根据特定列值将 SQL 列拆分为多列

Question

我想编写一个查询，列出我们在我的大学提供的课程。一个项目至少包括一个专业，可能还有一个 "option"、一个 "specialty" 和一个 "subspecialty"。这四个要素中的每一个都详细说明了将它们与专业相关联的代码。

一个专业可以有零个或多个选项，一个选项可以有零个或多个专业，一个专业可以有零个或多个子专业。相反，一个专业被允许没有与之相关的选项。

在结果集中，一行必须包含前一个元素才能有下一个元素，即一行将不包含专业、选项和专业。与专业相关的专业的出现意味着还有一个与该专业相关的选项。

我的问题在于数据的存储方式。所有程序数据都在一个 table 中，布局如下：

+----------------+---------------+------+
| program_name   | program_level | code |
+----------------+---------------+------+
| Animal Science | Major         |    1 |
| Equine         | Option        |    1 |
| Dairy          | Option        |    1 |
| CLD            | Major         |    2 |
| Thesis         | Option        |    2 |
| Non-Thesis     | Option        |    2 |
| Development    | Specialty     |    2 |
| General        | Subspecialty  |    2 |
| Rural          | Subspecialty  |    2 |
| Education      | Major         |    3 |
+----------------+---------------+------+

所需的输出将如下所示：

+----------------+-------------+----------------+-------------------+------+
| major_name     | option_name | specialty_name | subspecialty_name | code |
+----------------+-------------+----------------+-------------------+------+
| Animal Science | Equine      |                |                   |    1 |
| Animal Science | Dairy       |                |                   |    1 |
| CLD            | Thesis      | Development    | General           |    2 |
| CLD            | Thesis      | Development    | Rural             |    2 |
| CLD            | Non-Thesis  | Development    | General           |    2 |
| CLD            | Non-Thesis  | Development    | Rural             |    2 |
| Education      |             |                |                   |    3 |
+----------------+-------------+----------------+-------------------+------+

到目前为止，我已尝试创建四个连接此 "code" 的查询，每个查询都基于不同的 "program_level"。但是这些字段没有正确组合。

Answer 1

使用子查询来建立你想要的。

代码：

SELECT(SELECT m.program_name FROM yourtable m WHERE m.program_level = 'Major' AND y.program_name = m.program_name) AS major_name,
      (SELECT o.program_name FROM yourtable o WHERE o.program_level = 'Option' AND y.program_name = o.program_name) AS Option_name,
      (SELECT s.program_name FROM yourtable s WHERE s.program_level = 'Specialty' AND y.program_name = s.program_name) AS Specialty_name,
      (SELECT ss.program_name FROM yourtable ss WHERE ss.program_level = 'Subspecialty' AND y.program_name = ss.program_name) AS Subspecialty_name, code
FROM yourtable y

输出：

major_name      Option_name Specialty_name  Subspecialty_name   code
Animal Science  (null)      (null)          (null)              1
(null)          Equine      (null)          (null)              1
(null)          Dairy       (null)          (null)              1
CLD             (null)      (null)          (null)              2
(null)          Thesis      (null)          (null)              2
(null)          Non-Thesis  (null)          (null)              2
(null)          (null)      Development     (null)              2
(null)          (null)      (null)          General             2
(null)          (null)      (null)          Rural               2
Education       (null)      (null)          (null)              3

SQL Fiddle: http://sqlfiddle.com/#!3/9b75a/2/0

Answer 2

我找不到比这更简单的了:

/* Replace @Programs with the name of your table */

SELECT majors.program_name, options.program_name, 
    specs.program_name, subspecs.program_name, majors.code
FROM @Programs majors
LEFT JOIN @Programs options 
    ON majors.code = options.code AND options.program_level = 'Option'
LEFT JOIN @Programs specs 
    ON options.code = specs.code AND specs.program_level = 'Specialty'
LEFT JOIN @Programs subspecs 
    ON specs.code = subspecs.code AND subspecs.program_level = 'Subspecialty'
WHERE majors.program_level = 'Major'

编辑：更正了拼写错误 "Speciality"，现在应该可以使用了。

Answer 3

此处解决方案的关键是要意识到实际上有多个实体存储在同一个 table 中，并且这些实体需要先 'extracted'。

这可以通过过滤 program_level 列的简单子查询来完成。

select ma.major_name, op.option_name, sp.specialty_name,  ss.subspecialty_name, ma.code
from 
   (select code, program_name  as major_name 
    from programs where program_level = 'Major') ma
left outer join
    (select code, program_name as option_name
     from programs where program_level ='Option') op
     on ma.code = op.code    
left outer join
    (select code, program_name as specialty_name
     from programs where program_level ='Specialty') sp
    on op.code = sp.code
left outer join
    (select code, program_name as subspecialty_name
     from programs where program_level ='Subspecialty') ss
    on sp.code = ss.code
order by ma.code asc, 
      ma.major_name asc,
      op.option_name asc,
      sp.specialty_name asc,
      ss.subspecialty_name asc;

这为您提供了所需的输出：

MAJOR_NAME      OPTION_NAME SPECIALTY_NAME  SUBSPECIALTY_NAME   CODE
Animal Science  Dairy       ?               ?                   1   
Animal Science  Equine      ?               ?                   1   
CLD             Non-Thesis  Development     General             2   
CLD             Non-Thesis  Development     Rural               2   
CLD             Thesis      Development     General             2   
CLD             Thesis      Development     Rural               2   
Education       ?           ?               ?                   3

干杯，拉斯

根据特定列值将 SQL 列拆分为多列

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