Dynamic pivot query using MySQL - mysql

Question

I have the following table with the details:
Example:
CREATE TABLE Table1
(`PK` int, `Name` varchar(3), `Subject` varchar(9), `Grade` varchar(1));
INSERT INTO Table1
(`PK`, `Name`, `Subject`, `Grade`)
VALUES
(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');
Now I want to write a store procedure through which I want to pivot the table.
Attempt
DELIMITER $$
create PROCEDURE sptest1(IN nm varchar(50),IN sub varchar(50))
begin
SET #sql = NULL;
SELECT GROUP_CONCAT(DISTINCT
CONCAT('MAX(CASE WHEN ', nm ,' = ''', nm,
''' THEN grade END) `', nm, '`'))
INTO #sql
FROM table1;
SET #sql = CONCAT('SELECT ', sub ,',', #sql, '
FROM table1
GROUP BY ', sub ,'');
select #sql;
PREPARE stmt FROM #sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
END$$
DELIMITER ;
Calling Function:
call sptest1('name','subject')
But getting wrong result:
subject name
----------------
Biology null
Geography null
History null
Language null
Math null
Music null
Expected Result:
subject Bob Sue
-------------------------
Biology D null
Geography null C
History B C
Language C null
Math A A
Music null A

Answer 1

You can rewrite you dynamic sql query as
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;
Demo
In your stored procedure you have to hard code your name column for you dynamic sql if you pass it as parameter then you can use case to check for column, for example
MAX(CASE WHEN ', nm ,' = ''', nm, ..
Above part of procedure will evaluated as MAX(CASE WHEN name = 'name' .. because variable nm contains 'name' it will not recognize it as column
DROP PROCEDURE `sptest1`;
DELIMITER $$
CREATE
PROCEDURE `sib`.`sptest1`(IN nm VARCHAR(50),IN sub VARCHAR(50))
BEGIN
SET #sql = NULL;
SELECT GROUP_CONCAT( DISTINCT
CONCAT('MAX(CASE WHEN ',nm,' = ''',CASE WHEN nm = 'name' THEN `Name` END,''' THEN grade END) ',CASE WHEN nm = 'name' THEN `Name` END)
)
INTO #sql
FROM table1;
SET #sql = CONCAT('SELECT ', sub ,',', #sql, ' FROM table1 GROUP BY ', sub ,'');
/*select #sql; */
PREPARE stmt FROM #sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
END$$
DELIMITER ;
Demo