What is MySQL LEFT JOIN and how can we write MySQL query for it?

MySQL MySQLi Database

While joining two tables using a LEFT join, the concept of left table and right table are introduced and a join-predicate is also required. It returns all the rows in the left table including rows that satisfy join-predicate and also rows which do not satisfy join-predicate.

For the rows that do not match the join-predicate, NULL appears in the column of the right table in the result set. To understand it, we are taking the example of two tables named tbl_1 and tbl_2 which are having following data −

mysql> Select * from tbl_1;
+----+--------+
| Id | Name   |
+----+--------+
| 1  | Gaurav |
| 2  | Rahul  |
| 3  | Raman  |
| 4  | Aarav  |
+----+--------+
4 rows in set (0.00 sec)

mysql> Select * from tbl_2;
+----+---------+
| Id | Name    |
+----+---------+
| A  | Aarav   |
| B  | Mohan   |
| C  | Jai     |
| D  | Harshit |
+----+---------+
4 rows in set (0.00 sec)

Now, the query for LEFT JOIN can be as follows −

mysql> SELECT tbl_1.id,tbl_2.id FROM tbl_1 LEFT JOIN tbl_2 ON tbl_1.name = tbl_2.name;
+----+------+
| id | id   |
+----+------+
| 1  | NULL |
| 2  | NULL |
| 3  | NULL |
| 4  | A    |
+----+------+
4 rows in set (0.02 sec)

Nancy Den

Published on 20-Feb-2018 11:48:10

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