Select values that meet different conditions on different rows?

Question

This is a very basic query I can\'t figure out....

Let\'s say I have a two column table like this:

userid  |  roleid
--------|--------
   1    |    1         


        

Answer 1

The classic way to do this is to treat it as a relational division problem.

In English: Select those users for whom none of the desired roleid values is missing.

I'll assume you have a Users table to which the UserRole table refers, and I'll assume the desired roleid values are in a table:

create table RoleGroup(
  roleid int not null,
  primary key(roleid)
)
insert into RoleGroup values (1);
insert into RoleGroup values (2);
insert into RoleGroup values (3);

I'll also assume all the relevant columns are not NULLable, so there are no surprises with IN or NOT EXISTS. Here's a SQL query that expresses the English above:

select userid from Users as U
where not exists (
  select * from RoleGroup as G
  where not exists (
    select R.roleid from UserRole as R
    where R.roleid = G.roleid
    and R.userid = U.userid
  )
);

Another way to write it is this

select userid from Users as U
where not exists (
  select * from RoleGroup as G
  where G.roleid not in (
    select R.roleid from UserRole as R
    where R.userid = U.userid
  )
);

This may or may not end up being efficient, depending on indexes, platform, data, etc. Search the web for "relational division" and you'll find a lot.