如何計算SQL中每個值的表行？

Question

我是新來的，所以如果它的主題不固定，請修復我。 我有這張桌子：

id | uid | month | year
-----------------------
1 | 5 | 12 | 2018
2 | 5 | 12 | 2018
3 | 9 | 12 | 2018
4 | 3 | 01 | 2019

我想計算每個uid的month和year值等於month=12 AND year=2018

例如，我想得到這樣的響應：

uid=5 - count = 2
uid=9 - count = 1

怎么可能？

Answer 1

與group by uid ：

select uid, count(*) as counter
from table
where month = 12 and year = 2018
group by uid 

Answer 2

您可以使用條件聚合：

select uid,
       sum(case when year = 2018 and month = 12 then 1 else 0 end) as cnt
from t
group by uid;

這將包括計數為0的uid 。如果只希望計數大於0的uid，請使用where子句：

select uid, count(*)
from t
where year = 2018 and month = 12
group by uid;

Answer 3

選擇uid，從year = 2018和month = 12按uid分組的表中選擇count（*）