MySQL Deleting Rows When There is a Foreign Key

To delete a row with a foreign key in MySQL, use the CASCADE keyword while creating a foreign key.

Using the ON DELETE CASCADE clause in MySQL, all matching rows from the child table are deleted when the referencing rows from the parent table are deleted. This helps in maintaining the referential integrity of the table.

Syntax

FOREIGN KEY (col_name) REFERENCES customers(col_name) ON DELETE CASCADE

How to Delete Rows When There is a Foreign Key

Let’s look at the step-by-step process of how to delete rows from a table that has a foreign key constraint on it.

Step 1: Creating a Database

CREATE DATABASE student;
USE student;

Step 2: Creating two tables

We will create two tables, and insert values in them.

CREATE TABLE student_details
(
     student_id INT PRIMARY KEY,
     Student_name varchar(8),
     student_year varchar(8)
);
CREATE TABLE student_exam
(
  exam_id INT PRIMARY KEY,
  exam_name varchar(8),
  student_id INT,
  FOREIGN KEY(student_id) 
  REFERENCES student_details(student_id) 
  ON DELETE CASCADE
);

INSERT INTO student_details VALUES
(19102047, 'PRADEEP', '1st'),
(19102048, 'KOUSHIK', '1st'), 
(19102049, 'SATWIK', '2st'),
(19102050, 'VAMSI', '2nd');

INSERT INTO student_exam VALUES
(9001, 'DBMS', '19102047'),
(9002, 'C.N', '19102048'),
(9003, 'O.S', '19102049'),
(9004, 'O.S', '19102050');

the

SELECT * FROM student_details;

SELECT* FROM student_exam;

Note: If ON DELETE CASCADE constraint is not used then referencing error occurs.

Step 3: Delete rows when there is a foreign key Example

Query to delete a student with id 19102048 from the student_exam table where referenced tables are student_details.

Query:

 DELETE FROM student_details
 WHERE student_id=19102048;

Before Deleting-

After Deleting –
The rows with student id 19102048 are deleted in both tables.

Conclusion

Deleting rows with foreign keys gives a referencing error. To avoid this error, we can use ‘ON DELETE CASCADE‘ during foreign key creation. This constraint helps maintain the referential integrity of the table by deleting the referenced rows from the child table if their referencing rows from the parent table are deleted.