Question

i am curetly building a cms,
I have a tables called menus

Menus

MenuID | MenuName
1        Home
2        users
3        email
4        system

i have another table to control user access to these menus

call user_menu_access
MenuID | UserID
1           1
2           1

i am doing a left join like this

SELECT * 
FROM menus
LEFT JOIN user_menu_access 
    ON menus.MenuID = user_menu_access.MenuID 
WHERE UserID = 1

my result set dublicates rows as i will have to enties for 1 and 2 , i want to
have a result set that is as follows

MenuID  | UserID
1           1
2           1
3           NULL
4           NULL

Answer 1

You need to move the condition into the ON clause:

SELECT menus.menuid, userid 
FROM menus
LEFT JOIN user_menu_access 
    ON menus.MenuID = user_menu_access.MenuID 
    AND UserID = 1

See this query executing (giving exactly the results you asked for) here.

The reason this works is that the WHERE clause filters the rowset after the join has occurred, but because you’re doing a left join, the columns of the user_menu_access table are all null for menus that don’t have any user access, so your condition isn’t true for those rows.