mySQL 返回每个类别的前 5 个

2021-11-20 00:00:00 sql mysql greatest-n-per-group

我希望能够为每个菜单返回 5 个菜单项.我试过这几个脚本，但没有运气.这是桌子

I want to be able to return 5 menuitem per menu. I've tried this several script but had no luck. here are the tables

menus
-------
menuid int()
profileName varchar(35)

menuitems
-----------
itemid int()
name varchar(40)

这是我现在拥有的.我收到以下脚本的错误消息.错误:子查询返回多于 1 行.

Here what I have now. I'm getting the error message with the script below. Error: Subquery returns more then 1 row.

SELECT m.profilename, name
FROM menus m 
WHERE (SELECT name
        from menuitems s
        where m.menuid = s.menuid
        limit 5)

非常感谢任何建议.

推荐答案

你必须为此使用副作用变量

You have to use side effecting variables for this

SELECT profilename, name
FROM
(
    SELECT m.profilename, s.name,
        @r:=case when @g=m.profilename then @r+1 else 1 end r,
        @g:=m.profilename
    FROM (select @g:=null,@r:=0) n
    cross join menus m 
    left join menuitems s on m.menuid = s.menuid
) X
WHERE r <= 5

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