CODEWITHSUNDEEP
mysqljoinsyntaxpivot-table
ejsuncy
ejsuncy

Reputation: 449

turning table rows into columns in mysql

Say I have a database similar to the following:

Table student_info:

id    name
111   jon
112   dan
113   david
...

and table scores:

item_id    student_id    score
01         111           37
02         111           45
01         112           55
02         112           44
01         113           66
02         113           45
...

Is it possible to do a mysql query to generate the following table?:

Student_Name  ITEM_1_SCORE  ITEM_2_SCORE
jon           37            45
dan           55            44
david         66            45
...

If so, what would the syntax be? I don't know if this is a join operation or something else?

thanks.

Upvotes: 1

Views: 3089

Answers (2)

John Woo
John Woo

Reputation: 263833

If you have only two values for item_id, then it is fine to hard code values. Example

SELECT  a.Name AS Student_Name,
        MAX(CASE WHEN item_id = '01' THEN b.score END) Item_1_Score,
        MAX(CASE WHEN item_id = '02' THEN b.score END) Item_2_Score
FROM    student_info a
        LEFT JOIN scores b
            ON a.id = b.student_ID
GROUP   BY a.Name

Otherwise, when you have unknow number of scores, a Dynamic SQL is much prefered.

SELECT  GROUP_CONCAT(DISTINCT
        CONCAT('MAX(CASE WHEN item_id = ''',
               item_id,
               ''' THEN Score END) AS ',
               CONCAT('`Item_', item_id, '_Score`')
               )) INTO @sql
FROM scores;

SET @sql = CONCAT('SELECT   a.Name AS Student_Name, ', @sql, ' 
                    FROM    student_info a
                            LEFT JOIN scores b
                                ON a.id = b.student_ID
                    GROUP   BY a.Name');

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

Both queries will OUTPUT the same

╔══════════════╦══════════════╦══════════════╗
║ STUDENT_NAME ║ ITEM_1_SCORE ║ ITEM_2_SCORE ║
╠══════════════╬══════════════╬══════════════╣
║ dan          ║           55 ║           44 ║
║ david        ║           66 ║           45 ║
║ jon          ║           37 ║           45 ║
╚══════════════╩══════════════╩══════════════╝

Upvotes: 7

Mahmoud Gamal
Mahmoud Gamal

Reputation: 79969

Like this:

SELECT
  i.name AS Student_Name,
  MAX(CASE WHEN s.item_id = 1 THEN score END) AS ITEM_1_SCORE, 
  MAX(CASE WHEN s.item_id = 2 THEN score END) AS ITEM_2_SCORE
FROM student_info AS i
INNER JOIN scores AS s ON s.student_id = i.id
GROUP BY i.name;

See it in action here:

This will give you:

| STUDENT_NAME | ITEM_1_SCORE | ITEM_2_SCORE |
----------------------------------------------
|          dan |           55 |           44 |
|        david |           66 |           45 |
|          jon |           37 |           45 |

Upvotes: 4

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