Question

Ok so there are couple posts here already on this and fewer still out on the web. I’ve literally tried every one of them and can not get anything to work. Hopefully someone here can take pity on me 🙂

Here is the data I’m working with. I want to delete all these records.

SELECT
part_desc, count(*) as rec_num
FROM ag_master
GROUP BY part_desc HAVING COUNT(*) > 1000;

+--------------------------------------+---------+
| part_desc                            | rec_num |
+--------------------------------------+---------+
| SILICON DELAY LINE, TRUE OUTPUT      |    1092 |
| LOADABLE PLD                         |    1401 |
| 8-BIT, FLASH, 8 MHz, MICROCONTROLLER |    1411 |
| FPGA                                 |    1997 |
| 8-BIT, MROM, 8 MHz, MICROCONTROLLER  |    3425 |
+--------------------------------------+---------+
5 rows in set (0.00 sec)

The closest I’ve come to finding code that would do it is shown below. The syntax checks ok and it runs, however it just seems to hang the database up. I’ve let it run for as long as 10 minutes and nothing ever happens so I abort it.

DELETE
FROM ag_master
WHERE part_id IN (
  SELECT part_id
  FROM ag_master
  GROUP BY part_desc
  HAVING COUNT(*) > 1000
);

Here’s the explain plan on the tmp table

mysql> EXPLAIN SELECT * FROM ag_master WHERE part_desc IN (SELECT part_desc FROM tmp);
+----+--------------------+-----------+--------+---------------+------+---------+------+--------+-------------+
| id | select_type        | table     | type   | possible_keys | key  | key_len | ref  | rows   | Extra       |
+----+--------------------+-----------+--------+---------------+------+---------+------+--------+-------------+
|  1 | PRIMARY            | ag_master | ALL    | NULL          | NULL | NULL    | NULL | 177266 | Using where |
|  2 | DEPENDENT SUBQUERY | tmp       | system | NULL          | NULL | NULL    | NULL |      1 |             |
+----+--------------------+-----------+--------+---------------+------+---------+------+--------+-------------+
2 rows in set (0.00 sec)

Answer 1

As stated in the manual:

Currently, you cannot delete from a table and select from the same table in a subquery.

I think you’ll have to perform this operation via a temporary table:

CREATE TEMPORARY TABLE temp
  SELECT   part_desc
  FROM     ag_master
  GROUP BY part_desc
  HAVING   COUNT(*) > 1000;

DELETE FROM ag_master WHERE part_desc IN (SELECT part_desc FROM temp);

DROP TEMPORARY TABLE temp;