Question

我想使用sql查询来显示“佛罗里达”大学的“学费”和“费用”。

我使用下面的代码先查询学费，然后再结合查询第二笔费用。但是，我没有得到预期的结果。

所需的结果如下：

>    School      |     Tuition     |   Fees
> --------------------------------------------
>   School 1     |      2000       |    50
>   School 2     |      1000       |    80

但是我得到了以下结果：

>    School      |     Tuition     |   Fees
> --------------------------------------------
>   School 1     |      2000       |    50
>   School 1     |      1000       |    50
>   School 2     |      2000       |    80
>   School 2     |      1000       |    80

我对WordPress数据库使用了以下查询。 “学费”和“费用”的所有值都包含在名为“ entity_field_number”的表中。这些值在名为“值”的列中。学杂费由名为“ field_name”的列来区分。 “ entity_id”是分配给学校的ID号。

表“ entity_field_number”

>    entity_id   |     field_name     |   value
> --------------------------------------------
>        1       |     tuition       |    2000
>        2       |     tuition       |    1000
>        1       |      fees         |    50
>        2       |      fees         |    80

下面是我使用的查询。

SELECT *
    FROM
       (SELECT field_number.`value` as tuition,
           posts.`post_title` as school
    FROM field_number
      INNER JOIN posts
         ON posts.`post_type` = 'school_dir'
         AND field_number.`entity_id` = posts.`ID`
      INNER JOIN field_location
         ON field_location.`province` LIKE '%florida%'
         AND posts.`ID` = field_location.`entity_id`
         AND field_number.`field_name` = 'tuition') tuition,

       (SELECT field_number.`value` as fees,
           posts.`post_title` as school
    FROM field_number
      INNER JOIN posts
         ON posts.`post_type` = 'school_dir'
         AND field_number.`entity_id` = posts.`ID`
      INNER JOIN field_location
         ON field_location.`province` LIKE '%florida%'
         AND posts.`ID` = field_location.`entity_id`
         AND field_number.`field_name` = 'fees') fees

Answer 1

您所做的可以这样解释...

SELECT
   *
FROM
   (<sql>) tuition,
   (<sql>) fees

,是CROSS JOIN。所以你做到了...

SELECT
   *
FROM
   (<sql>) tuition
CROSS JOIN
   (<sql>) fees

CROSS JOIN从两个表（笛卡尔积）中获取行的每种组合。

您真正想要的是INNER JOIN ...

SELECT
   *
FROM
   (<sql>) tuition
INNER JOIN
   (<sql>) fees
        ON tuition.school = fees.school

现在，只有同一个学校的行才被加入。

但是，更好的方法是对查询进行重做以两次在field_number表上进行联接。为此，您需要每次（以避免歧义）。

SELECT
    posts.`post_title` as school,
    fees.`value`       as fees,
    tuition.`value`    as tuition
FROM
    field_location
INNER JOIN
    posts
        ON  posts.`ID` = field_location.`entity_id`
INNER JOIN
    field_number   AS fees
        ON  fees.`entity_id`  = posts.`ID`
        AND fees.`field_name` = 'fees'
INNER JOIN
    field_number   AS tuition
        ON  tuition.`entity_id`  = posts.`ID`
        AND tuition.`field_name` = 'tuition'
WHERE
        field_location.`province` LIKE '%florida%'
    AND posts.`post_type` = 'school_dir'

将多个结果合并为列

1 个答案: