Use count value in clause WHERE - mysql

Question

I want to add a WHERE clause to my SQL query. but if I add AND n! = 1, I get an error.
how do I proceed?
thank you
SELECT
COUNT(1) AS n,
`prescription`.`id_prescription`
FROM
`intervention`
LEFT JOIN
`prescription`
ON
`prescription`.`id_prescription` = `intervention`.`id_prescription`
WHERE
`intervention`.`status` != "finish" AND `intervention`.`status` != "canceled"
GROUP BY
id_prescription
ORDER BY `n` DESC
Erreur
Requête SQL : Documentation
SELECT
COUNT(1) AS n,
`prescription`.`id_prescription`
FROM
`intervention`
LEFT JOIN
`prescription`
ON
`prescription`.`id_prescription` = `intervention`.`id_prescription`
WHERE
`intervention`.`status` != "finish" AND `intervention`.`status` != "canceled" AND `n` !=1
GROUP BY
id_prescription
ORDER BY `n`
DESC LIMIT 0, 25
MySQL a répondu: Documentation
#1054 - Champ 'n' inconnu dans where clause

Answer 1

Use having to filter on aggregates:
SELECT COUNT(*) AS n, id_prescription
FROM intervention i
LEFT JOIN prescription p USING(`id_prescription`)
WHERE i.status <> 'finish' AND i.status <> 'canceled'
GROUP BY id_prescription
HAVING COUNT(*) <> 1
ORDER BY n DESC
Side notes:
table aliases make the query easier to write and read
USING() comes handy to shorten the join syntax when the matching column(s) are homonyms
backticks are probably not necessary here
<> is the standard operator for inequality
single quotes are the standard quoting character for literal strings
MySQL extends the SQL standard and allows using column aliases in the HAVING clause, so you could also phrase it: HAVING n <> 1

Answer 2

It seems very strange that you are aggregating by the second table in a LEFT JOIN. If you want to count the number of interventions for each prescription, then your query should look like this:
SELECT p.id_prescription, COUNT(i.id_prescription) as num_interventions
FROM prescription p
intervention i
ON p.id_prescription = i.id_prescription AND
i.stqatus NOT IN ('finish', 'canceled')
GROUP BY p.id_prescription
HAVING COUNT(i.id_prescription) <> 1
ORDER BY COUNT(i.id_prescription) DESC;
This version of the query correctly returns 0 for the count when there are no interventions. I suspect that the HAVING clause you really want is:
HAVING COUNT(i.id_prescription) > 1
And if this interpretation is correctly, then an INNER JOIN is a better option than a LEFT JOIN, because the logic requires a match.