MySQL LEFT JOIN returns empty resultset - mysql

Question

maybe I miss something stupid but...
I have three tables in m-to-m relation:
CREATE TABLE tbl_users (
usr_id INT NOT NULL AUTO_INCREMENT ,
usr_name VARCHAR( 64 ) NOT NULL DEFAULT '' ,
usr_surname VARCHAR( 64 ) NOT NULL DEFAULT '' ,
usr_pwd VARCHAR( 64 ) NOT NULL ,
usr_level INT( 1 ) NOT NULL DEFAULT 0,
PRIMARY KEY ( usr_id )
) ENGINE = InnoDB;
CREATE TABLE tbl_houses (
house_id INT NOT NULL AUTO_INCREMENT ,
city VARCHAR( 100 ) DEFAULT '' ,
address VARCHAR( 100 ) DEFAULT '' ,
PRIMARY KEY ( house_id )
) ENGINE = InnoDB;
CREATE TABLE tbl_users_houses (
user_id INT NOT NULL ,
house_id INT NOT NULL ,
INDEX user_key (user_id),
FOREIGN KEY (user_id) REFERENCES tbl_users(usr_id)
ON DELETE CASCADE
ON UPDATE CASCADE,
INDEX house_key (house_id) ,
FOREIGN KEY (house_id) REFERENCES tbl_houses(house_id)
ON DELETE CASCADE
ON UPDATE CASCADE
) ENGINE = InnoDB;
Into the link table I have two records:
user_id house_id
1 1
1 2
Now, trying to select all houses with:
select * from tbl_houses AS H
left join tbl_users_houses AS UH on H.house_id = UH.house_id
where UH.user_id = 2;
Why I get no data instead of all houses?

Answer 1

Because of this line:
where UH.user_id = 2;
This is only true if UH.user_id is non-null, so it effectively excludes any case where you have a house without a matching row in UH, which is the point of using a LEFT JOIN.
If you want all houses, and UH data where there is a match, use this:
select * from tbl_houses AS H
left join tbl_users_houses AS UH on H.house_id = UH.house_id and UH.user_id = 2;

Answer 2

Your WHERE clause is specifying that
UH.user_id = 2
What happens if you change it to H.user_id = 2 ?
To give this (all houses for user_id = 2):
select * from tbl_houses AS H
left join tbl_users_houses AS UH on H.house_id = UH.house_id
where H.user_id = 2;
Or if you want all houses regardless and data for user_id = 2 where it exists in tbl_User_houses try this:
select * from tbl_houses AS H
left join tbl_users_houses AS UH on H.house_id = UH.house_id and UH.user_id = 2;

Answer 3

Becasue you have no user with id 2.

Answer 4

Assuming your question "Why I get no data instead of all houses?" means you are wondering why you are not getting all the users when the users table is on on inner side of an outer join, this is happening because you placed the predicate condition after the join (in where clause) instead of in the join condition. This effectively converts the join to an inner join. change it to:
select * from tbl_houses H
left join tbl_users_houses UH
on uh.house_id = h.house_id
and UH.user_id = 2;
Conditions in where clauses are applied after all joins have been processed. At this point, values from rows from tables on the outer side of outer joins will all have nulls in them, so any predicate condition on such a value will cause these rows to be eliminated.