How do SQL EXISTS statements work?

Question

I\'m trying to learn SQL and am having a hard time understanding EXISTS statements. I came across this quote about \"exists\" and don\'t understand something:

Answer 1

EXISTS means that the subquery returns at least one row, that's really it. In that case, it's a correlated subquery because it checks the supplier_id of the outer table to the supplier_id of the inner table. This query says, in effect:

SELECT all suppliers For each supplier ID, see if an order exists for this supplier If the supplier is not present in the orders table, remove the supplier from the results RETURN all suppliers who have corresponding rows in the orders table

You could do the same thing in this case with an INNER JOIN.

SELECT suppliers.* 
  FROM suppliers 
 INNER 
  JOIN orders 
    ON suppliers.supplier_id = orders.supplier_id;

Ponies comment is correct. You'd need to do grouping with that join, or select distinct depending on the data you need.