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Pivot table in MySQL - convert to pivot table with values in varchar format

Tags:

sql

mysql

pivot

i would like convert data from Table1 as you can see on first picture on data in Pivot_table. Is possible to do that in MySQL? Because the values of pivot table(A, B, C, D) are in varchar data format and i cannot use any aggregation function of MySQL like SUM or others.

Table1:
PK        Name     Subject     Grade
-------------------------------------
1         Bob       Math        A
2         Bob       History     B
3         Bob       Language    C
4         Bob       Biology     D
5         Sue       History     C
6         Sue       Math        A
7         Sue       Music       A
8         Sue       Geography   C


Pivot_table:
Subject     Bob     Sue
-------------------------
Math        A        A
History     B        C
Language    C 
Biology     D
Music                A
Geography            C

Thanks for your help

like image 879
Bushwacka Avatar asked Sep 05 '13 06:09

Bushwacka


1 Answers

A static query (as far as only Bob and Sue are concerned) might look like this

SELECT subject, 
       MAX(CASE WHEN name = 'Bob' THEN grade END) `Bob`,
       MAX(CASE WHEN name = 'Sue' THEN grade END) `Sue` 
  FROM table1 
 GROUP BY subject

Now to be able to account for other names use dynamic SQL like this

SET @sql = NULL;

SELECT GROUP_CONCAT(DISTINCT
         CONCAT('MAX(CASE WHEN name = ''', name,
                ''' THEN grade END) `', name, '`'))
  INTO @sql
  FROM table1;

SET @sql = CONCAT('SELECT subject, ', @sql, ' 
                     FROM table1 
                    GROUP BY subject');

PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;

Output:

|   SUBJECT |    BOB |    SUE |
|-----------|--------|--------|
|   Biology |      D | (null) |
| Geography | (null) |      C |
|   History |      B |      C |
|  Language |      C | (null) |
|      Math |      A |      A |
|     Music | (null) |      A |

Here is SQLFiddle demo


You can wrap it into a stored procedure to simplify things on the calling end

DELIMITER $$
CREATE PROCEDURE sp_grade_report()
BEGIN
  SET @sql = NULL;

  SELECT GROUP_CONCAT(DISTINCT
           CONCAT('MAX(CASE WHEN name = ''', name,
                  ''' THEN grade END) `', name, '`'))
    INTO @sql
    FROM table1;

  SET @sql = CONCAT('SELECT subject, ', @sql, ' 
                       FROM table1 
                      GROUP BY subject');

  PREPARE stmt FROM @sql;
  EXECUTE stmt;
  DEALLOCATE PREPARE stmt;
END$$
DELIMITER ;

Sample usage:

CALL sp_grade_report();

Here is SQLFiddle demo

like image 57
peterm Avatar answered Sep 22 '22 02:09

peterm