Question

我有一个像这样的SQL查询 -

select * from (SELECT parent.item_name as name, parent.lft as lft
FROM item_tree AS item,
    item_tree AS parent
WHERE item.lft > parent.lft AND item.rgt < parent.rgt
    AND item.item_id = 6) 
      as ABC

这会返回一个表 -

name   |   lft

A      |     1
B      |     2
C      |     3

但是，我需要值C，因为它具有最大lft值。所以，我修改了下面的代码 -

select * from (SELECT parent.item_name as name, parent.lft as lft
FROM item_tree AS item,
    item_tree AS parent
WHERE item.lft > parent.lft AND item.rgt < parent.rgt
    AND item.item_id = 6) 
      as ABC
    having ABC.lft = MAX(ABC.lft)

现在，此查询不返回任何值。这段代码有什么问题？

提前致谢！

Answer 1

尝试此查询

    SELECT name, lft
    from (SELECT parent.item_name as name, parent.lft as lft
    FROM item_tree AS item,
        item_tree AS parent
    WHERE item.lft > parent.lft AND item.rgt < parent.rgt
        AND item.item_id = 6) 
          as ABC
    ORDER BY lft DESC
    LIMIT 1

Answer 2

MAX is agregate function, and not working without GROUP BY.

Try this:

select * from (SELECT parent.item_name as name, parent.lft as lft
FROM item_tree AS item,
    item_tree AS parent
WHERE item.lft > parent.lft AND item.rgt < parent.rgt
    AND item.item_id = 6) 
      as ABC
    order by ABC.lft desc limit 1

如何找到列中包含最大数字的行？

2 个答案: