PostgreSQL 多列输出使用 where 子句

Question

I would like to find the average of a column for 2 time periods but I'm unsure how to display both in single query. I know the below won't work but I'm looking for the proper syntax:

SELECT
AVG(amount) AS avg_amt
FROM table
WHERE day >= date('2019-05-05') AND day<= date('2019-07-04'),
AVG(amount) AS avg_amt2
WHERE day >= date('2019-07-05') AND day<= date('2019-09-04')

Answer 1

You can use the FILTER clause to aggregate functions:

SELECT
    AVG(amount) FILTER(WHERE day >= date '2019-05-05' AND day <= date '2019-07-04') AS avg_amt1,
    AVG(amount) FILTER(WHERE day >= date '2019-07-05' AND day <= date '2019-09-04') AS avg_amt2
FROM table

We can optimize the query a little with a WHERE clause:

SELECT
    AVG(amount) FILTER(WHERE day <= date '2019-07-04') AS avg_amt1,
    AVG(amount) FILTER(WHERE day >= date '2019-07-05') AS avg_amt2
FROM table
WHERE day >= date '2019-05-05' AND day <= date '2019-09-04'