I've discussed about the SQL EXISTS operator in this article:
The SQL EXISTS Operator
Let's take a look at the EXISTS and NOT EXISTS operator with subquery. Both SQL and MySQL supports EXISTS operator and the syntax is same in both as well.
SELECT Columns FROM table WHERE [NOT] EXISTS (Subquery);
Let's take a look at NOT EXISTS example:
SELECT seller_id, seller_name, seller_state FROM sellers WHERE NOT EXISTS (SELECT * FROM invoices WHERE invoices.seller_id = sellers.seller_id);
In the query above, we are using NOT EXISTS with a Subquery which checks if the subquery returns any results back.
All we check using EXISTS and NOT EXISTS is that if the result set exists.
Also, when we use EXISTS, the subquery won't return any results but it would just specify that whether any rows satisfy the conditions.
The query above fetches all the sellers that don't have invoices in the invoices table. The correlated subquery selects all the invoices that have same seller_id as in the table outside.
Do we need to code * or column names in the subquery?
It doesn't matter because the subquery actually doesn't return any result set.
After the execution of the subquery, the query will simple check the condition in WHERE clause in which it checks if any invoices found for any sellers. If so, then it won't include those sellers in the result set.
Let's write down query which gets the completely opposite result set using EXISTS.
SELECT seller_id, seller_name, seller_state FROM sellers WHERE EXISTS (SELECT * FROM invoices WHERE invoices.seller_id = sellers.seller_id);
This query will check whether the invoices EXISTS for any sellers and if so, it will include those sellers in the result set.