Question

    SELECT AVG(`col5`)
    FROM `table1`
    WHERE `id` NOT IN (
        SELECT `id` FROM `table2`
        WHERE `col4` = 5
    )
    group by `col2` having sum(`col3`) > 0
UNION
    SELECT MAX(`col5`)
    FROM `table1`
    WHERE `id` NOT IN (
        SELECT `id` FROM `table2`
        WHERE `col4` = 5
    )
    group by `col2` having sum(`col3`) = 0

For readability and performance reasons, I think this code could be refactored. But how?

EDITIONS

1. removed the outer select

2. made the first select to return a sum and the second one to return another value

3. replaced the SUM by AVG

Answer 1

SELECT * 
FROM table1 t1
left outer join table2 t2 on t1.id = t2.id and t2.col4 = 5
where t2.id is null
group by t1.col2 
having sum(col3) >= 0 

The outer select is missing the FROM clause and was not adding anything so I removed it. The NOT IN is inefficient compared to the LEFT OUTER JOIN method so I replaced that. The two UNIONs were easily combined into one by using >=.

Update:
Note the use of UNION ALL rather than UNION. I don’t think you want to remove duplicates, and it will perform faster this way.

SELECT AVG(t1.col5) 
FROM table1 t1 
left outer join table2 t2 on t1.id = t2.id and t2.col4 = 5 
where t2.id is null 
group by t1.col2  
having sum(t1.col3) > 0  
UNION ALL
SELECT MAX(t1.col5) 
FROM table1 t1 
left outer join table2 t2 on t1.id = t2.id and t2.col4 = 5 
where t2.id is null 
group by t1.col2  
having sum(t1.col3) = 0