Question

I have a MySQL db schema like so:

id    flags     views    done
-------------------------------
1     2         20       0
2     66        100      0
3     25        40       0
4     30        60       0 

... thousands of rows ...

I want to update all of the rows whose flags / views are >= 0.2. First as a test I want to try to SELECT to see how many rows would actually get updated. I tried:

SELECT flags/views AS poncho FROM mytable HAVING poncho > 0.2

But this only returns like 2 rows and I know there should be a lot more than that. It seems like it’s calculating the poncho value on all rows or something odd. What am I doing wrong?

Thanks!

Answer 1

Use the WHERE clause instead of HAVING:

SELECT flags/views AS poncho FROM mytable WHERE flags/views > 0.2

HAVING aggregates all identical values into one row. The SQL standard would not allow a HAVING clause without a corresponding GROUP BY, but MySQL is very permissive with the standard. What you have there is effectively the same as

SELECT flags/views AS poncho
FROM   mytable
GROUP  BY poncho
HAVING poncho > 0.2

More in the manual here.