Question

如何限制提取的子行仅为5？

这是SQLfiddle http://sqlfiddle.com/#!2/bd96a/2。现在，它使用parentid='4'和parentid='14'获取所有行。它应该只取每个parentid 5。假设我有数百行，每个parentid最多只能获取5个。

 "SELECT child.* FROM mytable as parent
     LEFT JOIN mytable as child on child.parentid=parent.id
     WHERE parent.pageid IN ( 1, 2) AND parent.submittype='1'
     ORDER BY child.id ASC";

我如何解决这个问题？

Answer 1

你想要每组最大的n ..这将做你想要的。抱歉，我在手机上，所以我无法对其进行格式化。如果有人可以为我编辑那就太棒了！

SELECT 
    child.*, 
    if(@a = child.parentid, @b :=@b+1, @b := 1) as counting_col, 
    @a := child.parentid
FROM mytable as parent
LEFT JOIN mytable as child on child.parentid=parent.id
CROSS JOIN(select @a :=0, @b:=1) t
WHERE parent.pageid IN ( 1, 2) 
  AND parent.submittype='1'
HAVING counting_col <=5
ORDER BY child.id ASC

限制JOIN获取的行数

1 个答案: