Question

我有一个多表查询，如下所示：

SELECT tree.name, tree.type, branch.name, branch.type, leaf.name, leaf.type 
FROM tree, branch, leaf 
WHERE leaf.branch_id = branch.id 
   AND branch.tree_id = tree.id
ORDER by field(tree.type, 'Poplar', 'Birch', 'Hazelnut')

因此，这为我提供了属于三个树条目中的任何一个的所有叶条目。

现在，我真的只想按照指定的顺序返回属于一棵树的叶条目。

因此，如果存在属于Poplar树的叶条目，则仅显示这些条目。但是，如果没有Poplar叶条目，则只显示属于Birch树的叶条目。

可能有任意数量的叶子条目。我只想要在我的优先级列表中出现的树上的那些。理想情况下只使用一个查询。

有什么想法吗？提前谢谢....

Answer 1

试试这个：

SELECT * FROM
(
    SELECT tree.name, tree.type, branch.name, branch.type, leaf.name, leaf.type, 
           IF(@var_type = '' OR a.type = @var_type, a.type := @var_type, 0) AS check_flag
    FROM tree, branch, leaf, (SELECT @var_type := '')
    WHERE leaf.branch_id = branch.id
          AND branch.tree_id = tree.id
    ORDER by field(tree.type, 'Poplar', 'Birch', 'Hazelnut')
) a
WHERE check_flag <> 0;

Answer 2

我建议使用http://dev.mysql.com/doc/refman/5.0/en/string-functions.html#function_find-in-set

你可以这样做

    SELECT * FROM (
    SELECT find_in_set(tree.type, "Poplar,Birch,Hazelnut") as sequence, tree.name, tree.type,    branch.name, branch.type, leaf.name, leaf.type 
    FROM tree, branch, leaf 
    WHERE leaf.branch_id = branch.id 
       AND branch.tree_id = tree.id
    ORDER by sequence
    ) WHERE sequence = 1

Answer 3

这是一种方法。此查询首先为每个叶子查找适当的树类型。然后它将这些信息加入：

select tree.name, tree.type, branch.name, branch.type, leaf.name, leaf.type
from (SELECT leaf.id,
             min(case when tree.type = 'Poplar' then tree.id end) as poplarid,
             min(case when tree.type = 'Birch' then tree.id end) as birchid,
             min(case when tree.type = 'Hazelnut' then tree.id end) as hazelid
      FROM leaf join
           branch
           on leaf.branch_id = branch.id join
           tree
           on branch.tree_id = tree.id join
      group by leaf.id
     ) lt join
     leaf
     on leaf.leaf_id = lt.leaf_id join
     branch
     on leaf.branch_id = branch.id join
     tree
     on tree.id = branch.tree_id
where tree.id = coalesce(lt.poplarid, lt.birchid, lt.hazelid)
ORDER by field(tree.type, 'Poplar', 'Birch', 'Hazelnut')

SQL - 具有优先级过滤器的SELECT查询

3 个答案: