Question

I have the next tables

Users {id, name}
Messages {id, user_id, cache_user_name}

What I want is to do a JOIN only when cache_user_name is NULL for performance reasons.

For example:

SELECT Messages.*, Users.name FROM Messages INNER JOIN Users ON (Messages.user_id = Users.id) 
// ON (ISNULL(Messages.cache_user_name) AND ...

The best way is doing 2 queries? 1 for rows without cache (join) and the other for cached rows with a join?

[EDIT]

The result I need is:

Users

ID: 1, NAME: Wiliam

Messages

ID: 1, USER_ID: 1, CACHE_USER_NAME: Wiliam
ID: 2, USER_ID: 1, CACHE_USER_NAME: null

Result

ID: 1,  USER_ID: 1,  CACHE_USER_NAME: Wiliam,  USERS.NAME: null   // No join, faster
ID: 2,  USER_ID: 1,  CACHE_USER_NAME: null,    USERS.NAME: Wiliam // Join

Answer 1

You can add WHERE ... IS NULLclause.

The optimizer will (try to) use the best performing plan.

SELECT   Messages.*
         , Users.name 
FROM     Messages 
         INNER JOIN Users ON (Messages.user_id = User.id)
WHERE    Users.cache_user_name IS NULL

Edit

Given following data, what would you expect as output?

DECLARE @Users TABLE (ID INTEGER, Name VARCHAR(32))
DECLARE @Messages TABLE (ID INTEGER, User_ID INTEGER, Cache_User_Name VARCHAR(32))

INSERT INTO @Users VALUES (1, 'Wiliam')
INSERT INTO @Users VALUES (2, 'Lieven')
INSERT INTO @Users VALUES (3, 'Alexander')

INSERT INTO @Messages VALUES (1, 1, NULL)
INSERT INTO @Messages VALUES (2, 1, 'Cached Wiliam')
INSERT INTO @Messages VALUES (3, 2, NULL)
INSERT INTO @Messages VALUES (4, 3, 'Cached Alexander')

SELECT  *
FROM    @Users u
        INNER JOIN @Messages m ON m.User_ID = u.ID
WHERE   m.Cache_User_name IS NULL