MYSQL SELECT to DELETE specified twice - mysql

Question

There is a problem when I change SELECT to DELETE:
DELETE
FROM mitarbeiter
WHERE mitarbeiter.pers_nr=
(SELECT mitarbeiter.pers_nr
FROM mitarbeiter
LEFT JOIN kunde ON kunde.betreuer=mitarbeiter.pers_nr
group by mitarbeiter.pers_nr
order by count(*)
limit 1);
The error says...
Table 'mitarbeiter' is specified twice, both as a target for 'DELETE' and as a separate source for data
How can I change that?

Answer 1

You can replace the logic with a JOIN:
DELETE m
FROM mitarbeiter m JOIN
(SELECT m2.pers_nr
FROM mitarbeiter m2 LEFT JOIN
kunde k
ON k.betreuer = m2.pers_nr
GROUP BY m2.pers_nr
ORDER BY COUNT(*)
LIMIT 1
) m2
ON m.pers_nr = m2.pers_nr;
That said, the logic can probably be simplified, but it is strange. You are using COUNT(*) with a LEFT JOIN, so even non-matches in the second table get a count of 1. Knowing your intentions -- with sample data and desired results -- would help others figure out if another approach would work better.

Answer 2

Instead of where and subquery you could try usin a join based and a table as alias for the same subquery
DELETE m.*
FROM mitarbeiter m
INNER JOIN (
SELECT mitarbeiter.pers_nr
FROM mitarbeiter
LEFT JOIN kunde ON kunde.betreuer=mitarbeiter.pers_nr
group by mitarbeiter.pers_nr
order by count(*)
limit 1
) t ON t.pers_nr = m.pers_nr

Answer 3

Another workaround you could try is saving your subquery in a temporary table, then calling on rows from that table.
DROP TABLE IF EXISTS tempTable;
CREATE TEMPORARY TABLE tempTable
SELECT mitarbeiter.pers_nr
FROM mitarbeiter
LEFT JOIN kunde
ON kunde.betreuer=mitarbeiter.pers_nr
GROUP BY mitarbeiter.pers_nr
ORDER BY COUNT(*)
LIMIT 1;
DELETE FROM mitarbeiter
WHERE mitarbeiter.pers_nr
IN (
SELECT * FROM tempTable
);
DROP TABLE tempTable; -- cleanup
Warning: this may take a much longer time and/or larger space than using JOIN as in the other solutions.

Answer 4

Welcome to Stack Overflow! Does this help point you in the right direction?
I often find that result visualization is easier for my feeble brain when I use Common Table Expressions
Please note that MySQL prior to version 8.0 doesn't support the WITH clause
CREATE TABLE IF NOT EXISTS mitarbeiter (
pers_nr varchar(10),
PRIMARY KEY (pers_nr)
) DEFAULT CHARSET=utf8;
INSERT INTO mitarbeiter (pers_nr) VALUES
('Scum'),
('Worker'),
('Manager'),
('President');
CREATE TABLE IF NOT EXISTS kunde (
kunde_id int(3) NOT NULL,
betreuer varchar(10) NOT NULL,
PRIMARY KEY (kunde_id)
) DEFAULT CHARSET=utf8;
INSERT INTO kunde (kunde_id, betreuer) VALUES
(1, 'Scum'),
(2, 'Worker'),
(3, 'Worker'),
(4, 'Manager'),
(5, 'Manager'),
(6, 'Manager'),
(7, 'President'),
(8, 'President'),
(9, 'President'),
(10, 'President');
WITH s1
AS
(SELECT betreuer
, count(1) AS kunde_count_by_pers_nr -- JJAUSSI: find the kunde count by pers_nr
FROM kunde
GROUP BY betreuer
),
s2
AS
(SELECT MIN(kunde_count_by_pers_nr) AS kunde_count_by_pers_nr_min -- JJAUSSI: find the lowest kunde_count
FROM s1),
s3
AS
(SELECT s1.betreuer
FROM s1 INNER JOIN s2
ON s1.kunde_count_by_pers_nr = s2.kunde_count_by_pers_nr_min -- JJAUSSI: Retrieve all the betreuer values with the lowest kunde_count
)
SELECT * -- JJAUSSI: Test this result and see if it contains the records you expect to delete
FROM s3;
--DELETE -- JJAUSSI: Once you are confident in the results from s3, this DELETE can work
-- FROM mitarbeiter
-- WHERE pers_nr IN (SELECT betreuer
-- FROM s3);
SELECT * -- JJAUSSI: Check for the desired results (we successfully got rid of 'Scum'!)
FROM mitarbeiter;