Conditionally delete last row in mysql - mysql

Question

How do I conditionally delete the last row in a mysql table?
me/my_machine#17:26:57>cat create_tables.sql
CREATE DATABASE IF NOT EXISTS test_db;
USE test_db;
CREATE TABLE IF NOT EXISTS err_hist_table (
id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
cl INT NOT NULL,
usr VARCHAR(16),
fault_code VARCHAR(10));
INSERT INTO err_hist_table (cl,usr,fault_code) VALUES (1,'pA','A'), (2,'pA','NULL'),(3,'pC','B'),(4,'pB','NULL');
The above SQL commands create a table that is like this:
MySQL [test_db]> SELECT * FROM err_hist_table;
+----+----+------+------------+
| id | cl | usr | fault_code |
+----+----+------+------------+
| 1 | 1 | pA | A |
| 2 | 2 | pA | NULL |
| 3 | 3 | pC | B |
| 4 | 4 | pB | NULL |
+----+----+------+------------+
4 rows in set (0.00 sec)
Now I want to DELETE the last row (biggest value of id) only when the value of fault_code is NULL. If it is not NULL, I want to skip the delete.
To my sql-newbie eyes, it seems like there should be a simple something like:
SELECT * IF err_history_table.id=max(err_history_table) AND fault_code = 'NULL';
I could not find my answer on the mysql docs page. Is there a simple solution for something like this?

Answer 1

You can do it with a subquery that returns the row of the maximum id where you check the value of fault_code:
DELETE FROM err_hist_table
WHERE id = (
SELECT id FROM (
SELECT * FROM err_hist_table
ORDER BY id DESC LIMIT 1
) t
WHERE fault_code IS NULL
);
See the demo.
Results:
| id | cl | usr | fault_code |
| --- | --- | --- | ---------- |
| 1 | 1 | pA | A |
| 2 | 2 | pA | |
| 3 | 3 | pC | B |

Answer 2

I'm not totally sure if this is what you want, but it should work. If you have a recent version of MySQL you should be able to use sub-queries, like this:
DELETE FROM err_hist_table WHERE id = (SELECT MAX(id) FROM err_hist_table) AND fault_code IS NULL
Untested on your specific table of course. I would even suggest performing a SELECT first to verify it does what you expect.

Answer 3

It's worth mentioning that you can acheive the desired result using a single JOIN, without needing a sub-sub-query and in a way which seems to me both concise and readable:
DELETE e1 FROM err_hist_table e1
JOIN (
SELECT MAX(id) AS id
FROM err_hist_table
) e2 USING (id)
WHERE e1.fault_code = 'NULL'
More on the topic of using joins in a DELETE statement here: https://dev.mysql.com/doc/refman/8.0/en/delete.html#idm45306662494864