Question

How do you do SQL query for the following condition? Suppose you have two tables: table1 and table2, where each entry in table1 can have multiple corresponding entries in table2. The pseudo code for the query that I want is:

for each $row in table1
   $rows = find all rows in table2 that corresponds to $row with $row.id == table2.foreign_id
   # $rows is an array of corresponding row in table2
   if all rows in $rows meet some condition 
   then 
     return $row
   else
     continue
   end
end

EDIT: note in the above pseudo code, I only want the row in table1 that has all its relations in TABLE2 that meets some conditions, not just some condition in table1.

PS: I want to do it in SQL due to efficiency problems that I may have otherwise.

Thanks a lot.

Answer 1

You can reformulate this with a where not exists ( .. ) type clause.

For example, pretending you want a list of customers whose orders are all completed:

 select * from customers c
 where not exists (
     select * from orders 
     where customerid=c.id 
       and status <> 'C'
 )

So you are asking for all customers who have no uncompleted orders – which is the same thing as all customers whose orders are all completed.

Instead of:

if all rows in $rows meet some condition

You are saying:

if NO rows in $rows DO NOT meet some condition

Edit: As pointed out in the comments, this will also return customers who have no orders. You could add and exists (select * from orders where customerid=c.id) to the end of the above to exclude these rows.