Question

Here is the query i am using to join two tables

SELECT `rf_popup`.* 
  FROM `rf_popup` LEFT JOIN 
       `g_metadata` ON (`rf_popup`.`name` = `g_metadata`.`name`) 
 WHERE (`g_metadata`.`g_id` = '2009112305475443' AND 
        `g_metadata`.`value` < rf_popup.cardinality OR 
        `g_metadata`.`g_id` IS NULL) AND 
       `category` IN ('S', 'all') AND 
       `field` IN ('descr', 'all') AND 
       `filler_type` IN ('F2', 'all') 
 ORDER BY `rf_popup`.`priority` DESC LIMIT 5 

THIS IS TABLE rf_popup

+--------------+------------------------------------------------------------------------------------+------+-----+---------+----------------+
| Field        | Type                                                                               | Null | Key | Default | Extra          |
+--------------+------------------------------------------------------------------------------------+------+-----+---------+----------------+
| id           | int(11)                                                                            | NO   | PRI | NULL    | auto_increment |
| name         | varchar(200)                                                                       | YES  |     | NULL    |                |
| text         | text                                                                               | YES  |     | NULL    |                |
| cardinality  | int(11)                                                                            | NO   |     | 1       |                |
| field        | varchar(200)                                                                       | YES  |     | NULL    |                |
| category     | enum('A','H','N','R','S','D','all') | YES  |     | NULL    |                |
| time_to_show | enum('START','END')                                                                | YES  |     | START   |                |
| filler_type  | enum('F1','F2','F3','R1','R2','R3','all')   | YES  |     | FILLER1 |                |
| priority     | int(11)                                                                            | NO   |     | 0       |                |
+--------------+------------------------------------------------------------------------

————+——+—–+———+—————-+

This is TABLE g_metadata

+-------+--------------+------+-----+---------+----------------+
| Field | Type         | Null | Key | Default | Extra          |
+-------+--------------+------+-----+---------+----------------+
| g_id  | varchar(50)  | YES  | MUL | NULL    |                |
| name  | varchar(50)  | YES  | MUL | NULL    |                |
| value | varchar(200) | YES  |     | NULL    |                |
| id    | int(11)      | NO   | PRI | NULL    | auto_increment |
+-------+--------------+------+-----+---------+----------------+

But the Query Returns Empty Result.but if i replace WHERE in query with AND. The query returns the results.Whats the difference??
There shouldn’t be any difference so i am confused

Answer 1

Simply speaking, WHERE filters the entire result set and JOIN ... ON filters only the joined table.

Since you have a LEFT JOIN here, putting the filter in the ON clause restricts the results returned for the g_metadata table, but does not effect the results returned for rf_popup. That is, you will get null columns in the joined table (g_metadata) but the rows for rf_popup will still be pulled.

On the other hand, putting the filter in the WHERE clause performs the join first, then filters the resulting (joined) result set. That is, rows that do not match the entire WHEREclause are simply not returned.

---

Consider the following simplified examples:

TableA:

id
-----
1
2
3

TableB:

id   | yesNo
-----+------
1    | yes
2    | no
3    | yes

1. A simple join:

   SELECT * 
   FROM TableA 
   LEFT JOIN TableB ON TableA.id = TableB.id
   

   returns

   A.id | B.id | yesNo
   -----+------+------
   1    | 1    | yes
   2    | 2    | no
   3    | 3    | yes
   

2. Filtering in the ON clause:

   SELECT * 
   FROM TableA 
   LEFT JOIN TableB ON TableA.id = TableB.id
       AND yesNo = 'yes'
   

   returns

   A.id | B.id | yesNo
   -----+------+------
   1    | 1    | yes
   2    | NULL | NULL
   3    | 3    | yes
   

3. Filtering in the WHERE clause:

   SELECT * 
   FROM TableA 
   LEFT JOIN TableB ON TableA.id = TableB.id
   WHERE yesNo = 'yes'
   

   returns

   A.id | B.id | yesNo
   -----+------+------
   1    | 1    | yes
   3    | 3    | yes