because constraints are applied to every modification of their respective tables or columns.

Name constraints on new tables

You can name constraints applied to new tables using the CONSTRAINT clause before defining the constraint:

> CREATE TABLE foo (a INT CONSTRAINT another_name PRIMARY KEY);

> CREATE TABLE bar (a INT, b INT, CONSTRAINT yet_another_name PRIMARY KEY (a,b));

View constraints

To view a table's constraints, use SHOW CONSTRAINTS or SHOW CREATE.

Remove constraints

The procedure for removing a constraint depends on its type:

Constraint Type	Procedure
`CHECK`	Use `DROP CONSTRAINT`.
`DEFAULT` value	Use `ALTER COLUMN`.
`FOREIGN KEY`	Use `DROP CONSTRAINT`.
`NOT NULL`	Use `ALTER COLUMN`.
`PRIMARY KEY`	Primary key constraints can be dropped with `DROP CONSTRAINT` if an `ADD CONSTRAINT` statement follows the `DROP CONSTRAINT` statement in the same transaction.
`UNIQUE`	The `UNIQUE` constraint cannot be dropped directly. To remove the constraint, drop the index that was created by the constraint, e.g., `DROP INDEX my_unique_constraint`.

Change constraints

The procedure for changing a constraint depends on its type:

Constraint Type	Procedure
`CHECK`	Issue a transaction that adds a new `CHECK` constraint (`ADD CONSTRAINT`), and then remove the existing one (`DROP CONSTRAINT`).
`DEFAULT` value	The `DEFAULT` value can be changed through `ALTER COLUMN`.
`FOREIGN KEY`	Issue a transaction that adds a new `FOREIGN KEY` constraint (`ADD CONSTRAINT`), and then remove the existing one (`DROP CONSTRAINT`).
`NOT NULL`	The `NOT NULL` constraint cannot be changed, only added and removed with `ALTER COLUMN`.
`PRIMARY KEY`	To change a primary key, use an `ALTER TABLE ... ALTER PRIMARY KEY` statement. When you change a primary key with `ALTER PRIMARY KEY`, the old primary key index becomes a secondary index. If you do not want the old primary key to become a secondary index, use `DROP CONSTRAINT`/`ADD CONSTRAINT` to change the primary key.
`UNIQUE`	Issue a transaction that adds a new `UNIQUE` constraint (`ADD CONSTRAINT`), and then remove the existing one (`DROP CONSTRAINT`).

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