foreign key mysql

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;

1
2        
3            
4        
5     CREATE TABLE products(
6    productId INT AUTO_INCREMENT PRIMARY KEY,
7    productName varchar(100) not null,
8    categoryId INT NOT NULL,
9    CONSTRAINT fk_category
10    FOREIGN KEY (categoryId) 
11    REFERENCES categories(categoryId)
12        ON UPDATE CASCADE
13        ON DELETE CASCADE
14) ENGINE=INNODB;

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

1ALTER TABLE database.table
2  ADD COLUMN columnname INT DEFAULT(1),
3  ADD FOREIGN KEY fk_name(fk_column) REFERENCES reftable(refcolumn) ON DELETE CASCADE;