Question

I’m sorry if this is a newbie question, but it seems I don’t get why this doesn’t work like I would like:

mysql> select t.id,t.date_fin_val,tc.date_fin_val
from tiers t
join tiers_critere_int tc on tc.id_tiers=t.id
where (t.date_fin_val is null) and (tc.date_fin_val is null);
+----+---------------------+---------------------+
| id | date_fin_val        | date_fin_val        |
+----+---------------------+---------------------+
|  1 | 0000-00-00 00:00:00 | 0000-00-00 00:00:00 | 
|  1 | 0000-00-00 00:00:00 | 0000-00-00 00:00:00 | 
|  1 | 0000-00-00 00:00:00 | 0000-00-00 00:00:00 | 
+----+---------------------+---------------------+
3 rows in set (0.00 sec)

mysql> select t.id,t.date_fin_val,tc.date_fin_val
from tiers t
left outer join tiers_critere_int tc on tc.id_tiers=t.id
where (t.date_fin_val is null) and (tc.date_fin_val is null);
Empty set (0.00 sec)

mysql> 

I thought that “left outer joins” means: “if there is no result on the right side, but one on the left, go on anyway with the one on the left and put “null” values on the right.
If I were right, the second query with “left outer join” instead of “join” should return values. But it doesn’t. Why?

Here are my datas:

mysql> select * from tiers t where date_fin_val is null;
+----+---------------------+--------------------+
| id | date_fin_val        | est_tiers_physique |
+----+---------------------+--------------------+
|  1 | 0000-00-00 00:00:00 |                  1 | 
+----+---------------------+--------------------+
1 row in set (0.00 sec)

mysql> select * from tiers_critere_int  where date_fin_val is null;
+----+---------------------+----------+------------+---------+
| id | date_fin_val        | id_tiers | id_critere | critere |
+----+---------------------+----------+------------+---------+
|  1 | 0000-00-00 00:00:00 |        1 |          2 |      86 |
|  2 | 0000-00-00 00:00:00 |        1 |          6 |     170 |
|  3 | 0000-00-00 00:00:00 |        1 |          7 |      65 |
+----+---------------------+----------+------------+---------+
3 rows in set (0.00 sec)

mysql>

Thank you very much!

Answer 1

My results are correct with you data and queries.

CREATE TABLE `tiers` (
  `id` int(11) DEFAULT NULL,
  `date_fin_val` datetime DEFAULT NULL,
  `est_tiers_physique` int(11) DEFAULT NULL
) ENGINE=MyISAM DEFAULT CHARSET=latin1

insert into tiers (id, est_tiers_physique) values (1, 1);

select * from tiers;
+------+--------------+--------------------+
| id   | date_fin_val | est_tiers_physique |
+------+--------------+--------------------+
|    1 | NULL         |                  1 |
+------+--------------+--------------------+
1 row in set (0.00 sec)

CREATE TABLE `tiers_critere_int` (
  `id` int(11) DEFAULT NULL,
  `date_fin_val` datetime DEFAULT NULL,
  `id_tiers` int(11) DEFAULT NULL,
  `id_critere` int(11) DEFAULT NULL,
  `critere` int(11) DEFAULT NULL
) ENGINE=MyISAM DEFAULT CHARSET=latin1

insert into tiers_critere_int (id, id_tiers, id_critere, critere) 
values 
(1, 1, 2, 86),
(2, 1, 6, 170),
(3, 1, 7, 65)
;

select * from tiers_critere_int;
+------+--------------+----------+------------+---------+
| id   | date_fin_val | id_tiers | id_critere | critere |
+------+--------------+----------+------------+---------+
|    1 | NULL         |        1 |          2 |      86 |
|    2 | NULL         |        1 |          6 |     170 |
|    3 | NULL         |        1 |          7 |      65 |
+------+--------------+----------+------------+---------+
3 rows in set (0.00 sec)

select t.id,t.date_fin_val,tc.date_fin_val
from tiers t
join tiers_critere_int tc on tc.id_tiers=t.id
where (t.date_fin_val is null) and (tc.date_fin_val is null);
+------+--------------+--------------+
| id   | date_fin_val | date_fin_val |
+------+--------------+--------------+
|    1 | NULL         | NULL         |
|    1 | NULL         | NULL         |
|    1 | NULL         | NULL         |
+------+--------------+--------------+
3 rows in set (0.00 sec)

select t.id,t.date_fin_val,tc.date_fin_val
from tiers t
left outer join tiers_critere_int tc on tc.id_tiers=t.id
where (t.date_fin_val is null) and (tc.date_fin_val is null);
+------+--------------+--------------+
| id   | date_fin_val | date_fin_val |
+------+--------------+--------------+
|    1 | NULL         | NULL         |
|    1 | NULL         | NULL         |
|    1 | NULL         | NULL         |
+------+--------------+--------------+
3 rows in set (0.02 sec)

Now if you insert an empty string you will have:

insert into tiers (id, date_fin_val, est_tiers_physique) values (2, '', 1);
Query OK, 1 row affected, 1 warning (0.00 sec)

select * from tiers;
+------+---------------------+--------------------+
| id   | date_fin_val        | est_tiers_physique |
+------+---------------------+--------------------+
|    1 | NULL                |                  1 |
|    2 | 0000-00-00 00:00:00 |                  1 |
+------+---------------------+--------------------+
2 rows in set (0.00 sec)