SQL query to find where ID does not exist in another column - mysql

Question

I have a table: Accounts
+-----------+-----------------+------+
| AccountNo | ParentAccountNo | name |
+-----------+-----------------+------+
| 1 | null | ABC |
| 2 | 1 | ABCD |
| 3 | 1 | CDE |
| 4 | 2 | DEF |
| 5 | null | GHI |
| 6 | 3 | MNO |
| 7 | 5 | JKL |
+-----------+-----------------+------+
I need to get results where AccountNo does not exist in ParentAccountNo.
For example, based on the data above, the results I expect are:
+-----------+-----------------+------+
| AccountNo | ParentAccountNo | name |
+-----------+-----------------+------+
| 4 | 2 | DEF |
| 6 | 3 | MNO |
| 7 | 5 | JKL |
+-----------+-----------------+------+
Will accept answer for MySQL or SQL-Server.

Answer 1

By joining the table to itself with a LEFT JOIN on the AccountNo = ParentAccountNo, you can then filter out the rows where there were no matching ID's.
WITH Accounts AS(
SELECT *
FROM (VALUES (1,null,'ABC'),
(2,1,'ABCD'),
(3,1,'CDE'),
(4,2,'DEF'),
(5,null,'GHI'),
(6,3,'MNO'),
(7,5,'JKL')) V(AccountNo,ParentAccountNo,name))
SELECT t1.*
FROM Accounts t1
LEFT JOIN Accounts t2 ON t1.AccountNo = t2.ParentAccountNo
WHERE t2.AccountNo IS NULL
[DEMO HERE]
You were curious about the usage of WITH. . .
Here is an equivalent to my shorthand usage of WITH:
CREATE TABLE Accounts (
AccountNo INT NOT NULL,
ParentAccountNo INT NULL,
Name VARCHAR(200))
INSERT INTO Accounts
SELECT *
FROM (VALUES (1,null,'ABC'),
(2,1,'ABCD'),
(3,1,'CDE'),
(4,2,'DEF'),
(5,null,'GHI'),
(6,3,'MNO'),
(7,5,'JKL')) V(AccountNo,ParentAccountNo,name)