Aggregation condition

Since v0.7.0, HAVING has been added to support the conditional filtering of aggregation results directly. By using HAVING, we can avoid using a subquery after the GROUP BY statement. See the following example:

> SELECT 
> gender_age.age 
> FROM employee
> GROUP BY gender_age.age 
> HAVING count(*)=1;
+----------------+
| gender_age.age |
+----------------+
| 27             |
| 30             |
| 35             |
| 57             |
+----------------+
4 rows selected (25.829 seconds)

> SELECT 
> gender_age.age, 
> count(*) as cnt -- Support use column alias in HAVING, like ORDER BY
> FROM employee 
> GROUP BY gender_age.age HAVING cnt=1; 
+----------------+-----+
| gender_age.age | cnt |
+----------------+-----+
| 27             | 1   |
| 30             | 1   |
| 35             | 1   |
| 57             | 1   |
+----------------+-----+ 
4 rows selected (25...