Question

I want to select some rows with a specific filter, but don’t limit if I don’t get, at least, 40 rows.

It’s a pseudo-example:

SELECT * FROM `table`
WHERE
   SUM(1) < 40 OR
   `age` > 18

It’s similar to LIMIT, but LIMIT will consider the WHERE filter ever. I want to ignore the filter if I don’t have at least 40 rows (but accept the firsts rows).

How I do that?

Edit: a lot of people had doubts what I really wanted.

This is an example:

ID AGE
1  10
2  20
3  30
4  10
5  20
6  30
7  10

I want to get the first 2 rows EVER. And only after at least two rows, get new rows that match the given conditions (WHERE).

For example: I want the first 2 rows more rows whose age is 30. The result would be equivalent to:

ID AGE
1  10 <first>
2  20 <second>
3  30 <conditional>
6  30 <conditional>

Answer 1

You can use an increasing variable @rownum to simulate the same functionality. However, this is much less efficient than limit because the server brings the filtered-out rows into memory and continuously performs the @rownum:=@rownum+1 calculation.

SELECT @rownum:=@rownum+1, t.*
FROM `table` t, (SELECT @rownum:=0) r
WHERE
   @rownum <= 40 OR
   `age` > 18