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Is it possible to see the result of left join of two tables?


colap

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http://www.sitepoint.com/understanding-sql-joins-mysql-database/

 

Is it possible to see the result of left join of two tables(without on clause)?

mysql> select * from user;
+----+----------+--------+
| id | name     | course |
+----+----------+--------+
|  1 | Alice    |      1 |
|  2 | Bob      |      1 |
|  3 | Caroline |      2 |
|  4 | David    |      5 |
|  5 | Emma     |   NULL |
+----+----------+--------+
5 rows in set (0.00 sec)
mysql> select * from course;
+----+------------+
| id | name       |
+----+------------+
|  1 | HTML5      |
|  2 | CSS3       |
|  3 | Javascript |
|  4 | PHP        |
|  5 | MySQL      |
+----+------------+
5 rows in set (0.00 sec)
mysql> SELECT * FROM `user` left join course on user.course=course.id;
+----+----------+--------+------+-------+
| id | name     | course | id   | name  |
+----+----------+--------+------+-------+
|  1 | Alice    |      1 |    1 | HTML5 |
|  2 | Bob      |      1 |    1 | HTML5 |
|  3 | Caroline |      2 |    2 | CSS3  |
|  4 | David    |      5 |    5 | MySQL |
|  5 | Emma     |   NULL | NULL | NULL  |
+----+----------+--------+------+-------+
5 rows in set (0.00 sec)

Why/how is id=NULL and name=NULL for Emma,NULL?

Edited by php-coder
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