Mysql Convert Column to row (Pivot table )

Question

I have a table like this

+---+-----+----+----+----+----+
|id |month|col1|col2|col3|col4|
+---+-----+----+----+----+----+
|101|Jan  |A   |B   |NULL|B   |
+---         


        

Answer 1

Although this question is suuuper old and someone marked it as "very common", people still seem to find it (me included) and find it helpfull. I developed a more generalized version for unpivoting a row and thought it might be helpful to someone.

SET @target_schema='schema';
SET @target_table='table';
SET @target_where='`id`=1';
SELECT
    GROUP_CONCAT(qry SEPARATOR ' UNION ALL ')
    INTO @sql
FROM (
    SELECT
        CONCAT('SELECT `id`,', QUOTE(COLUMN_NAME), ' AS `key`,`', COLUMN_NAME, '` AS `value` FROM `', @target_table, '` WHERE ', @target_where) qry
    FROM (
        SELECT `COLUMN_NAME` 
        FROM `INFORMATION_SCHEMA`.`COLUMNS` 
        WHERE `TABLE_SCHEMA`=@target_schema 
            AND `TABLE_NAME`=@target_table
    ) AS `A`
) AS `B`;
PREPARE s FROM @sql; EXECUTE s; DEALLOCATE PREPARE s;

I use this query on a MySQL 8.x server and aggregate it to a JSON object there, hence the id, key, value result structure.

Answer 2

What you need to do is first, unpivot the data and then pivot it. But unfortunately MySQL does not have these functions so you will need to replicate them using a UNION ALL query for the unpivot and an aggregate function with a CASE for the pivot.

The unpivot or UNION ALL piece takes the data from your col1, col2, etc and turns it into multiple rows:

select id, month, col1 value, 'col1' descrip
from yourtable
union all
select id, month, col2 value, 'col2' descrip
from yourtable
union all
select id, month, col3 value, 'col3' descrip
from yourtable
union all
select id, month, col4 value, 'col4' descrip
from yourtable

See SQL Fiddle with Demo.

Result:

|  ID | MONTH |  VALUE | DESCRIP |
----------------------------------
| 101 |   Jan |      A |    col1 |
| 102 |   feb |      C |    col1 |
| 101 |   Jan |      B |    col2 |
| 102 |   feb |      A |    col2 |
| 101 |   Jan | (null) |    col3 |
| 102 |   feb |      G |    col3 |
| 101 |   Jan |      B |    col4 |
| 102 |   feb |      E |    col4 |

You then wrap this in a subquery to apply the aggregate and the CASE to convert this into the format you want:

select descrip, 
  max(case when month = 'jan' then value else 0 end) jan,
  max(case when month = 'feb' then value else 0 end) feb
from
(
  select id, month, col1 value, 'col1' descrip
  from yourtable
  union all
  select id, month, col2 value, 'col2' descrip
  from yourtable
  union all
  select id, month, col3 value, 'col3' descrip
  from yourtable
  union all
  select id, month, col4 value, 'col4' descrip
  from yourtable
) src
group by descrip

See SQL Fiddle with demo

The result is:

| DESCRIP | JAN | FEB |
-----------------------
|    col1 |   A |   C |
|    col2 |   B |   A |
|    col3 |   0 |   G |
|    col4 |   B |   E |