SQL Left Join a Table on a Left Joined Table - mysql

Question

Iam currently trying to left join a table on a left joined table as follows.
I have the tables:
accounts (id, vorname, nachname)
projektkurse (id, accounts_id, projektwochen_id)
projektkurs_einzel (id, projektkurse_id)
projektkurs_einzel_zeiten (id, date, shift, projektkurs_einzel_id)
Now I want to get every account and the amount times they have an entry inside of projektkurs_einzel_zeiten, which should also be unique. So having the same date and shift multiple times does not count as multiple entries. The result should also be limited by the column projektwochen_id from the table projektkurse. This column should match a certain value for example 8.
Some Accounts don't have any entries in projektkurse, projektkurs_einzel and projektkurs_einzel_zeiten, this is why my first thought was using LEFT JOIN like this:
SELECT accounts.id, accounts.vorname, accounts.nachname, COUNT(DISTINCT projektkurs_einzel_zeiten.date, projektkurs_einzel_zeiten.shift) AS T
FROM accounts
LEFT JOIN projektkurse on accounts.id = projektkurse.creator_id
LEFT JOIN projektkurs_einzel on projektkurse.id = projektkurs_einzel.projektkurs_id
LEFT JOIN projektkurs_einzel_zeiten ON projektkurs_einzel.id = projektkurs_einzel_zeiten.projektkurs_einzel_id
WHERE projektkurse.projektwochen_id = 8
GROUP BY accounts.id
This query does not achieve exactly what I want. It only returns accounts that have atleast one entry in projektkurse even if they have none in projektkurs_einzel and projektkurs_einzel_zeiten. The Count is obviously 0 for them but the accounts that have no entries in projektkurse are being ignored completly.
How can I also show the accounts that don't have entries in any other table with the Count 0 aswell?

Answer 1

I would recommend writing the query like this:
SELECT a.id, a.vorname, a.nachname,
COUNT(DISTINCT pez.date, pez.shift) AS T
FROM accounts a LEFT JOIN
projektkurse
ON a.id = pk.creator_id AND
pk.projektwochen_id = 8 LEFT JOIN
projektkurs_einzel pe
ON pk.id = pe.projektkurs_id LEFT JOIN
projektkurs_einzel_zeiten pez
ON pe.id = pez.projektkurs_einzel_id
GROUP BY a.id, a.vorname, a.nachname;
Notes:
Your problem is fixed by moving the WHERE condition to the ON clause. Your WHERE turns the outer join into an inner join, because NULL values do not match.
Table aliases make the query easier to write and to read.
It is a best practice to include all unaggregated columns in the GROUP BY. However, assuming that id is unique, your formulation is okay (due to something called "functional dependencies").

Answer 2

You should not use eft join table's column ins where condition this work as inner join
You should move the where condition for a left joined table in the corresponding ON clause
SELECT accounts.id, accounts.vorname, accounts.nachname, COUNT(DISTINCT projektkurs_einzel_zeiten.date, projektkurs_einzel_zeiten.shift) AS T
FROM accounts
LEFT JOIN projektkurse on accounts.id = projektkurse.creator_id
AND projektkurse.projektwochen_id = 8
LEFT JOIN projektkurs_einzel on projektkurse.id = projektkurs_einzel.projektkurs_id
LEFT JOIN projektkurs_einzel_zeiten ON projektkurs_einzel.id = projektkurs_einzel_zeiten.projektkurs_einzel_id
GROUP BY accounts.id