SQL嵌套查询可能吗？

Question

我发现select语句基本上使用不同的where子句来计算。我的问题是，如何将结果合并到一个语句中，以便这些计数可以成为列？

1. 从table1中选择count（*）作为c1，其中city ='nyc'
2. 从table1中选择count（*）作为c2，其中city ='boston'
3. 从table1中选择count（*）作为c3，其中city ='sf'

Answer 1

SELECT
  COUNT(CASE WHEN city = 'nyc' THEN 1 END) AS Nyc,
  COUNT(CASE WHEN city = 'boston' THEN 1 END) AS Boston,
  COUNT(CASE WHEN city = 'sf' THEN 1 END) AS Sf
FROM table

Answer 2

使用sum()和filtering only required cities

select sum(case when city = 'nyc' then 1 end) c1,
       sum(case when city = 'boston' then 1 end) c2,
       sum(case when city = 'sf' then 1 end) c3
from table1
where city in ('nyc','boston','sf')

Answer 3

select count(CASE WHEN city = 'nyc' THEN 1 END) as c1,
       count(CASE WHEN city = 'boston' THEN 1 END) as c2,       
       count(CASE WHEN city = 'sf' THEN 1 END) as c3
from table1

SQLFiddle上的演示

同样在SQLServer2005 +，Oracle中，您可以使用PIVOT操作

SELECT *
FROM table1
PIVOT (
COUNT(city) FOR city IN ([nyc], [boston], [sf])
) p

SQLFiddle上的演示

Answer 4

你可以给GROUP BY一个机会，

SELECT city, gender, count(*)
WHERE gender = "male"
GROUP BY city, gender;

Answer 5

完整性）

select
    (select count(*) as c1 from table1 where city = 'nyc') as c1,
    (select count(*) as c2 from table1 where city = 'boston') as c2,
    (select count(*) as c3 from table1 where city = 'sf') as c3