How can I simplify the following SQL query?

Question

I have a query that does everything I need it to, but it feels fragile and definitely really really complicated and I'd love to simplify it. I think it's complicated because of the need to pivot rows to columns for meta data.

SELECT
  O.order_item_id,
  O.order_item_name,
  MAX(CASE WHEN OIM.meta_key = 'class' THEN OIM.meta_value END) AS 'class',
  MAX(CASE WHEN OIM.meta_key = 'fee-type' THEN OIM.meta_value END) AS 'fees',
  MAX(CASE WHEN OIM.meta_key = 'enrolment_id' THEN (
    SELECT firstname FROM wp_enrolments E WHERE OIM.meta_value=E.id) END) as 'name',
  MAX(CASE WHEN OIM.meta_key = 'enrolment_id' THEN (
    SELECT surname FROM wp_enrolments E WHERE OIM.meta_value=E.id) END) as 'surname',
  MAX(CASE WHEN OIM.meta_key = 'enrolment_Id' THEN (
    SELECT UM.meta_value
    FROM wp_usermeta UM, wp_enrolments E
    WHERE UM.meta_key='_user_phone'
    AND UM.user_id=E.memberid
    AND E.memberid=OIM.meta_value
    AND OIM.meta_key='enrolment_id' LIMIT 1) END) as 'phone',
  MAX(CASE WHEN OIM.meta_key = 'enrolment_id' THEN (
    SELECT U.user_email
    FROM wp_users U, wp_enrolments E
    WHERE U.id=E.memberid
    AND E.memberid=OIM.meta_value
    AND OIM.meta_key='enrolment_id' LIMIT 1) END) as 'email',
FROM wp_woocommerce_order_items O
INNER JOIN wp_woocommerce_order_itemmeta OIM ON O.order_item_id = OIM.order_item_id
GROUP BY O.order_item_id

Answer 1

Consider LEFT JOIN of all the tables of subqueries, then use those columns in CASE statements. NB: Without data this is untested and may require adjustment, but can help you re-factor the SQL query. Two joins of wp_enrolments were needed due to different join fields ( id and memberid ):

SELECT
  O.order_item_id,
  O.order_item_name,
  MAX(CASE WHEN OIM.meta_key = 'class' THEN OIM.meta_value END) AS `class`,
  MAX(CASE WHEN OIM.meta_key = 'fee-type' THEN OIM.meta_value END) AS `fees`,
  MAX(CASE WHEN OIM.meta_key = 'enrolment_id' THEN idE.firstname END) as `name`,
  MAX(CASE WHEN OIM.meta_key = 'enrolment_id' THEN idE.surname END) as `surname`,
  MAX(CASE WHEN OIM.meta_key = 'enrolment_Id' AND UM.meta_key='_user_phone' THEN UM.meta_value END) as `phone`,
  MAX(CASE WHEN OIM.meta_key = 'enrolment_id' THEN U.user_email END) as `email`

FROM wp_woocommerce_order_items O
INNER JOIN wp_woocommerce_order_itemmeta OIM 
  ON O.order_item_id = OIM.order_item_id
LEFT JOIN wp_enrolments idE
  ON OIM.meta_value = idE.id
LEFT JOIN wp_enrolments memE
  ON OIM.meta_value = memE.memberid
  AND OIM.meta_key = 'enrolment_id'
LEFT JOIN wp_users U
  ON U.id = memE.memberid
LEFT JOIN wp_usermeta UM
  ON UM.user_id = memE.memberid

GROUP BY O.order_item_id,
         O.order_item_name