postgresql - 合并两个分组？

Question

我有两个问题:

SELECT to_char(pubdate, 'YYYY-MM') pub_month, COUNT(*) total 
FROM books 
GROUP BY pub_month;

和

SELECT to_char(pubdate, 'YYYY-MM') pub_month, COUNT(*) total_bought 
FROM books 
WHERE purchased=true 
GROUP BY pub_month;

有什么办法可以把它们组合起来使得结果是

+-----------+-------+--------------
| pub_month | total | total_bought 
+-----------+-------+--------------
|   2018-10 |     5 |            2
|   2018-09 |    10 |            7

Answer 1

你可以使用 FILTER子句:

The filter clause extends aggregate functions (sum, avg, count, …) by an additional where clause. The result of the aggregate is built from only the rows that satisfy the additional where clause too.

SELECT to_char(pubdate, 'YYYY-MM') pub_month
    , COUNT(*) FILTER (WHERE purchased) total_bought
    , COUNT(*) total 
FROM books 
GROUP BY pub_month;

或条件聚合:

SELECT to_char(pubdate, 'YYYY-MM') pub_month
    , SUM(CASE WHEN purchased THEN 1 ELSE 0 END) total_bought
    , COUNT(*) total 
FROM books 
GROUP BY pub_month;

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补充说明:WHERE purchased=true <=> WHERE purchased