Does it make sense to use "LIMIT 1" in a query "SELECT 1 ..."?

Question

The question should be clear enough, but is it of any advantage to use instead of

SELECT 1 FROM table ...

SELECT 1 FROM table ... LIMIT 1?

Answer 1

Let's say your table has a million rows...

SELECT 1 FROM table ...

Will return the value 1, a million times...

SELECT 1 FROM table ... LIMIT 1

Will return the value 1, one time.

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EDIT

You mention that you're specifically interested in regards to an EXISTS check. EXISTS stops processing after the first row is found (which is why EXISTS is more efficient than IN in this case), so with this in mind, there isn't a functional difference between these two queries:

SELECT *
FROM tableA ta
WHERE EXISTS (
    SELECT 1
    FROM TableB tb
    WHERE tb.ID = ta.ID
);

And

SELECT *
FROM tableA ta
WHERE EXISTS (
    SELECT 1
    FROM TableB tb
    WHERE tb.ID = ta.ID
    LIMIT 1
);