php - Mysql 限制列值重复N次

Question

我有两张 table

Customer (idCustomer, ecc.. ecc..)
Comment (idCustomer, idComment, ecc.. ecc..)

显然这两个表是连在一起的，例如

SELECT * FROM Comment AS co
  JOIN Customer AS cu ON cu.idCustomer = co.idCustomer

有了这个，我从与客户相关联的表中选择所有评论，但现在我想限制评论的数量，每个客户最多 2 个评论。

我看到的第一件事是使用 GROUP BY cu.idCustomer 但它只限制每个客户 1 条评论，但我想每个客户 2 条评论。

我怎样才能做到这一点？

Answer 1

MySQL 中的一个选项是服务器端变量。例如:

set @num := 0, @customer := -1;

select  *
from    (
        select  idCustomer
        ,       commentText
        ,       @num := if(@customer = idCustomer, @num + 1, 1) 
                    as row_number
        ,       @customer := idCustomer
        from    Comments
        order by 
                idCustomer, PostDate desc
        ) as co
join    Customer cu
on      co.idCustomer = cu.idCustomer
where   co.row_number <= 2