过滤器是什么意思？

Question

select driverid, count(*) 
from f1db.results
    where position is  null
group by driverid order by driverid 

My thought: first find out all the records that position is null then do the aggregate function.

select driverid, count(*) filter (where position is null) as outs
   from f1db.results 
group by driverid order by driverid

First time, I meet with filter clause, not sure what does it mean? Two code block results are different. Already googled, seems don't have many tutorials about the FILTER. Kindly share some links.

Answer 1

A more helpful term to search for is aggregate filters , since FILTER (WHERE) is only used with aggregates.

Filter clauses are an additional filter that is applied only to that aggregate and nowhere else. That means it doesn't affect the rest of the columns!

You can use it to get a subset of the data, like for example, to get the percentage of cats in a pet shop:

SELECT shop_id,
       CAST(COUNT(*) FILTER (WHERE species = 'cat')
            AS DOUBLE PRECISION) / COUNT(*) as "percentage"
FROM animals
GROUP BY shop_id

For more information, see the docs

Answer 2

FILTER ehm... filters the records which should be aggregated - in your case, your aggregation counts all position s that are NULL .

Eg you have 10 records:

SELECT COUNT(*)

would return 10.

If 2 of the records have position = NULL

than

SELECT COUNT(*) FILTER (WHERE position IS NULL)

would return 2.

What you want to achieve is to count all non- NULL records:

SELECT COUNT(*) FILTER (WHERE position IS NOT NULL)

In the example, it returns 8.