How can we convert subqueries to INNER 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 reserved a car.

mysql> Select Name from customers WHERE customer_id IN (Select id from reserve);
+----------+
| Name     |
+----------+
| Rahul    |
| Yashpal  |
| Gaurav   |
+----------+
3 rows in set (0.00 sec)

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

• Move the ‘Reserve’ table named in the subquery to the FROM clause.

• The WHERE clause compares the customer_id column to the ids returned from the subquery. 

Hence convert the expression to an explicit direct comparison between id columns of two tables.

mysql> SELECT Name from customers, reserve WHERE customer_id = id;
+----------+
| Name     |
+----------+
| Rahul    |
| Yashpal  |
| Yashpal  |
| Rahul    |
| Gaurav   |
+----------+
5 rows in set (0.00 sec)

As we can see that the above result is not exactly the same as the result of subquery so use DISTINCT keyword to get the same result as follows:

mysql> SELECT DISTINCT name from customers,reserve WHERE customer_id = id;
+----------+
| Name     |
+----------+
| Rahul    |
| Yashpal  |
| Gaurav   |
+----------+
3 rows in set (0.03 sec)