How do I compare SUM and COUNT()s in SQL? - sql

Question

I am building a small query to find all CustomerNumbers where all of their policies are in a certain status (terminated).
Here is the query I am working on
select
a.cn
,p.pn
, tp = COUNT(p.pn)
, tp2 = SUM(case when p.status = 4 then 1 else 0 end)
from
(
select cn, cn2
from bc
union
select cn, cn2= fn
from ic
) as a
left join p as p
on a.cn = p.cn
group by
a.cn,
pn
My issue is when I add the clause:
WHERE cn = tp
It says the columns are invalid. Am I missing something incredibly obvious?

Answer 1

You can't use aliases at the same level of a query. The reason is that the where clause is logically evaluated before the select, so the aliases defined in the select are not available in the where.
A typical solution is to repeat the expression (other answers) or use a subquery or cte:
with cte as (
<your query here>
)
select cte.*
from cte
where TotalPolicies = TermedPolicies;
However, in your case, you have an easier solution, because you have an aggregation query. So just use:
having TotalPolicies = TermedPolicies

Answer 2

You cannot use the aliased aggregate column names in the where clause. You have to use the expression itself instead. Also you cannot use it as where cluase, but use it in the having clause
HAVING COUNT(p.PolicyNumber) = SUM(case when p.status = 4 then 1 else 0 end)

Answer 3

You can also make the whole query as a subquery then add your where statement:
select CustomerNumber
,PolicyNumber
,TotalPolicies
,TermedPolicies
from (
select
a.CustomerNumber
,p.PolicyNumber
, TotalPolicies = COUNT(p.PolicyNumber)
, TermedPolicies = SUM(case when p.status = 4 then 1 else 0 end)
from
(
select CustomerNumber, CompanyName
from BusinessClients
union
select CustomerNumber, CompanyName = FullName
from IndividualClients
) as a
left join Policies as p
on a.CustomerNumber = p.CustomerNumber
group by
a.CustomerNumber,
PolicyNumber
) tb
where TotalPolicies = TermedPolicies

Answer 4

select
a.CustomerNumber
,p.PolicyNumber
, COUNT(p.PolicyNumber) as TotalPolicies
, SUM(case when p.status = 4 then 1 else 0 end) as TermedPolicies
from
(
select CustomerNumber, CompanyName
from BusinessClients
union
select CustomerNumber, CompanyName = FullName
from IndividualClients
) as a
left join Policies as p
on a.CustomerNumber = p.CustomerNumber
WHERE COUNT(p.PolicyNumber)= SUM(case when p.status = 4 then 1 else 0 end)
group by
a.CustomerNumber,
PolicyNumber
This should work. But it is not tested.

Answer 5

In order to filter by an aggregate function, you must include it in the HAVING clause, rather than the WHERE clause.
select
a.CustomerNumber
,p.PolicyNumber
, TotalPolicies = COUNT(p.PolicyNumber)
, TermedPolicies = SUM(case when p.status = 4 then 1 else 0 end)
from
(
select CustomerNumber, CompanyName
from BusinessClients
union
select CustomerNumber, CompanyName = FullName
from IndividualClients
) as a
left join Policies as p
on a.CustomerNumber = p.CustomerNumber
having COUNT(p.PolicyNumber) = SUM(case when p.status = 4 then 1 else 0 end)
group by
a.CustomerNumber,
PolicyNumber
The reason for this has to do with the way SQL engines evaluate queries. The contents of the WHERE clause are used to filter out rows before the aggregate functions are applied. If you could reference aggregate functions there, the engine would have to have some way to determine which predicates to apply before aggregation and which to apply after. The HAVING clause allows the engine to have a clear demarcation between the two: WHERE applies before aggregation and HAVING applies after aggregation.

Answer 6

When dealing with aggregations in a query that has grouping, you will need to use HAVING. This should work:
select
a.CustomerNumber
,p.PolicyNumber
, TotalPolicies = COUNT(p.PolicyNumber)
, TermedPolicies = SUM(case when p.status = 4 then 1 else 0 end)
from
(
select CustomerNumber, CompanyName
from BusinessClients
union
select CustomerNumber, CompanyName = FullName
from IndividualClients
) as a
left join Policies as p
on a.CustomerNumber = p.CustomerNumber
group by
a.CustomerNumber,
PolicyNumber
HAVING TermedPolicies = SUM(case when p.status = 4 then 1 else 0 end)