Constraints¶
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.
Syntax:
[CONSTRAINT <name>] PRIMARY KEY [ column_name [, ... ] ]
For example, a table with a named PRIMARY KEY constraint can be created
with:
cr> CREATE TABLE person (
... firstname TEXT,
... lastname TEXT,
... CONSTRAINT c PRIMARY KEY (firstname, lastname)
... );
CREATE OK, 1 row affected (... sec)
The named PRIMARY KEY constraints can be inlined:
cr> CREATE TABLE person2 (
... firstname TEXT CONSTRAINT c PRIMARY KEY,
... lastname TEXT CONSTRAINT c PRIMARY KEY
... );
CREATE OK, 1 row affected (... sec)
If a new column is required to be added as a PRIMARY KEY column:
cr> ALTER TABLE person2 ADD COLUMN middleName text PRIMARY KEY;
ALTER OK, -1 rows affected (... sec)
The PRIMARY KEY constraint can also be unnamed, e.g.:
cr> CREATE TABLE person3 (
... firstname TEXT PRIMARY KEY,
... lastname TEXT PRIMARY KEY
... );
CREATE OK, 1 row affected (... sec)
If CONSTRAINT <name> is omitted, CrateDB generates a unique name
automatically. This name is visible in
table_constraints.
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 CONSTRAINT <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 inlined 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 inlined, 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)
You can reference multiple columns using table constraints:
cr> CREATE TABLE metrics5 (
... weight REAL,
... qty INTEGER,
... CHECK (weight * qty != 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.
Note
To add a CHECK constraint to a sub-column of an object column you must
address the sub-column by it’s full path:
cr> CREATE TABLE metrics6 (properties OBJECT AS (weight INTEGER CHECK (properties['weight'] >= 0)))
CREATE OK, 1 row affected (... sec)
Column constraints¶
Column constraints are constraints that are applied on each column of the table separately.
The supported column constraints are:
NULL¶
The NULL constraint specifies that a column of a table can also contain
null values.
The columns that are part of the primary key of a table cannot be declared as
NULL.
A column cannot be declared both as NULL and NOT NULL.
Note
NULL constraint is not shown in SHOW CREATE TABLE, as is the
default for every column.
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.