SQL - DELETE JOIN

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Simple deletion operation in SQL can be performed on a single record or multiple records of a table. And to delete records from multiple tables, the most straightforward approach would be to delete records from one table at a time.

However, SQL makes it easier by allowing the deletion operation to be performed on multiple tables simultaneously. This is achieved using Joins.

The SQL DELETE... JOIN Clause

The purpose of Joins in SQL is to combine records of two or more tables based on common columns/fields. Once the tables are joined, performing the deletion operation on the obtained result-set will delete records from all the original tables at a time.

For example, consider a database of an educational institution. It consists of various tables: Departments, StudentDetails, LibraryPasses, LaboratoryPasses etc. When a set of students are graduated, all their details from the organizational tables need to be removed, as they are unwanted. However, removing the details separately from multiple tables can be cumbersome.

To make it simpler, we will first retrieve the combined data of all graduated students from all the tables using Joins; then, this joined data is deleted from all the tables using DELETE statement. This entire process can be done in one single query.

Syntax

Following is the basic syntax of the SQL DELETE... JOIN statement −

DELETE table(s)
FROM table1 JOIN table2
ON table1.common_field = table2.common_field;

When we say JOIN here, we can use any type of Join: Regular Join, Natural Join, Inner Join, Outer Join, Left Join, Right Join, Full Join etc.

Example

To demonstrate this deletion operation, we must first create tables and insert values into them. We can create these tables using CREATE TABLE queries as shown below.

Create a table named CUSTOMERS, which contains the personal details of customers including their name, age, address and salary etc. Using the following query −

CREATE TABLE CUSTOMERS (
   ID INT NOT NULL,
   NAME VARCHAR (20) NOT NULL,
   AGE INT NOT NULL,
   ADDRESS CHAR (25),
   SALARY DECIMAL (18, 2),       
   PRIMARY KEY (ID)
);

Now, insert values into this table using the INSERT statement as follows −

INSERT INTO CUSTOMERS VALUES
(1, 'Ramesh', 32, 'Ahmedabad', 2000.00 ),
(2, 'Khilan', 25, 'Delhi', 1500.00 ),
(3, 'Kaushik', 23, 'Kota', 2000.00 ),
(4, 'Chaitali', 25, 'Mumbai', 6500.00 ),
(5, 'Hardik', 27, 'Bhopal', 8500.00 ),
(6, 'Komal', 22, 'Hyderabad', 4500.00 ),
(7, 'Muffy', 24, 'Indore', 10000.00 );

The table will be created as −

Let us create another table ORDERS, containing the details of orders made and the date they are made on.

CREATE TABLE ORDERS (
   OID INT NOT NULL,
   DATE VARCHAR (20) NOT NULL,
   CUSTOMER_ID INT NOT NULL,
   AMOUNT DECIMAL (18, 2)
);

Using the INSERT statement, insert values into this table as follows −

INSERT INTO ORDERS VALUES 
(102, '2009-10-08 00:00:00', 3, 3000.00),
(100, '2009-10-08 00:00:00', 3, 1500.00),
(101, '2009-11-20 00:00:00', 2, 1560.00),
(103, '2008-05-20 00:00:00', 4, 2060.00);

The table is displayed as follows −

Following DELETE... JOIN query removes records from these tables at once −

DELETE a
FROM CUSTOMERS AS a INNER JOIN ORDERS AS b
ON a.ID = b.CUSTOMER_ID;

Output

The output will be displayed in SQL as follows −

Query OK, 3 rows affected (0.01 sec)

Verification

We can verify whether the changes are reflected in a table by retrieving its contents using the SELECT statement as follows −

SELECT * FROM CUSTOMERS;

The table is displayed as follows −

Since, we only deleted records from CUSTOMERS table, the changes will not be reflected in the ORDERS table. We can verify it using the following query.

SELECT * FROM ORDERS;

The ORDERS table is displayed as −

DELETE... JOIN with WHERE Clause

The ON clause in DELETE... JOIN query is used to apply constraints on the records. In addition to it, we can also use the WHERE clause to make the filtration stricter. Observe the query below. Here, we are deleting the records of customers, in the CUSTOMERS table, whose salary is lower than Rs. 2000.00.

DELETE a
FROM CUSTOMERS AS a INNER JOIN ORDERS AS b
ON a.ID = b.CUSTOMER_ID
WHERE a.SALARY < 2000.00;

Output

On executing the query, following output is displayed.

Query OK, 1 row affected (0.01 sec)

Verification

We can verify whether the changes are reflected in a table by retrieving its contents using the SELECT statement as follows −

SELECT * FROM CUSTOMERS;

The CUSTOMERS table after deletion is as follows −

Since we only deleted records from the CUSTOMERS table, the changes will not be reflected in the ORDERS table. We can verify it using the following query −

SELECT * FROM ORDERS;

The ORDERS table is displayed as −