Creating and managing check constraints

A CHECK constraint allows you to specify that the values of one or more columns must satisfy a boolean expression. In this article, we'll describe how to manage this type of constraint in your database.

Adding a check constraint to a new table

In the following CREATE TABLE snippet, we create a table to store information about concerts. To require that the end time of a concert is later than its start time, we include a check constraint.

CREATE TABLE Concert (
  ConcertId STRING(20),
  StartTime Timestamp,
  EndTime Timestamp,
  CONSTRAINT start_before_end CHECK(StartTime < EndTime),
) PRIMARY KEY (ConcertId);

The constraint definition begins with the CONSTRAINT keyword. We've explicitly named the constraint as start_before_end in this example, which makes it easy to identify in error messages and whenever we need to refer to it. If no name is given, Cloud Spanner provides one, with the generated name beginning with the prefix CK_. Constraint names are scoped to the schema, along with the names for tables and indexes, and must be unique within the schema. The check constraint definition consists of the keyword CHECK followed by an expression in parentheses. The expression can only reference columns of this table. In this example, it references StartTime and EndTime, and the check here makes sure that the start time of a concert is always less than the end time.

The value of the check constraint expression is evaluated when a new row is inserted or when StartTime or EndTime of an existing row are updated. If the expression evaluates to TRUE or NULL, the data change is allowed by the check constraint. If the expression evaluates to FALSE, the data change is not allowed.

  • The following restrictions apply to a check constraint expression term.

    • The expression can only reference columns in the same table.

    • The expression must reference at least one non-generated column, whether directly or through a generated column which references a non-generated column.

    • The expression can't reference columns that have set the allow_commit_timestamp option.

    • The expression can't contain subqueries.

    • The expression can't contain non-deterministic functions, such as CURRENT_DATE() and CURRENT_TIMESTAMP().

Adding a check constraint to an existing table

Using the following ALTER TABLE statement, we add a constraint to make sure that all concert ids are greater than zero.

ALTER TABLE Concerts
ADD CONSTRAINT concert_id_gt_0 CHECK (ConcertId > 0);

Once again, we've given the constraint a name, concert_id_gt_0. Adding a CHECK constraint to an existing table starts the enforcement of the constraint immediately for new data and starts a long-running operation to validate that existing data conforms to the new constraint. Because this validation is performed as a long-running operation, ongoing transactions on the table are not impacted. For more information, see Schema update performance. If there are any violations on existing data, the constraint is rolled back.

Removing a check constraint

The following DDL statement drops a CHECK constraint from the Concerts table.

ALTER TABLE Concerts
DROP CONSTRAINT concert_id_gt_0;

Modifying a check constraint expression

Modifying the expression of a CHECK constraint is not allowed. Instead, you need to drop the existing constraint and create a new constraint with the new expression.

Viewing properties of a check constraint

Cloud Spanner's INFORMATION_SCHEMA contains information about the check constraints on your database. The following are some examples of the questions you can answer by querying the information schema.

What check constraints are defined in my database?

SELECT tc.CONSTRAINT_NAME, tc.TABLE_NAME, tc.CONSTRAINT_TYPE
FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS as tc
WHERE tc.CONSTRAINT_TYPE = "CHECK";

What is the current state of the check constraints in my database?

If you have added a check constraint to an existing table, you might want to view its current state to determine, for example, whether all existing data has been validated against the constraint. If SPANNER_STATE returns VALIDATING_DATA in the following query, it means Cloud Spanner is still in the process of validating existing data against that constraint.

SELECT cc.CONSTRAINT_NAME, cc.SPANNER_STATE
FROM INFORMATION_SCHEMA.CHECK_CONSTRAINTS as cc;

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