MySQL LEFT JOIN with Parent ID in an if statement

I'm attempting to perform a different join for fields if the post_parent of an ID is other than a 0.

An Example: A field has a parent id of 1234, I intend to query the wp_postmeta table by the parent ID, not the post ID for the intended fields because the parent ID is not a 0. ( if this makes sense )

I have attempted to achieve what I intend for the first Billing Address 1 field, but I'm getting a syntax issue with the "THEN" usage in the IF statement, please see below

SELECT 
  wp_posts.ID, 
  wp_posts.post_title, 
  pm1.meta_value as 'Billing Address 1', 
  pm2.meta_value as 'Billing Address 2', 
  pm3.meta_value as 'Billing City',
  pm4.meta_value as 'Billing State',
  pm5.meta_value as 'Billing Postcode',
  pm6.meta_value as 'Billing Email',
  pm7.meta_value as 'Billing Phone',
  CONCAT_WS(' ',pm9.meta_value,pm8.meta_value) as 'Billing Full Name',
  pm10.meta_value as 'Shipping First Name',
  pm11.meta_value as 'Shipping Last Name',
  pm12.meta_value as 'Shipping Address 1',
  pm13.meta_value as 'Shipping Address 2',
  pm14.meta_value as 'Shipping City',
  pm15.meta_value as 'Shipping State',
  pm16.meta_value as 'Shipping Postcode',
  pm17.meta_value as 'Shipping Cost',
  pm18.meta_value as 'Order Subtotal',
  pm19.meta_value as 'Discount',
  pm20.meta_value as 'Refund Total'
FROM wp_posts 
  LEFT JOIN wp_postmeta AS pm1  ON IF(pm1.post_parent = 0 THEN wp_posts.ID = pm1.post_id AND pm1.meta_key='_billing_address_1' ELSE wp_posts.post_parent = pm1.post_id AND pm1.meta_key='_billing_address_1')
  LEFT JOIN wp_postmeta AS pm2  ON (wp_posts.ID = pm2.post_id  AND pm2.meta_key='_billing_address_2') 
  LEFT JOIN wp_postmeta AS pm3  ON (wp_posts.ID = pm3.post_id  AND pm3.meta_key='_billing_city') 
  LEFT JOIN wp_postmeta AS pm4  ON (wp_posts.ID = pm4.post_id  AND pm4.meta_key='_billing_state') 
  LEFT JOIN wp_postmeta AS pm5  ON (wp_posts.ID = pm5.post_id  AND pm5.meta_key='_billing_postcode') 
  LEFT JOIN wp_postmeta AS pm6  ON (wp_posts.ID = pm6.post_id  AND pm6.meta_key='_billing_email') 
  LEFT JOIN wp_postmeta AS pm7  ON (wp_posts.ID = pm7.post_id  AND pm7.meta_key='_billing_phone') 
  LEFT JOIN wp_postmeta AS pm8  ON (wp_posts.ID = pm8.post_id  AND pm8.meta_key='_billing_last_name')
  LEFT JOIN wp_postmeta AS pm9  ON (wp_posts.ID = pm9.post_id  AND pm8.meta_key='_billing_first_name')
  LEFT JOIN wp_postmeta AS pm10 ON (wp_posts.ID = pm10.post_id AND pm10.meta_key='_shipping_first_name') 
  LEFT JOIN wp_postmeta AS pm11 ON (wp_posts.ID = pm11.post_id AND pm11.meta_key='_shipping_last_name') 
  LEFT JOIN wp_postmeta AS pm12 ON (wp_posts.ID = pm12.post_id AND pm12.meta_key='_shipping_address_1') 
  LEFT JOIN wp_postmeta AS pm13 ON (wp_posts.ID = pm13.post_id AND pm13.meta_key='_shipping_address_2') 
  LEFT JOIN wp_postmeta AS pm14 ON (wp_posts.ID = pm14.post_id AND pm14.meta_key='_shipping_city') 
  LEFT JOIN wp_postmeta AS pm15 ON (wp_posts.ID = pm15.post_id AND pm15.meta_key='_shipping_state') 
  LEFT JOIN wp_postmeta AS pm16 ON (wp_posts.ID = pm16.post_id AND pm16.meta_key='_shipping_postcode') 
  LEFT JOIN wp_postmeta AS pm17 ON (wp_posts.ID = pm17.post_id AND pm17.meta_key='_order_shipping') 
  LEFT JOIN wp_postmeta AS pm18 ON (wp_posts.ID = pm18.post_id AND pm18.meta_key='_order_total')
  LEFT JOIN wp_postmeta AS pm19 ON (wp_posts.ID = pm19.post_id AND pm19.meta_key='_cart_discount')
  LEFT JOIN wp_postmeta AS pm20 ON (wp_posts.ID = pm20.post_id AND pm20.meta_key='_refund_amount')
WHERE 
  #wp_posts.post_type   = 'shop_order' AND 
  wp_posts.post_status = 'wc-completed' AND 
  wp_posts.post_date > '2016-08-01 13:13:00' AND 
  wp_posts.post_date < '2017-05-31 13:13:00'
GROUP BY wp_posts.ID ORDER BY wp_posts.post_date DESC

Thank you