问题

I'd like to transpose the results of a MySQL query from a per row key => value resultset to a resultset where the key is the column header and the value is a row entry for that column.

i.e. Given the following data

|------------------+-------------|
|  CLASS_LESSON    | ATTENDANTS  |
|------------------+-------------|
|  class1art       | 1           |
|  class1history   | 1           |
|  class2geography | 2           |
|------------------+-------------|

I'd like to transform it to

|------------+---------------+------------------|
|  class1art | class1history | class2geography  |
|------------+---------------+------------------|
|  1         | 1             | 2                |
|------------+---------------+------------------|

Assume that the class/lesson pairs are dynamic; they can be added or removed at any time. I don't want to explicitly call them out as suggested in the typical 'pivot table' sql solution.

select 
  MAX(CASE WHEN class_lesson = 'class1art' THEN attendants ELSE 0 END) AS class1art,
  MAX(CASE WHEN class_lesson = 'class1history' THEN attendants ELSE 0 END) AS class1history,
  MAX(CASE WHEN class_lesson = 'class2geography' THEN attendants ELSE 0 END) AS class2geography,
  MAX(CASE WHEN class_lesson = 'class7art' THEN attendants ELSE 0 END) AS class7art,
  MAX(CASE WHEN class_lesson = 'class7history' THEN attendants ELSE 0 END) AS class7history

from
    (select 
        group_concat(distinct class, lesson) as class_lesson,
            count(*) as attendants
    from
        TableName
    group by class , lesson
    ) a

Here's an SQLFiddle environment with sample data. Is this possible without utilizing stored procedures?

回答1:

Try this

SELECT  
MAX(CASE WHEN t.CLASS_LESSON = 'class1art' THEN t.ATTENDANTS ELSE NULL END) AS class1art,
MAX(CASE WHEN t.CLASS_LESSON = 'class1history' THEN t.ATTENDANTS ELSE NULL END) AS class1history,
MAX(CASE WHEN t.CLASS_LESSON = 'class2geography' THEN t.ATTENDANTS ELSE NULL END) AS class2geography
FROM 
(
 select 
 group_concat(distinct class, lesson) as class_lesson, count(*) as attendants

 from 
 TableName

 group by 
 class, lesson
) as t

FIDDLE DEMO

回答2:

Try this one, Why you want 1 for class2geography it should be 2.

SELECT  
MAX(CASE WHEN t.clsubject = 'class1art' THEN t.attndee ELSE NULL END) AS class1art,
MAX(CASE WHEN t.clsubject = 'class1history' THEN t.attndee ELSE NULL END) AS class1history,
MAX(CASE WHEN t.clsubject = 'class2geography' THEN t.attndee ELSE NULL END) AS class2geography
FROM 
(
SELECT CONCAT(class,lesson) AS clsubject, COUNT(*) AS attndee FROM TableName GROUP BY clsubject 
) t

来源：https://stackoverflow.com/questions/21304446/transpose-the-results-of-a-mysql-query

标签

mysql

sql

transpose

Transpose the results of a MySQL query

问题

回答1:

FIDDLE DEMO

回答2: