Need help to understand EXISTS behaviour/return-CodePudding

I am a beginner. As I understand, EXISTS will return true if the subquery contains any rows. Does that mean there's only one boolean returned?

 SELECT * FROM TABLE1 T1 WHERE EXISTS (
 SELECT 1 FROM TABLE2 T2 WHERE T1.ID= T2.ID
);

For example: this query should return tuples of Table 1 only where the IDs from both table match. But if EXISTS returns only 1 true (because the subquery is not empty - there's at least one row) then shouldn't it be equivalent to:

SELECT * FROM TABLE1 T1 WHERE TRUE;

Shouldn't it return all the rows in Table 1? If EXISTS returns a boolean every time a row is not empty then it would make much more sense to me. Thank you.

CodePudding user response：

You might be misinterpreting the logic of your first query. In plain English, your exists query says to return every record from TABLE1 for which we can find a record in TABLE2 having the same ID. Note that the query inside the exists clause is logically evaluated for every record in TABLE1. It does not just execute once.