# VALIDATE CONSTRAINT

The `VALIDATE CONSTRAINT` statement is part of `ALTER TABLE` and checks whether values in a column match a constraint on the column.

This statement is especially useful after applying a constraint to an existing column via `ADD CONSTRAINT`. In this case, `VALIDATE CONSTRAINT` can be used to find values already in the column that do not match the constraint.

## Required privileges

The user must have the `CREATE` privilege on the table.

## Synopsis

## Parameters

Parameter | Description
--- | ---
`table_name` | The name of the table in which the constraint you'd like to validate lives.
`constraint_name` | The name of the constraint on `table_name` you'd like to validate.

## Examples

In `ADD CONSTRAINT`, we added a foreign key constraint like so:

    > ALTER TABLE orders ADD CONSTRAINT customer_fk FOREIGN KEY (customer_id) REFERENCES customers (id) ON DELETE CASCADE;

In order to ensure that the data added to the `orders` table prior to the creation of the `customer_fk` constraint conforms to that constraint, run the following:

    > ALTER TABLE orders VALIDATE CONSTRAINT customer_fk;

Note:

If present in a `CREATE TABLE` statement, the table is considered validated because an empty table trivially meets its constraints.

## See also

- Constraints
- `ADD CONSTRAINT`
- `CREATE TABLE`