Question

我有下表，描述了一些植物

p_id   p_type  p_name      p_size  p_tasty
   1   veggie  potato           1        0
   2   veggie  carrot           2        0
   3   veggie  cabbage          3        0
   4   fruit   strawberry       1        1
   5   fruit   apple            2        0
   6   fruit   watermelon       3        0

如果我想获得每种类型的最大工厂，查询将是这样的：

SELECT a.p_id,a.p_type, a.p_name,a.p_size,a.p_tasty
FROM test.plants a
INNER JOIN (
    SELECT b.p_type, MAX(b.p_size) p_size
    FROM test.plants b
    GROUP BY b.p_type
) b ON a.p_type = b.p_type AND a.p_size = b.p_size

并会给我这个：

p_id    p_type  p_name      p_size  p_tasty
3       veggie  cabbage     3       0
6       fruit   watermelon  3       0

但我怎样才能获得各种最大或最美味的植物？

即选择p_tasty值为1的工厂，如果没有此类工厂，请选择最大工厂。

我想我可以使用case when exists then ...子句，但也许有更简单的方法可以做到这一点？

Answer 1

LEFT JOIN您对美味植物的查询，如果没有美味的植物，请使用最大的默认植物。

SELECT IFNULL(c.p_id, a.p_id) AS p_id, a.p_type, IFNULL(c.p_name, a.p_name) AS p_name, IFNULL(c.p_size, a.p_size) AS p_size, IFNULL(c.p_tasty, a.p_tasty) AS p_tasty
FROM test.plants a
INNER JOIN (
    SELECT b.p_type, MAX(b.p_size) p_size
    FROM test.plants b
    GROUP BY b.p_type
) b ON a.p_type = b.p_type AND a.p_size = b.p_size
LEFT JOIN test.plants c ON a.p_type = c.p_type AND c.tasty = 1

Answer 2

或者根据需要在row_number（）顺序上使用partition by：

    if($form->isSubmitted() && $form->has('get_away_from_form')){
        if($form->get('get_away_from_form')->isClicked() == 1){
            $isClicked = 'It works as expected: $form->get(get_away_from_form)->isClicked() = '.$form->get('get_away_from_form')->isClicked();
        } else {
            $isClicked = 'It DOESN\'T work as expected: $form->get(get_away_from_form)->isClicked() = '.$form->get('get_away_from_form')->isClicked();
        }
    }

从列中选择最大值，但在其他列中具有特定值

2 个答案: