Showing posts with label Foreign Key constraints. Show all posts
Showing posts with label Foreign Key constraints. Show all posts

Friday, September 23, 2016

MySQL list all foreign key and other constraint names


SELECT  TABLE_NAME, COLUMN_NAME, CONSTRAINT_NAME, 
        REFERENCED_TABLE_NAME, REFERENCED_COLUMN_NAME

FROM    INFORMATION_SCHEMA.KEY_COLUMN_USAGE

WHERE   REFERENCED_TABLE_SCHEMA = 'database_name'

Sunday, December 8, 2013

How to Set up a Foreign Key Constraint in MySQL

The following is a simple example that illustrates Foreign Key constraints:

Creating a table for manager:
CREATE TABLE manager (id integer primary key auto_increment, name text) ENGINE=InnoDB;

Creating a table for club and creating a foreign key constraint for manager_id with key 'manager_id_reference':
CREATE TABLE club (id integer primary key auto_increment, is_accessible char, manager_id integer NOT NULL) ENGINE=InnoDB;
ALTER TABLE club ADD CONSTRAINT manager_id_reference FOREIGN KEY(manager_id) REFERENCES manager (id);

Inserting some managers:
INSERT INTO manager(name) VALUES('Pritom');
INSERT INTO manager(name) VALUES('Kumar');

Inserting some clubs:
INSERT INTO club(is_accessible, manager_id) VALUES(true, 1);
INSERT INTO club(is_accessible, manager_id) VALUES(false, 2);

Let see manager table data (SELECT * FROM manager):
ID NAME
1 Pritom
2 Kumar

Let see club table data (SELECT * FROM club):
ID IS_ACCESSIBLE MANAGER_ID
1 1 1
2 0 2
Let see manager table structure (DESCRIBE manager):
FIELD TYPE NULL KEY DEFAULT EXTRA
id int(11) NO PRI (null) auto_increment
name text YES (null)
Let see club table structure (DESCRIBE club):
FIELD TYPE NULL KEY DEFAULT EXTRA
id int(11) NO PRI (null) auto_increment
is_accessible char(1) YES (null)
manager_id int(11) NO MUL (null)

Now try to delete manager:
DELETE FROM USER WHERE id = 1;

will error as:
Schema Creation Failed: Cannot delete or update a parent row: a foreign key constraint fails (`db_2_8d4a1`.`club`, CONSTRAINT `manager_id_reference` FOREIGN KEY (`manager_id`) REFERENCES `manager` (`id`)):

This happens because the data in the manager table depends on the data in the club table.

Full sqlFiddle