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Where an ID doesn’t exist

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Mar 23.2021

I have two tables, a main table with data and indexed by and ID.

I have a second table, which has this id and another linking the two.

I want to find rows in my main table that aren’t in my linking second table. Essentially orphan rows.

I thought maybe like this but doesn’t work. Any ideas?

“`SELECT m.ID, COUNT(l.ID) FROM `mainTable` m JOIN `linkingTable` l ON m.ID = l.ID“`

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PHP

3 Comments(s) ↴

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@NogDogMar 23.2021 — I might do...
[code=sql]
select ID from mainTable where ID not in(
select ID from linkingTable
)
[/code]
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@kiwisauthorMar 23.2021 — Ended up with this?

SELECT m.ID FROM mainTable m<br/>
LEFT JOIN linkingTable l ON l.Dd = m.ID<br/>
WHERE l.ID IS NULL
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@NogDogMar 23.2021 — Should work, too. Not sure if either would be more efficient -- or if the database engine would essentially optimize both to do the same thing.
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