MySQL 基于 GROUP BY SELECT n 条记录

Question

假设我有 SQL 记录：

Country | Number
USA | 300
USA | 450
USA | 500
USA | 100
UK  | 300
UK  | 400
UK  | 1000

我正在做这样的事情：SELECT * FROM table GROUP BY Country。
如果我想选择只显示每个国家中 2 个最大数字的结果怎么办？我怎样才能存档这个？

结果将是：

Country | Number
USA | 450
USA | 500
UK  | 400
UK  | 1000

Answer 1

示例数据

create table data (Country varchar(10), Number int);
insert into data select
'USA' , 300 union all select
'USA' , 450 union all select
'USA' , 500 union all select
'USA' , 100 union all select
'FR'  , 100 union all select
'FR'  , 420 union all select
'UK'  , 300 union all select
'UK'  , 400 union all select
'UK'  , 1000;

第一个选项是使用像 Scrum Meister 所示的变量的伪排名，但在这里作为单个语句呈现

SELECT Country, Number
FROM (
    SELECT
        Number,
        @r := case when @c=country then @r+1 else 1 end rownum,
        @c := Country Country 
    FROM (select @r :=0 , @c := '') x, data
    ORDER BY Country, Number DESC
) y
WHERE rownum < 3;

如果您在前端使用它，并且只需要 2 个计数，那么您可以使用此形式返回列表中的计数（单列）

SELECT
    Country,
    left(x,locate(',',concat(x,','),locate(',',x)+1)-1) Numbers
FROM (
    SELECT
        a.Country,
        Group_Concat(a.Number) x
    From (
        select country, number
        from data
        order by country, number desc) a
    group by a.Country
) b

结果是

"Country";"Numbers"
"FR";"420,100"
"UK";"1000,400"
"USA";"500,450"

如果有可能出现平局，则第二种形式的这种变体将删除平局并显示“每个国家/地区前 2 个不同的数字”作为记录。

SELECT distinct x.Country, x.Number
From data x
inner join
(
    SELECT
        Country,
        left(x,locate(',',concat(x,','),locate(',',x)+1)-1) Numbers
    FROM (
        SELECT
            a.Country,
            Group_Concat(a.Number) x
        From (
            select distinct country, number
            from data
            order by country, number desc) a
        group by a.Country
    ) b
) y on x.Country=y.Country
    and concat(',',y.Numbers,',') like concat('%,',x.Number,',%')
order by x.Country, x.Number Desc

结果

"Country";"Number"
"FR";"420"
"FR";"100"
"UK";"1000"
"UK";"400"
"USA";"500"
"USA";"450"

Answer 2

由于MySql没有内置的RANK函数，查询可能会很慢：

SET @cRank = 0;
SET @cCoutnry = '';

SELECT Country, Number
FROM (
    SELECT Number, @cRank := IF(@cCoutnry = Country, @cRank+1, 1) AS rank, @cCoutnry := Country Country 
    FROM table
    ORDER BY Country, Number DESC
) rs
WHERE rank < 3

Answer 3

让我们将您的表命名为 TName，然后他们的查询将是。

SELECT * FROM (SELECT ROW_NUMBER() OVER (PARTITION BY X.Country) AS RowNo, *
FROM (SELECT Country, Name FROM TName ORDER BY Country, Number) X ) Y WHERE Y.RowNo <= 2