有没有办法计算 PostgreSQL 中的列组合？

Question

我有一个名为 MEMBER 的 Postgres 表，其中包含 3 列，如下所示：

id_musician	id_band	instrument
1	1	Guitar
1	1	Vocals
2	3	Vocals
2	4	Vocals
2	4	Guitar
3	1	Guitar

我需要计算每个成员是/曾经是多少个乐队的成员。有没有办法计算这个？

我尝试了下一个代码：

SELECT DISTINCT e.id_musician,count(id_band)
FROM MEMBER e
GROUP BY e.id_musician, e.instrument
ORDER BY e.id_musician;

但它给了我这个结果：

id_musician	count
1	1
2	1
2	2
3	1

例如，我想知道每个成员有多少个乐队

id_musician	count
1	1
2	2
3	1

但音乐家 2 上没有双排

有什么建议吗？

Answer 1

你可以使用count(distinct id_band):

select id_musician, count(distinct id_band) from members group by id_musician;