Pivot queries

Remarks#

Pivot query creation in MySQL relies upon the GROUP_CONCAT() function. If the result of the expression that creates the columns of the pivot query is expected to be large, the value of the group_concat_max_len variable must be increased:

set session group_concat_max_len = 1024 * 1024; -- This should be enough for most cases

Creating a pivot query

MySQL does not provide a built-in way to create pivot queries. However, these can be created using prepared statements.

Assume the table tbl_values:

Request: Create a query that shows the sum of Value for each Name; the Group must be column header and Name must be the row header.

-- 1. Create an expression that builds the columns
set @sql = (
    select group_concat(distinct 
        concat(
            "sum(case when `Group`='", Group, "' then `Value` end) as `", `Group`, "`"
        )
    ) 
    from tbl_values
);

-- 2. Complete the SQL instruction
set @sql = concat("select Name, ", @sql, " from tbl_values group by `Name`");

-- 3. Create a prepared statement
prepare stmt from @sql;

-- 4. Execute the prepared statement
execute stmt;

Result:

Important: Deallocate the prepared statement once it’s no longer needed:

deallocate prepare stmt;

Example on SQL Fiddle