alter table add foreign key mysql

1ALTER TABLE orders
2ADD 
3FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE; 

1ALTER TABLE table_name
2DROP CONSTRAINT fk_name;

1-- On Create
2CREATE TABLE tableName (
3    ID INT,
4    SomeEntityID INT,
5    PRIMARY KEY (ID),
6    FOREIGN KEY (SomeEntityID)
7        REFERENCES SomeEntityTable(ID)
8        ON DELETE CASCADE
9);
10
11-- On Alter, if the column already exists but has no FK
12ALTER TABLE
13  tableName
14ADD
15  FOREIGN KEY (SomeEntityID) REFERENCES SomeEntityTable(ID) ON DELETE CASCADE;
16  
17 -- Add FK with a specific name
18 -- On Alter, if the column already exists but has no FK
19ALTER TABLE
20  tableName
21ADD CONSTRAINT fk_name
22  FOREIGN KEY (SomeEntityID) REFERENCES SomeEntityTable(ID) ON DELETE CASCADE;

1CREATE TABLE parent (
2    id INT NOT NULL,
3    PRIMARY KEY (id)
4) ENGINE=INNODB;
5
6CREATE TABLE child (
7    id INT,
8    parent_id INT,
9    INDEX par_ind (parent_id),
10    FOREIGN KEY (parent_id)
11        REFERENCES parent(id)
12        ON DELETE CASCADE
13) ENGINE=INNODB;

1ALTER TABLE `TABLE_NAME`
2ADD COLUMN `COLUMN_NAME` BIGINT(20) UNSIGNED NULL DEFAULT NULL AFTER `AFTER_COLUMN_NAME`, 
3ADD FOREIGN KEY `FOREIGN_RELATION_NAME`(`COLUMN_NAME`) REFERENCES `FOREIGN_TABLE`(`FOREIGN_COLUMN`)  ON UPDATE SET NULL ON DELETE SET NULL

1ALTER TABLE tryholpz_demo07.core_modules 
2ADD COLUMN belongs_to_role INT, 
3ADD FOREIGN KEY core_modules(belongs_to_role) REFERENCES role_specific_modules_info(id) ON DELETE CASCADE