alter table add column forigen key mysql code example
Example 1: drop foreign key
ALTER TABLE table_name
DROP CONSTRAINT fk_name;
Example 2: mysql add foreign key
-- On Create
CREATE TABLE tableName (
ID INT,
SomeEntityID INT,
PRIMARY KEY (ID),
FOREIGN KEY (SomeEntityID)
REFERENCES SomeEntityTable(ID)
ON DELETE CASCADE
);
-- On Alter, if the column already exists but has no FK
ALTER TABLE
tableName
ADD
FOREIGN KEY (SomeEntityID) REFERENCES SomeEntityTable(ID) ON DELETE CASCADE;
-- Add FK with a specific name
-- On Alter, if the column already exists but has no FK
ALTER TABLE
tableName
ADD CONSTRAINT fk_name
FOREIGN KEY (SomeEntityID) REFERENCES SomeEntityTable(ID) ON DELETE CASCADE;
Example 3: sql foreign key
# A foreign key is essentially a reference to a primary
# key in another table.
# A Simple table of Users,
CREATE TABLE users(
userId INT NOT NULL,
username VARCHAR(64) NOT NULL,
passwd VARCHAR(32) NOT NULL,
PRIMARY KEY(userId);
);
# Lets add a LEGIT user!
INSERT INTO users VALUES(1000,"Terry","Teabagface$2");
# We will create an order table that holds a reference
# to an order made by our Terry
CREATE TABLE orders(
orderId INT NOT NULL,
orderDescription VARCHAR(255),
ordererId INT NOT NULL,
PRIMARY KEY(orderId),
FOREIGN KEY (ordererId) REFERENCES users(userId)
);
# Now we can add an order from Terry
INSERT INTO orders VALUES(0001,"Goat p0rn Weekly",1000);
# Want to know more about the plight of Goats?
# See the link below
Example 4: alter table add column forigen key mysql
ALTER TABLE tryholpz_demo07.core_modules
ADD COLUMN belongs_to_role INT,
ADD FOREIGN KEY core_modules(belongs_to_role) REFERENCES role_specific_modules_info(id) ON DELETE CASCADE