有没有办法计算 PostgreSQL 中列的组合？
Is there a way to count combination of columns in PostgreSQL?
我有一个名为 MEMBER 的 Postgres table，它有 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 is/was 成员所属的乐队数量。有没有办法算这个？
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
我想知道每个成员有多少乐队
id_musician
count
1
1
2
2
3
1
但没有音乐家 2 的双排
有什么建议吗？
您可以使用 count(distinct id_band):
select id_musician, count(distinct id_band) from members group by id_musician;
我有一个名为 MEMBER 的 Postgres table，它有 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 is/was 成员所属的乐队数量。有没有办法算这个？
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 |
我想知道每个成员有多少乐队
| id_musician | count |
|---|---|
| 1 | 1 |
| 2 | 2 |
| 3 | 1 |
但没有音乐家 2 的双排
有什么建议吗？
您可以使用 count(distinct id_band):
select id_musician, count(distinct id_band) from members group by id_musician;