Ambiguous column name error, how do I fix it?

Question

1. Users 4 Cols
UserID - UserName - RealName - Flags

2. UsersGroups 2 Cols
UserID - GroupID

3. Groups 3 Cols
GroupID - GroupName - Flags

What I want to do is select a specific UserName ie USERA and update the Flags column. but I also want to update the Flags column in the Groups table to the same value.

UPDATE dbo.Users
SET Flags = @var
WHERE UserName = 'UserA'

UPDATE dbo.Groups
SET Flags = @var
FROM dbo.Users u INNER JOIN dbo.UsersGroups ug ON u.UserID = ug.UserID
INNER JOIN dbo.Groups g ON g.GroupID = ug.GroupID
WHERE u.UserName = 'UserA'

but I keep getting : Ambiguous column name 'Flags'.

if I do Set Groups.Flags = @Var i get : Msg 4104, Level 16, State 1, Line 1 The multi-part identifier "Groupy.Flags" could not be bound.

Answer 1

You need to add the alias for the Groups table. Change this:

UPDATE dbo.Groups
SET Flags = @var
FROM dbo.Users u INNER JOIN dbo.UsersGroups ug ON u.UserID = ug.UserID
INNER JOIN dbo.Groups g ON g.GroupID = ug.GroupID
WHERE u.UserName = 'UserA'

To this:

UPDATE g -- change dbo.Groups here to simply 'g'
SET g.Flags = @var
FROM dbo.Users u INNER JOIN dbo.UsersGroups ug ON u.UserID = ug.UserID
INNER JOIN dbo.Groups g ON g.GroupID = ug.GroupID
WHERE u.UserName = 'UserA'

Answer 2

The problem is that you haven't specified the table name for the field "Flags" and it probably exists in more than one table in the query. Add the table name in the format "Tablename.flags" to the front of all references to fix the problem.

Answer 3

UPDATE g
SET g.Flags = @var
FROM
  dbo.Groups g
    INNER JOIN
  dbo.UsersGroups ug
    ON g.GroupID = ug.GroupID
    INNER JOIN
  dbo.Users u
    ON u.UserID = ug.UserID
WHERE u.UserName = 'UserA'

• In the from clause - the update target needs to be the first table there.
• In the update clause - use the table alias created in the from clause.
• In the set clause - use the table alias created in the from clause.

I once knew the reasons that this dance needs to be done this way - now I just do it out of habit. I suspect it has something to do with TSQL's double FROM clause in DELETE statements, and the possibility of talking about Two different instances of the Groups table between the FROM and UPDATE clause... or even Two different instances of the Groups table in the from clause (think self-join).