检查其他表中的关系状态

I have in database:

User:
id | name
1  | one
2  | two
3  | three
4  | four
5  | five


House:
id | name
1  | London
2  | Barcelona

UserHouse:
id_user | id_house
 1      |  1
 2      |  2
 4      |  1

How can i use in SQL query IF and ISSET?

SELECT * from User

I would like receive:

users:

name | has_house
1    | yes
2    | yes
3    | no
4    | yes
5    | no

I must start my query with SELECT * from User. I can't star from House or UserHouse. Is possible? If yes, how?

I can use also DOCTRINE and Symfony.

Try using LEFT JOIN

SELECT  a.id, IF(COALESCE(b.id_user, 1) = 1, 'no', 'yes') has_house
FROM    user a
        LEFT JOIN userhouse b
            on a.id = b.id_user

If you realy can start your query only with SELECT * from User, you can try this:

SELECT * FROM User
  JOIN (
    SELECT
      User.id id_user,
      IF(UserHouse.id_house > 0, 'yes', 'no') AS has_house
    FROM
        User
      LEFT JOIN
        UserHouse ON UserHouse.id_user = User.id
    ) HasHouse ON HasHouse.id_user = User.id;

if you are using sql server, you should use instead of the if statement a case one.

SELECT   user.id, 'Has House' =
  CASE COALESCE(b.id_user, 0)
     WHEN 0 THEN 'No'
     ELSE 'Yes'
  END
FROM user as u leftjoin userhouse as uh on u.id = uh.id_user

http://msdn.microsoft.com/en-us/library/ms181765.aspx