How can we convert subqueries to LEFT JOIN?

To make it understand we are using the data from the following tables −

mysql> Select * from customers;
+-------------+----------+
| Customer_Id | Name     |
+-------------+----------+
| 1           | Rahul    |
| 2           | Yashpal  |
| 3           | Gaurav   |
| 4           | Virender |
+-------------+----------+
4 rows in set (0.00 sec)

mysql> Select * from reserve;
+------+------------+
| ID   | Day        |
+------+------------+
| 1    | 2017-12-30 |
| 2    | 2017-12-28 |
| 2    | 2017-12-25 |
| 1    | 2017-12-24 |
| 3    | 2017-12-26 |
+------+------------+
5 rows in set (0.00 sec)

Now, the following is a subquery which will find the name of all the customers who have not to reserve any car.

mysql> Select Name from customers where customer_id NOT IN (Select id From reserve);
+----------+
| Name     |
+----------+
| Virender |
+----------+
1 row in set (0.00 sec)

Now, with the help of followings steps, we can convert the above subquery into RIGHT join −

• Move the ‘Reserve’ table named in the subquery to the FROM clause and join it to ‘Customers’ using LEFT JOIN.
• The WHERE clause compares the customer_id column to the ids returned from the subquery. Hence convert the IN expression to an explicit direct comparison between id columns of two tables in the FROM clause.
• In the WHERE clause, restrict the output to those rows having NULL in the ‘Reserve’ table.

mysql> SELECT Name from customers LEFT JOIN reserve ON customer_id = Id WHERE Id IS NULL;
+----------+
| Name     |
+----------+
| Virender |
+----------+
1 row in set (0.00 sec)

Sai Nath

Updated on 22-Jun-2020 10:55:57

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