How can I join two tables but get results from table one where theres no entry in table two?

Question

I have a menu table and a user favourites table. It is possible to add a favourite to a specific user in the favourites table.

I want to get a list of all the menu entries that the user doesn't have a record for in the favourites table.

I am having problems because I can never get the complete list when one of the favourites is being used by another user to the one I am testing with.

--Data
SELECT DISTINCT MM.MenuName
FROM Portal.dbo.ModuleMenu MM
LEFT OUTER JOIN dbo.UserMenuFavourites MF
ON MM.MenuId = MF.MenuItemID
AND MF.UserID = NULL

WHERE [IsFavourite?] = 1

The code above returns the full list but does not remove the ones that user one has already entered in the MF table.

Answer 1

You need to add a condition on the user to the join:

SELECT MM.MenuName
FROM Portal.dbo.ModuleMenu MM
LEFT JOIN dbo.UserMenuFavourites MF
  ON MM.MenuId = MF.MenuItemID
  AND MF.UserID = ? -- Specify the user you're interested in here
WHERE [IsFavourite?] = 1
AND MF.UserID IS NULL -- This condition selects only non-joins

Notes:

• You can have non-key conditions in the join condition (many folks don't realise this)
• For left joins all column values are null, so having the condition of a joined column being null filters out all but the non-joins
• No need for DISTINCT , because there's only ever one row from the first table when there's no join

Answer 2

The problem is that the where clause is referring to the second table in the left outer join . When there is no match, this will be NULL .

The best way to solve this is to move the condition to the on clause:

SELECT DISTINCT MM.MenuName
FROM Portal.dbo.ModuleMenu MM LEFT OUTER JOIN
     dbo.UserMenuFavourites MF
     ON MM.MenuId = MF.MenuItemID and
        MF.[IsFavourite?] = 1
WHERE MF.UserID = NULL;

The comparison to NULL can be in the where clause.