Understanding NOT EXISTS - mysql

Question

I'm trying to get the hang of NOT EXISTS and am having some trouble.
Say I have a 2 tables.
Employees:
+------+------+
| eid | name |
+------+------+
| 1 | Bob |
| 2 | Alice|
| 3 | Jill |
+------+------+
Transactions:
+----------+----------+----------+-----------+
| tid | eid | type | amount |
+----------+----------+----------+-----------+
| 1 | 1 | Deposit | 50 |
| 2 | 1 | Open | 500 |
| 3 | 3 | Open | 200 |
| 4 | 2 | Withdraw | 25 |
| 5 | 2 | Open | 100 |
+----------+----------+----------+-----------+
Let's say I want to find the names of all the employees that have not opened any account with the amount of $250 or higher. This means that I only want the rows where an employee has opened an account of amount < $250.
Right now I have something like this...
SELECT name FROM Employees e
WHERE NOT EXISTS (
SELECT * FROM Transactions t
WHERE t.type <> 'Open' AND t.amount >= 250 AND t.eid = e.eid);
This is obviously wrong and I don't really understand why.

Answer 1

You need to combine an EXISTS with a NOT EXISTS since you "only want the rows where an employee has opened an account of amount < $250.":
SELECT name FROM Employees e
WHERE EXISTS (
SELECT 1 FROM Transactions t
WHERE t.amount < 250 AND t.type='Open' AND t.eid = e.eid)
AND NOT EXISTS (
SELECT 1 FROM Transactions t
WHERE t.amount >= 250 AND t.eid = e.eid);
You need the EXISTS to ensure that only employee are returned which have an open account with amount < 250 at all. The NOT EXISTS is required to ensure that not employee are included which have additional accounts with amount >= 250.
Here's a sql-fiddle demo

Answer 2

The only issue I see - is that you've used <> for transaction type, not =
SELECT name FROM Employees e
WHERE NOT EXISTS (
SELECT null FROM Transactions t
WHERE t.transaction_type = 'Open' AND t.amount >= 250 AND t.eid = e.eid);
After you edited your question the answer would be:
SELECT name FROM Employees e
WHERE EXISTS (
SELECT null FROM Transactions t
WHERE t.transaction_type = 'Open' AND t.amount < 250 AND t.eid = e.eid);

Answer 3

I'd recommend using an LEFT JOIN instead of a sub select.
SELECT name FROM Employees e
LEFT JOIN Transactions t
ON e.eid = t.eid
WHERE t.tid IS NULL
OR t.type <> 'Open'
OR t.amount <= 250;
This will join all transaction records, and then only include records where a transaction does not exist, the user has a non-open transaction, or the amount doesn't meet the reuiqred $250