SQL-深入了解子句

Question

I'm trying to understand what condition can be placed after the having clause - it is usually a condition, that uses the functions from the select statement, but it also can contain a column, that is typed in the select statement.

Here's an example:

create table r(a int, b int, c int, d int, e int primary key)

select a, min(b), sum(c)
from r
where b > 5
group by a
having "condition"

which of the following can replace the "condition"

a) b = 5

b) a = sum(e)

c) min(b) >= 6

When I execute this in SQL, only a) doesn't work, but what's the logic behind it?

Answer 1

This will be correct one

create table r(a int, b int, c int, d int, e int primary key)

select a, min(b), sum(c)
from r
where b > 5
group by a
having "min(b) >= 6"

Answer 2

The HAVING clause is applied as a filter after the GROUP BY .

If you keep that in mind, it becomes obvious that:

b=5 fails because you don't GROUP BY b in the query and b is used as-is and not in an aggregation function.

a=SUM(e) succeeds because you GROUP BY a and compare a with an aggregate function.

MIN(b) >= 6 succeeds because you compare an aggregate function with a constant.

Hope this clarifies ...