Aggregate Nested Pgsql query - postgresql

Question

I'm trying to query a table to get me some results, but the way I'm doing it gives me the error: ERROR: aggregate function calls cannot be nested.
The table looks like this:
ID | canal_c1 | tarifacao | date | ativo_id
The query I'm trying is this:
SELECT
SUM(case when tarifacao = 'ForaPonta' then canal_c1 else 0 end) as ConsForaPonta,
MAX(case when tarifacao = 'ForaPonta' then canal_c1 else 0 end) as DemForaPonta,
ativo_id as ativo_id,
data_leitura_inicio::date as date
FROM
medicao
WHERE
medicao.ativo_id in (45) AND
medicao.tipo_leitura = 'Consumo' AND
medicao.data_leitura_inicio >= '2017-01-01' AND
medicao.data_leitura_inicio < '2017-01-10'
GROUP BY
medicao.ativo_id,
medicao.data_leitura_inicio::date
That gives me result like these:
query result
And that's fine what I need now is the datetime from the DemForaPonta field, in order to do that I trying this, but got that error.
MAX(case when tarifacao = 'ForaPonta' and
canal_c1 = MAX(case when tarifacao = 'ForaPonta' then canal_c1 else 0 end)
then data_leitura_inicio end) as DateDemForaPonta
Do you know how I could achieve this?
Thanks.
Edit:
Here's an example, the query result is the intended result.
example

Answer 1

It would be helpful to have an example with real data. But anyway, you can do what you need with nested selects, among other options.
Nested SELECTs
SELECT *,
MAX(case when tarifacao = 'ForaPonta' and canal_c1 = DemForaPonta then data_leitura_inicio END)
AS DateDemForaPonta
FROM(
SELECT
SUM(case when tarifacao = 'ForaPonta' then canal_c1 else 0 end) as ConsForaPonta,
MAX(case when tarifacao = 'ForaPonta' then canal_c1 else 0 end) as DemForaPonta,
ativo_id as ativo_id,
data_leitura_inicio::date as date
FROM
medicao
WHERE
medicao.ativo_id in (45) AND
medicao.tipo_leitura = 'Consumo' AND
medicao.data_leitura_inicio >= '2017-01-01' AND
medicao.data_leitura_inicio < '2017-01-10'
GROUP BY
medicao.ativo_id,
medicao.data_leitura_inicio::date
)data
Since I don't have data to test it, don't really know if it does the trick, but it should.