如何计算条件语句中按另一列分组的 mySQL 中的列的平均值？

Question

I need to calculate the average of one column where a second column has the same values. For example, you have

Col1 | Col2
a    | 2
a    | 9
b    | 6
b    | 10

I need to retrieve the average of Col2 grouped by Col1, so (9+2)/2 and (6+10)/2. I am able to retrieve it in its own separate query, but it's part of a condition where the average has to be greater than 7. The sample code in mySQL is

SELECT Col1, AVG(Col2) FROM tableName
GROUP BY Col1;

SELECT Col1 FROM tableName
WHERE (SELECT AVG(Col2) GROUP BY Col1) >= 7;

The first query correctly gives me the average of Col2 grouped by Col1, but the second query simply takes the average of the entire Col2 and returns the values that are greater than the calculated average of all columns. Why is the second query not grouping by col1, and how should I change my code to correctly group the tuples?

Answer 1

You seem to want a having clause:

select col1, avg(col2) as avg_col2
from tablename
group by col1
having avg(col2) >= 7

MySQL supports column aliases in the having clause (this is an extension to the SQL standard), so you can simplify it as:

having avg_col2 >= 7