Question

I am having some difficulties wrapping my head around this issue. I’m sure there’s a work-around but I’m trying to use a single query. I am joining two tables with a couple of conditions. I need to return all rows matching the ‘t_id’ (even if NULL). I also need to join only the data matching’e_id’. If the ‘e_id’ does not exist, it should return NULL. The following query returns all of the rows that I need:

SELECT b.*, ev.* FROM blocks b 
LEFT OUTER JOIN element_values ev ON ev.b_id = b.id
WHERE t_id = 1;

+----+------+-----------+-----+------+------+-----------------+
| id | t_id | type      | pos | e_id | b_id | value           |
+----+------+-----------+-----+------+------+-----------------+
|  1 |    1 | textinput |   1 |    1 |    1 | this is it edit |
|  2 |    1 | textinput |   0 |    1 |    2 | what is it      |
|  5 |    1 | template  |   2 | NULL | NULL | NULL            |
+----+------+-----------+-----+------+------+-----------------+

But adding another condition for ‘e_id’ will return the accurate data, but will exclude the third row if it is NULL (of course).

SELECT b.*, ev.* FROM blocks b 
LEFT OUTER JOIN element_values ev ON ev.b_id = b.id
WHERE t_id = 1 AND e_id = 1;

+----+------+-----------+-----+------+------+-----------------+
| id | t_id | type      | pos | e_id | b_id | value           |
+----+------+-----------+-----+------+------+-----------------+
|  1 |    1 | textinput |   1 |    1 |    1 | this is it edit |
|  2 |    1 | textinput |   0 |    1 |    2 | what is it      |
+----+------+-----------+-----+------+------+-----------------+

I’m looking for a solution where all three rows are still returned if ‘e_id’ doesn’t exist. For example, with the last query, when I pass ‘e_id = 2’, the set will be empty. But, I need it to still return the all of the rows that equal the ‘t_id’.

Any thoughts are helpful.

Thanks!

Answer 1

You can inlcude the e_id = 1 in the LEFT JOIN clause to get the rows you want.

Change your query to as follows:

SELECT b.*, ev.* 
  FROM blocks b  LEFT OUTER JOIN element_values ev 
    ON ev.b_id = b.id 
   AND e_id = 1
 WHERE t_id = 1;