What is MySQL LEFT JOIN and how can we write MySQL query for it?
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)
Published on 20-Feb-2018 11:48:10
