FK Constraints

foreign constraint

foreign constraint 1

What should the DB engine do if execution SQL statement collides with a FOREIGN KEY constraint?
For example: DELETE FROM Company WHERE name = ‘lmao’;
and the Products Table contains Foreign Key entries for cname = ‘lmao’

ON DELETE / ON UPDATE Options

NO ACTION
Default behavior; Raises an error and the Delete/Update is rolled back
CASCADE
Automatically fixes the referencing Table; on UPDATE, corresponding rows are changed
SET NULL
Automatically fixes the referencing Table; FOREIGN KEY column must be NULLABLE
SET DEFAULT
Automatically fixes the referencing Table; FOREIGN KEY column must have DEFAULT value defined

CREATE TABLE Company (
    name VARCHAR(100) PRIMARY KEY);
    CREATE TABLE Product (
    name VARCHAR(100) PRIMARY KEY,
    cname VARCHAR(100) REFERENCES Company (name)
    ON UPDATE CASCADE
    ON DELETE SET NULL);

Weak Entity Set -> Foreign Key + Cascade Delete

CREATE TABLE Team (  
    univname CHAR(30) REFERENCES University(name)  
    ON DELETE CASCADE ON UPDATE CASCADE,  
    number int,
    sport CHAR(30))