Constraints¶
Table of contents
Table constraints¶
Table constraints are constraints that are applied to the table as a whole.
PRIMARY KEY¶
The PRIMARY KEY constraint specifies that a column or columns of a table
can contain only unique (non-duplicate), non-null values.
Using columns of type object, geo_point, geo_shape or array as
PRIMARY KEY is not supported.
To use a whole object as PRIMARY KEY each column within the object
can be declared as PRIMARY KEY instead.
Adding a PRIMARY KEY column is only possible if the table is empty.
Warning
The verification if the table is empty and the schema update isn’t atomic. That means that it could be possible to add a primary key column to a table that isn’t empty.
If that is the case queries that contain the primary key columns in the
WHERE clause will not behave as expected.
INDEX¶
The INDEX constraint specifies a specific index method on one or more
columns.
It is possible to define more than one index per table, whether as a column constraint or a table constraint.
See also
CHECK¶
The CHECK constraint specifies that the values of certain columns must
satisfy a boolean expression on INSERT and
UPDATE.
Syntax:
[CONSTRAINT check_name>] CHECK (boolean_expression)
If CONSTAINT check_name is omitted, CrateDB generates a unique name
automatically. This name is visible in
table_constraints. This name can be used with
DROP CONSTRAINT to remove the constraint.
The CONSTRAINT definition can either be inline with a column, like this:
cr> CREATE TABLE metrics1 (
... weight REAL CONSTRAINT weight_is_positive CHECK (weight >= 0),
... );
CREATE OK, 1 row affected (... sec)
Or, also inline, but without explicit name:
cr> CREATE TABLE metrics2 (
... weight REAL CHECK (weight >= 0),
... );
CREATE OK, 1 row affected (... sec)
Or, on a table level with explicit name:
cr> CREATE TABLE metrics3 (
... weight REAL,
... CONSTRAINT weight_is_positive CHECK (weight >= 0),
... );
CREATE OK, 1 row affected (... sec)
Or without name:
cr> CREATE TABLE metrics4 (
... weight REAL,
... CHECK (weight >= 0),
... );
CREATE OK, 1 row affected (... sec)
Multiple columns can be referenced:
cr> CREATE TABLE metrics5 (
... weight REAL,
... qty INTEGER,
... CHECK (weight * qty != 1918),
... );
CREATE OK, 1 row affected (... sec)
cr> CREATE TABLE metrics6 (
... weight REAL,
... qty INTEGER CHECK (qty * weight != 1918)
... );
CREATE OK, 1 row affected (... sec)
Warning
The CHECK constraint conditions must be deterministic, always yielding
the same result for the same input.
A way to break this is to reference a user-defined function in a CHECK expression, and then change the
behavior of that function. Some existing rows in the table could now violate
the CHECK constraint. That would cause a subsequent database dump and
reload to fail.
Column constraints¶
Column constraints are constraints that are applied on each column of the table separately.
The supported column constraints are:
NOT NULL¶
The NOT NULL constraint specifies that a column of a table can contain only
non-null values.
The columns that are part of the primary key of a table are NOT NULL by
default.