Question

I have a MySQL table (tbl_filters) with 3 columns: id, cat, val

id & val are numeric, cat is varchar. There are multiple rows for each id.

I also have another table (tbl_info) with multiple columns, including an id which corresponds to the id from tbl_filters. There is a column called name, which is what I’m looking for.

I would like to select the name of all the rows which match a set value for cat, but only if the val for cat is the maximum for this id, and only if it is above a minimum set val.

In pseudocode it would be something like:

SELECT tbl_info.name FROM tbl_info,tbl_filters
WHERE (tbl_info.id=tbl_filters.id) AND (cat="mycat") AND (val>=0.3)
AND (there are no other rows for this id in tbl_info with a higher value for val)

Example:

tbl_filters
id,cat,val
1 eg1 0.43
1 eg2 0.60
1 eg3 0.78

tbl_info
id name
1 MyName

In the above example, a value should only be returned if I am looking for the cat called eg3, since that has the highest value. For the other cats, nothing should be returned, since they are not the highest value.

Another option would be to make a column in tbl_info just for the cat with the highest value, but that is a messy solution I would prefer to avoid.

Answer 1

I THINK I’m following you… The INNER-MOST query pre-qualifies the HIGHEST Value per ID of your minimum value qualification, and the category that qualifies. ONCE you get that list, re-join back to get the name from the tbl_info. I’ve re-joined to the tbl_filters a second time in case there were other elements on that record you want, such as the date of the rate, or other things. If you DONT need that, you can ignore the second “tf2” join and just change the fields list from tf2.val to PreQualified.HighestQualVal.

select
      ti.id,
      ti.name,
      tf2.val
   from
      ( select 
              tf.id,
              max( tf.val ) as HighestQualVal
           from
              tbl_filters tf
           where
                  tf.cat = "mycat"
              and tf.val >= 0.3
      ) PreQualified

      JOIN tbl_info ti
         on PreQualified.id = ti.id

      JOIN tbl_filters tf2
         on PreQualified.id = tf2.id
         AND PreQualified.HighestQualVal = tf2.val