Constraint Definition

Description

You can define NOT NULL, UNIQUE, PRIMARY KEY, FOREIGN KEY as the constraints. You can also create an index by using INDEX or KEY.

<column_constraint> ::=

NOT NULL | UNIQUE | PRIMARY KEY | FOREIGN KEY <referential definition>

 

<table_constraint> ::=

[ CONSTRAINT [ <constraint_name> ] ] UNIQUE [ KEY | INDEX ]( column_name_comma_list ) |

[ { KEY | INDEX } [ <constraint_name> ]( column_name_comma_list ) |

[ PRIMARY KEY ( column_name_comma_list )] |

[ <referential_constraint> ]

 

<referential_constraint> ::=

FOREIGN KEY ( column_name_comma_list ) <referential definition>

 

<referential definition> ::=

REFERENCES [ referenced_table_name ] ( column_name_comma_list )

[ <referential_triggered_action> ... ]

 

<referential_triggered_action> ::=

{ ON UPDATE <referential_action> } |

{ ON DELETE <referential_action> } |

{ ON CACHE OBJECT cache_object_column_name }

 

<referential_action> ::=

CASCADE | RESTRICT | NO ACTION  | SET NULL

NOT NULL Constraints
Description

A column for which the NOT NULL constraint has been defined must have a certain value that is not NULL. The NOT NULL constraint can be defined for all columns. An error occurs if you try to insert a NULL value into a column with the NOT NULL constraint by using the INSERT or UPDATE statement.

Example

CREATE TABLE const_tbl1(id INT NOT NULL, INDEX i_index(id ASC), phone VARCHAR);

 

CREATE TABLE const_tbl2(id INT NOT NULL PRIMARY KEY, phone VARCHAR);

INSERT INTO const_tbl2 (NULL,'000-0000');

 

In line 2, column 25,

 

ERROR: syntax error, unexpected Null

UNIQUE Constraint
Description

The UNIQUE constraint enforces a column to have a unique value. An error occurs if a new record that has the same value as the existing one is added by this constraint.

You can place a UNIQUE constraint on either a column or a set of columns. If the UNIQUE constraint is defined for multiple columns, the uniqueness is ensured not for each column, but the combination of multiple columns.

Example

If a UNIQUE constraint is defined on a set of columns, this ensures the uniqueness of the values in all the columns. As shown below, the second INSERT statement succeeds because the value of column a is the same, but the value of column b is unique. The third INSERT statement causes an error because the values of column a and b are the same as those in the first INSERT statement.

--UNIQUE constraint is defined on a single column only

CREATE TABLE const_tbl5(id INT UNIQUE, phone VARCHAR);

INSERT INTO const_tbl5(id) VALUES (NULL), (NULL);

INSERT INTO const_tbl5 VALUES (1, '000-0000');

SELECT * FROM const_tbl5;

 

           id  phone

===================================

         NULL  NULL

         NULL  NULL

            1  '000-0000'

 

INSERT INTO const_tbl5 VALUES (1, '111-1111');

 

ERROR: Operation would have caused one or more unique constraint violations.

 

 

--UNIQUE constraint is defined on several columns

CREATE TABLE const_tbl6(id INT, phone VARCHAR, CONSTRAINT UNIQUE(id,phone));

INSERT INTO const_tbl6 VALUES (1,NULL), (2,NULL), (1,'000-0000'), (1,'111-1111');

SELECT * FROM const_tbl6;

 

           id  phone

===================================

            1  NULL

            2  NULL

            1  '000-0000'

            1  '111-1111'

PRIMARY KEY Constraint
Description

A key in a table is a set of column(s) that uniquely identifies each row. A candidate key is a set of columns that uniquely identifies each row of the table. You can define one of such candidate keys a primary key. That is, the column defined as a primary key is uniquely identified in each row.

By default, the index created by defining the primary key is created in ascending order, and you can define the order by specifying ASC or DESC keyword next to the column.

Syntax

CREATE TABLE pk_tbl (a INT, b INT, PRIMARY KEY (a, b DESC));

Example

CREATE TABLE const_tbl7(

id INT NOT NULL,

phone VARCHAR,

CONSTRAINT pk_id PRIMARY KEY(id));

 

--CONSTRAINT keyword

CREATE TABLE const_tbl8(

id INT NOT NULL PRIMARY KEY,

phone VARCHAR);

 

--primary key is defined on multiple columns

CREATE TABLE const_tbl8 (

host_year    INT NOT NULL,

event_code   INT NOT NULL,

athlete_code INT NOT NULL,

medal        CHAR(1)  NOT NULL,

score        VARCHAR(20),

unit         VARCHAR(5),

PRIMARY KEY(host_year, event_code, athlete_code, medal)

);

FOREIGN KEY Constraint
Description

A foreign key is a column or a set of columns that references the primary key in other tables in order to maintain reference relationship. The foreign key and the referenced primary key must have the same data type. Consistency between two tables is maintained by the foreign key referencing the primary key, which is called referential integrity.

Syntax

[ CONSTRAINT < constraint_name > ]

FOREIGN KEY [ <foreign_key_name> ] ( column_name_comma_list )

REFERENCES [ referenced_table_name ] ( column_name_comma_list )

[ <referential_triggered_action> ]

 

<referential_triggered_action> :

ON UPDATE <referential_action>

[ ON DELETE <referential_action> [ ON CACHE OBJECT cache_object_column_name ]]

 

<referential_action> :

CASCADE | RESTRICT | NO ACTION | SET NULL

Example

--creaing two tables where one is referencing the other

CREATE TABLE a_tbl(

id INT NOT NULL DEFAULT 0 PRIMARY KEY,

phone VARCHAR(10));

 

CREATE TABLE b_tbl(

ID INT NOT NULL,

name VARCHAR(10) NOT NULL,

CONSTRAINT pk_id PRIMARY KEY(id),

CONSTRAINT fk_id FOREIGN KEY(id) REFERENCES a_tbl(id)

ON DELETE CASCADE ON UPDATE RESTRICT);

 

INSERT INTO a_tbl VALUES(1,'111-1111'), (2,'222-2222'), (3, '333-3333');

INSERT INTO b_tbl VALUES(1,'George'),(2,'Laura'),(3,'Max');

SELECT a.id, b.id, a.phone, b.name FROM a_tbl a, b_tbl b WHERE a.id=b.id;

 

           id           id                   phone                 name

==============================================================================

            1            1                   '111-1111'            'George'

            2            2                   '222-2222'            'Laura'

            3            3                   '333-3333'            'Max'

 

--when deleting primay key value, it cascades foreign key value  

DELETE FROM a_tbl WHERE id=3;

 

1 rows affected.

 

SELECT a.id, b.id, a.phone, b.name FROM a_tbl a, b_tbl b WHERE a.id=b.id;

 

           id           id                   phone                 name

==============================================================================

            1            1                   '111-1111'            'George'

            2            2                   '222-2222'            'Laura'

 

--when attempting to update primay key value, it restricts the operation

UPDATE  a_tbl SET id = 10 WHERE phone = '111-1111';

 

In the command from line 1,

 

ERROR: Update/Delete operations are restricted by the foreign key 'fk_id'.

 

0 command(s) successfully processed.

Caution
KEY or INDEX
Description

KEY and INDEX are used interchangeably. They create an index that uses the corresponding column as a key. You can specify the index name. If omitted, a name is assigned automatically.

Example

CREATE TABLE const_tbl3(id INT, phone VARCHAR, INDEX(id DESC, phone ASC));

 

CREATE TABLE const_tbl4(id INT, phone VARCHAR, KEY i_key(id DESC, phone ASC));