我如何在mysql中对多个值进行自我联接？

Question

具体来说，我想基于表的键/值类型中的N个元值查询新闻发布。

我只是找不到一个有效的查询示例。

用简单的英语来说，查询就是这样。

选择所有城市=达拉斯，风格=牧场，价格在100k和200k之间，pool = true的帖子

我可能需要比较一些或更少的元值。

对于非wordpress用户，元值位于一个表中，每个元数据都与posts表中的post ID相关联。

Answer 1

啊，EAV的乐趣。 简而言之，您需要执行多个子查询或一个交叉表。

Select ...
From Posts
Where post_id In    (
                    Select post_id
                    From Meta
                    Where Attribute = 'City'
                        And Value = 'Dallas'
                    )
    And post_id In  (
                    Select post_id
                    From Meta
                    Where Attribute = 'Style'
                        And Value = 'Ranch'
                    )
    And post_id In  (
                    Select post_id
                    From Meta
                    Where Attribute = 'Price'
                        And Cast(Value As int) Between 100000 And 200000
                    )
    And post_id In  (
                    Select post_id
                    From Meta
                    Where Attribute = 'Pool'
                        And Value = 'True'
                    )   

这是构建交叉表的另一种形式。 它更紧凑，但性能可能不佳：

Select ...
From Posts As P
    Join    (
            Select post_id
                , Min( Case When Attribute = 'City' Then Value End ) As City
                , Min( Case When Attribute = 'Style' Then Value End ) As Style
                , Min( Case When Attribute = 'Price' Then Cast(Value As int) End ) As Price
                , Min( Case When Attribute = 'Pool' Then Value End ) As Pool
            From Meta
            Where Attribute In('City','Style','Price','Pool')
            Group By post_id
            ) As Attributes
        On Attributes.post_id = P.post_id
Where Attributes.City = 'Dallas'
    And Attributes.Style = 'Ranch'
    And Attributes.Price Between 100000 And 200000
    And Attributes.Pool = 'True'