如何减去按字段

Question

我有一张表格：

id  year type  amount
 1  2015   in      10
 2  2015  out       5
 3  2016   in      20
 4  2016  out       1
 ...

以下查询将为我提供＆＃39;中＆type =＆＃39;的类型总和。按年份分组：

SELECT year, sum(amount)
FROM table
WHERE type = in
GROUP BY year

我将如何获得以下结果？

year sum(in) sum(out) "in-out"
2015     10       5        5
2016     20       1       19

总和（in）是＆＃39;金额的总和＆＃39;其中type =＆＃39; in＆＃39;。

Answer 1

使用CASE语句处理type。

的值

SELECT year,
       SUM(CASE WHEN type = 'in' THEN amount ELSE 0 END) AS sum_in,
       SUM(CASE WHEN type = 'out' THEN amount ELSE 0 END) AS sum_out,
       SUM(CASE WHEN type = 'in' THEN amount ELSE -amount END) AS in_out
    FROM table
    GROUP BY year;