Question

有没有办法做一个单一的选择从汽车桌上获得10辆最新的汽车，其中5辆是红色的，5辆是蓝色的？

我能想到的最好的是两个选择和一个程序化合并：

red  = query("select * from cars where color = 'red'  order by when desc limit 5")
blue = query("select * from cars where color = 'blue' order by when desc limit 5")
all = merge(red, blue)

Answer 1

对于MySQL，请尝试：

select * from cars where color = 'blue' order by releasedate desc limit 5
Union
select * from cars where color = 'red' order by releasedate desc limit 5

Answer 2

因为，mysql不像任何其他RDBMS一样支持CTE和Window Function，所以可以在此使用相关子查询，

SELECT  a.*
FROM    Cars a
WHERE   a.Color IN ('Red', 'Blue') AND
        (
            SELECT  COUNT(*)
            FROM    Cars b
            WHERE   b.Color IN ('Red', 'Blue') AND
                    a.Color = b.Color AND
                    a.time >= b.time
        ) <= 5

<强> POSTGRESQL

WITH carList
AS
(
    SELECT  Col1, Col2, Col3, Color,
            ROW_NUMBER() OVER (PARTITION BY Color ORDER BY time DESC) rn
    FROM    tableName
)
SELECT  Col1, Col2, Col3, Color
FROM    carList
WHERE   Color IN ('Red', 'Blue') AND
        RN <= 5

SQL选择其中的五个和五个

2 个答案: