I have People linked to Roles with three tables: People, Links, Roles. People has PersonName and PersonID columns, Roles has RoleID, and RoleName columns, and Links has PersonID and RoleID columns. Each person can have zero or more roles, each role can have zero or more people. How do I select (distinct) all names that are NOT linked to a specified role?
I'm not totally sure whether you want to list people that are not linked to one specific role, or people that are not linked to any role at all. So let's answer both questions. Here's some sample data:
People PersonID PersonName 24 Homer 26 Marge 31 Bart 33 Lisa 36 Maggie 45 Apu
Roles RoleID RoleName 101 Technician 103 Homemaker 107 Student 111 Teacher
Links PersonID RoleID 24 101 26 103 31 107 33 107
Let's try to find people without a specific role, say Student. If we already know the RoleID, 107, then the query is:
select PersonName from People left outer join Links on People.PersonID = Links.PersonID and Links.Roleid = 107 where Links.PersonID is null
This query works by specifying the RoleID condition in the join, and then testing for no match. To see why this works, let's ignore the WHERE clause for a moment, and look at the rows produced by the left outer join. Remember, the only possible joins are to Links rows with Roleid 107:
People People Links Links PersonID PersonName PersonID RoleID 24 Homer null null 26 Marge null null 31 Bart 31 107 33 Lisa 33 107 36 Maggie null null
The WHERE clause can now be applied, to select those people without a match to a RoleID 107 Links row. Homer, Marge, and Maggie are not Students.
The correct results are not produced if the Links.Roleid = 107 condition is moved from the ON clause to the WHERE clause. You can try this on your own if you wish.
Now, if the specific RoleID is not known, but the RoleName is, then just simply extend the left outer join to the third table:
select PersonName from People left outer join Links on People.PersonID = Links.PersonID left outer join Roles on Links.RoleID = Roles.RoleID and Roles.RoleName = 'Student' where Roles.RoleID is null
Again, the technique here is to specify the restricting condition in an ON clause, rather than in the WHERE clause.
Finally, if you want people with no role at all, then use the first left outer join query, joining only People and Links, without any Roleid condition in the ON clause, again selecting unmatched rows in the WHERE clause. This will give you Apu.
By the way, you may have assumed from the title of this answer, Many-to-many links that aren't there, that the solution might involve a NOT EXISTS subquery. Yes, it's possible to do it that way:
select PersonName from People where not exists ( select 1 from Links where PersonID = People.PersonID and Roleid = 107 )
However, I prefer the left outer join syntax.
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