Mysql group by top N的问题(转)
在日常工作中,经常要查询分组的前几名.oracle中可以通过row_num来支持查询,
mysql暂时不支持row_num.那么如何来完成这个需求呢?
例如：
表中的数据：
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+
--------+-------+-----+
| Person |
Group
| Age |
+
--------+-------+-----+
| Bob | 1 | 32 |
| Jill | 1 | 34 |
| Shawn | 1 | 42 |
| Jake | 2 | 29 |
| Paul | 2 | 36 |
| Laura | 2 | 39 |
+
--------+-------+-----+
|
期望的结果：
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+
--------+-------+-----+
| Shawn | 1 | 42 |
| Jill | 1 | 34 |
| Laura | 2 | 39 |
| Paul | 2 | 36 |
+
--------+-------+-----+
|
方式一:借鉴oracle中row_num的思想,在sql中增加伪列.
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set
@num := 0, @
group
:=
''
;
select
person, `
group
`, age
from
(
select
person, `
group
`, age,
@num := if(@
group
= `
group
`, @num + 1, 1)
as
row_number,
@
group
:= `
group
`
as
dummy
from
mytable
order
by
`
Group
`, Age
desc
, person
)
as
x
where
x.row_number <= 2;
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方式二:利用关联子查询
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SELECT
a.person, a.
group
, a.age
FROM
person
AS
a
WHERE
(
SELECT
COUNT
(*)
FROM
person
AS
b
WHERE
b.
group
= a.
group
AND
b.age >= a.age) <= 2
ORDER
BY
a.
group
ASC
, a.age
DESC
|