ADD CONSTRAINT clause

Applies to: Databricks SQL Databricks Runtime

Adds an informational primary key, informational foreign key, or an enforced check constraint to an existing Delta Lake table.

Syntax

ADD [check_constraint | key_constraint ]

check_constraint
    CONSTRAINT name CHECK ( condition ) [ ENFORCED ]

key_constraint
    { [ CONSTRAINT name ]
      {
        PRIMARY KEY ( key_column [ TIMESERIES ] [, ...] ) [ constraint_option [...] ] |
        { FOREIGN KEY (foreign_key_column [, ...] )
          REFERENCES parent_table [ ( parent_column [, ...] ] )
          [ constraint_option | foreign_key_option ] [...]
      }
    }

constraint_option
    { NOT ENFORCED |
      DEFERRABLE |
      INITIALLY DEFERRED |
      { NORELY | RELY } }

foreign_key_option
    { MATCH FULL |
      ON UPDATE NO ACTION |
      ON DELETE NO ACTION }

For compatibility with non-standard SQL dialects you can specify ENABLE NOVALIDATE instead of NOT ENFORCED DEFERRABLE INITIALLY DEFERRED.

Parameters

• check_constraint

  Defines a check constraint for a Delta Lake table.

  • CONSTRAINT name

    Specifies a name for the constraint. The name must be unique within the table. If no name is provided Azure Databricks will generate one.

  • CHECK ( condition )

    condition must be a deterministic expression returning a BOOLEAN.

    condition may be composed of literals, column identifiers within the table, and deterministic, built-in SQL functions or operators except:

    • Aggregate functions
    • Analytic window functions
    • Ranking window functions
    • Table valued generator functions

    Also condition must not contain any subquery.

    For a CHECK constraint to be satisfied in Azure Databricks it must evaluate to true.

    Delta Lake verifies the validity of the check constraint against both new and existing data. If any existing row violates the constraint an error will be raised.

• key_constraint

  Important

  This feature is in Public Preview.

  Applies to: Databricks SQL Databricks Runtime 11.3 LTS and above

  Defines an informational primary key or informational foreign key constraint for a Delta Lake table.

  • CONSTRAINT name

    Optionally specifies a name for the constraint. The name must be unique within the schema. If no name is provided Azure Databricks will generate one.

  • PRIMARY KEY ( key_column [ TIMESERIES ] [, ...] ) [ constraint_option [...] ]

    Applies to: Unity Catalog only

    Adds a primary key constraint to the Delta Lake table. A table can have at most one primary key.

    Primary key constraints are not supported for tables in the hive_metastore catalog.

  • key_column

    A column of the subject table defined as NOT NULL. Column names must not be repeated.

  • TIMESERIES

    Applies to: Databricks SQL Databricks Runtime 13.3 LTS and above

    Optionally labels the primary key column component as representing a timeseries.

  • FOREIGN KEY ( foreign_key_column [, ...] ) REFERENCES parent_table [ ( parent_column [, ...] ) ] foreign_key_option

    Applies to: Unity Catalog only

    Adds a foreign key (referential integrity) constraint to the Delta Lake table.

    Foreign key constraints are not supported for tables in the hive_metastore catalog.

    • foreign_key_column

      A column of the subject table. Column names must not be repeated. The data type of each column must match the type of the matching parent_column. The number of columns must match the number of parent_columns. Two foreign keys cannot share an identical set of foreign key columns.

    • parent_table

      Specifies the table the foreign key refers to. The table must have a defined PRIMARY KEY constraint, and you must have the SELECT privilege on the table.

    • parent_column

      A column in the parent table which is part of its primary key. All primary key columns of the parent table must be listed.

      If parent columns are not listed they are implied to be specified in the order given in the PRIMARY KEY definition.

    Foreign key constraints which only differ in the permutation of the foreign key columns are not allowed.

  • constraint_option

    Lists the properties of the constraints. All properties are optional but implied by default. Each property can at most be specified once.

    • NOT ENFORCED

      Azure Databricks takes no action to enforce it for existing or new rows.

    • DEFERRABLE

      The constraint enforcement can be deferred.

    • INITIALLY DEFERRED

      Constraint enforcement is deferred.

    • NORELY or RELY

      Applies to: Databricks SQL Databricks Runtime 14.2 and above for PRIMARY KEY constraints

      Applies to: Databricks SQL Databricks Runtime 15.4 and above for FOREIGN KEY constraints

      If RELY, Azure Databricks may exploit the constraint to rewrite queries. It is the user’s responsibility to ensure the constraint is satisfied. Relying on a constraint that is not satisfied may lead to incorrect query results.

      The default is NORELY.

  • foreign_key_option

    Lists the properties specific to foreign key constraints. All properties are optional but implied by default. Each property can at most be specified once.

    • MATCH FULL

      For the constraint to be considered true all column values must be NOT NULL.

    • ON UPDATE NO ACTION

      If the parent PRIMARY KEY is updated Azure Databricks takes no action to restrict the update or update the foreign key.

    • ON DELETE NO ACTION

      If the parent row is deleted Azure Databricks takes no action to restrict the action, update the foreign key, or delete the dependent row.

Important

Azure Databricks does not enforce primary key or foreign key constraints. Confirm key constraints before adding a primary or foreign key. Your ingest process may provide such assurance, or you can run checks against your data.

Examples

-- Add a primary key
> CREATE TABLE persons(first_name STRING NOT NULL, last_name STRING NOT NULL, nickname STRING);
> ALTER TABLE persons ADD CONSTRAINT persons_pk PRIMARY KEY(first_name, last_name);

-- Add a foreign key which Databricks does not enforce, but can rely upon.
> CREATE TABLE pets(name STRING, owner_first_name STRING, owner_last_name STRING);
> ALTER TABLE pets ADD CONSTRAINT pets_persons_fk
    FOREIGN KEY(owner_first_name, owner_last_name) REFERENCES persons
    NOT ENFORCED RELY;

-- Add a check contraint
> ALTER TABLE pets ADD CONSTRAINT pets_name_not_cute_chk CHECK (length(name) < 20);

Related articles

• ALTER TABLE
• DROP CONSTRAINT