Is there a way to use aggregate COUNT() values within CASE? - mysql

Question

I need to retrieve unique yet truncated part numbers, with their description values being conditionally determined.
DATA:
Here's some simplified sample data:
(the real table has half a million rows)
create table inventory(
partnumber VARCHAR(10),
description VARCHAR(10)
);
INSERT INTO inventory (partnumber,description) VALUES
('12345','ABCDE'),
('123456','ABCDEF'),
('1234567','ABCDEFG'),
('98765','ZYXWV'),
('987654','ZYXWVU'),
('9876543','ZYXWVUT'),
('abcde',''),
('abcdef','123'),
('abcdefg','321'),
('zyxwv',NULL),
('zyxwvu','987'),
('zyxwvut','789');
TRIED:
I've tried too many things to list here.
I've finally found a way to get past all the 'unknown field' errors and at least get SOME results, but:
it's SUPER kludgy!
my results are not limited to unique prods.
Here's my current query:
SELECT
LEFT(i.partnumber, 6) AS prod,
CASE
WHEN agg.cnt > 1
OR i.description IS NULL
OR i.description = ''
THEN LEFT(i.partnumber, 6)
ELSE i.description
END AS `descrip`
FROM inventory i
INNER JOIN (SELECT LEFT(ii.partnumber, 6) t, COUNT(*) cnt
FROM inventory ii GROUP BY ii.partnumber) AS agg
ON LEFT(i.partnumber, 6) = agg.t;
GOAL:
My goal is to retrieve:
prod
descrip
12345
ABCDE
123456
123456
98765
ZYXWV
987654
987654
abcde
abcde
abcdef
abcdef
zyxwv
zyxwv
zyxwvu
zyxwvu
QUESTION:
What are some cleaner ways to use the COUNT() aggregate data with a CASE type conditional?
How can I limit my results so that all prods are UNIQUE?

Answer 1

You can check if a left(partnumber, 6) is not unique in the result by checking if count(*) > 1. In such a case let descrip be left(partnumber, 6). Otherwise you can use max(description) (or min(description)) to get the single description but satisfy the needs to use an aggregation function on columns not in the GROUP BY. To replace empty or NULL descriptions, nullif() and coalesce() can be used.
That would lead to the following using just one level of aggregation and no joins:
SELECT left(partnumber, 6) AS prod,
CASE
WHEN count(*) > 1 THEN
left(partnumber, 6)
ELSE
coalesce(nullif(max(description), ''), left(partnumber, 6))
END AS descrip
FROM inventory
GROUP BY left(partnumber, 6)
ORDER BY left(partnumber, 6);
But there seems to be a bug in MySQL and this query fails. The engine doesn't "see" that, in the list after SELECT partnumber is only used in the expression left(partnumber, 6), which is also in the GROUP BY. Instead the engine falsely complains about partnumber not being in the GROUP BY and not subject to an aggregation function.
As a workaround, we can use a derived table, that does the shortening of partnumber to its first six characters. We then use use that column of the derived table instead of left(partnumber, 6).
SELECT l6pn AS prod,
CASE
WHEN count(*) > 1 THEN
l6pn
ELSE
coalesce(nullif(max(description), ''), l6pn)
END AS descrip
FROM (SELECT left(partnumber, 6) AS l6pn,
description
FROM inventory) AS x
GROUP BY l6pn
ORDER BY l6pn;
Or we slap some actually pointless max()es around the left(partnumber, 6) other than the first, to work around the bug.
SELECT left(partnumber, 6) AS prod,
CASE
WHEN count(*) > 1 THEN
max(left(partnumber, 6))
ELSE
coalesce(nullif(max(description), ''), max(left(partnumber, 6)))
END AS descrip
FROM inventory
GROUP BY left(partnumber, 6)
ORDER BY left(partnumber, 6);
db<>fiddle (Change the DBMS to some other like Postgres or MariaDB to see that they also accept the first query.)