Question

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How to do a Select in a Select

I have 2 tables:

TABLE1 Table1Id  TABLE2 Table2Id Table1Id UserId 

TABLE2 has thousands of entries in it. I want to return a list of TABLE1 entries where there isn’t an entry in TABLE2 for it for a particular user. So, where there isn’t a foreign key entry in TABLE2. A query like:

select count(*) from TABLE1 where Table1Id not in ( select Table1Id from TABLE2 where id_user = 1) 

However, that query runs very slowly. What would be the most efficient way of getting the results I require?

Answer 1

There is a similar question

I think it would be better

SELECT COUNT(*)  FROM TABLE1  WHERE NOT EXISTS (SELECT Table1Id FROM TABLE2 WHERE TABLE2.Table1Id = TABLE1.Table1Id AND UserID = 1) 

I would check the indexes also, as ck suggested