SQL:transforma filas en columnas en MySQL (sentencia SELECT)

Puede usar variables para este tipo de pivote, pero la consulta es un poco más complicada, porque necesita enumerar los valores para cada pedido:

SELECT o.order_id,
       MAX(case when rank = 1 then created_at end) as created_at_1,
       MAX(case when rank = 1 then email end) as email_1,
       MAX(case when rank = 1 then content end) as content_1,
       MAX(case when rank = 2 then created_at end) as created_at_2,
       MAX(case when rank = 2 then email end) as email_2,
       MAX(case when rank = 2 then content end) as content_2,
FROM orders o LEFT JOIN
     (SELECT oc.*,
             (@rn := if(@o = order_fk, @rn + 1,
                        if(@o := order_fk, 1, 1)
                       )
             ) as rank
      FROM order_comments oc CROSS JOIN
           (SELECT @rn := 0, @o := 0) vars
      ORDER BY order_fk, created_at
     ) oc
     ON o.order_id = oc.order_fk
GROUP BY o.order_id;