MySQL DELETE JOIN

 MySQL DELETE JOIN

Summary: in this tutorial, we will show you how to delete data from multiple tables by using the MySQL DELETE JOIN statement.

In the previous tutorial, you learned how to delete rows of multiple tables by using:

• A single DELETE statement on multiple tables.
• A single DELETE statement on multiple related tables which the child table has an ON DELETE CASCADE referential action for the foreign key.

This tutorial introduces to you a more flexible way to delete data from multiple tables using INNER JOIN or LEFT JOIN clause with the DELETE statement.

MySQL DELETE JOIN with INNER JOIN

MySQL also allows you to use the INNER JOIN clause in the DELETE statement to delete rows from a table and the matching rows in another table.

For example, to delete rows from both T1 and T2 tables that meet a specified condition, you use the following statement:

DELETE T1, T2
FROM T1
INNER JOIN T2 ON T1.key = T2.key
WHERE condition;

Notice that you put table names T1 and T2 between the DELETE and FROM keywords. If you omit T1 table, the DELETE the statement only deletes rows in T2 table. Similarly, if you omitT2 table, the DELETE the statement will delete only rows in T1 table.

The expression T1.key = T2.key specifies the condition for matching rows between T1 andT2 tables that will be deleted.

The condition in the  WHERE clause determine rows in the T1 and T2 that will be deleted.

MySQL DELETE JOIN with INNER JOIN example

Suppose, we have two tables t1 and t2 with the following structures and data:

DROP TABLE IF EXISTS t1, t2;

CREATE TABLE t1 (
    id INT PRIMARY KEY AUTO_INCREMENT
);

CREATE TABLE t2 (
    id VARCHAR(20) PRIMARY KEY,
    ref INT NOT NULL
);

INSERT INTO t1 VALUES (1),(2),(3);

INSERT INTO t2(id,ref) VALUES('A',1),('B',2),('C',3);

The following statement deletes the row with id 1 in the t1 table and also row with ref 1 in the t2 table using DELETE...INNER JOIN statement:

DELETE t1,t2 FROM t1
        INNER JOIN
    t2 ON t2.ref = t1.id 
WHERE
    t1.id = 1;

The statement returned the following message:

2 row(s) affected

It indicated that two rows have been deleted.

MySQL DELETE JOIN with LEFT JOIN

We often use the LEFT JOIN clause in the SELECT statement to find rows in the left table that have or don’t have matching rows in the right table.

We can also use the LEFT JOIN clause in the DELETE statement to delete rows in a table (left table) that does not have matching rows in another table (right table).

The following syntax illustrates how to use DELETE statement with LEFT JOIN clause to delete rows from T1 the table that does not have corresponding rows in the T2 table:

DELETE T1 
FROM T1
        LEFT JOIN
    T2 ON T1.key = T2.key 
WHERE
    T2.key IS NULL;

Note that we only put T1 table after the DELETE keyword, not both T1 and T2 tables like we did with the INNER JOIN clause.

MySQL DELETE JOIN with LEFT JOIN example

See the following customers and orders tables in the sample database:

Each customer has zero or more orders. However, each order belongs to one and only one customer.

We can use DELETE a statement with LEFT JOIN clause to clean up our customer's master data. The following statement removes customers who have not placed any order:

DELETE customers 
FROM customers
        LEFT JOIN
    orders ON customers.customerNumber = orders.customerNumber 
WHERE
    orderNumber IS NULL;

We can verify the delete by finding whether  customers who do not have any order exists using the following query:

SELECT 
    c.customerNumber, 
    c.customerName, 
    orderNumber
FROM
    customers c
        LEFT JOIN
    orders o ON c.customerNumber = o.customerNumber
WHERE
    orderNumber IS NULL;

The query returned an empty result set which is what we expected.

In this tutorial, you have learned how to use the MySQL DELETE JOIN statement to delete data from two or more tables.