我编写了两个结果相同的查询，在运行它们时，它们的性能是相同的。虽然我很好奇效率是不是两者都一样，因为我无法想象它们是一样的。
我觉得这个效率比较低：

SELECT post.ID, 
    post.post_title ticket_id, 
    (SELECT meta_value 
        FROM wp_postmeta post_meta
        WHERE post_meta.post_id = post.ID 
            && post_meta.meta_key = "WooCommerceEventsPurchaserFirstName") first_name,
    (SELECT meta_value 
        FROM wp_postmeta post_meta
        WHERE post_meta.post_id = post.ID 
            && post_meta.meta_key = "WooCommerceEventsPurchaserLastName") last_name,
    (SELECT meta_value 
        FROM wp_postmeta post_meta
        WHERE post_meta.post_id = post.ID 
            && post_meta.meta_key = "WooCommerceEventsPurchaserEmail") email,
    (SELECT meta_value 
        FROM wp_postmeta post_meta
        WHERE post_meta.post_id = post.ID 
            && post_meta.meta_key = "WooCommerceEventsPurchaserPhone") phone,
    (SELECT meta_value 
        FROM wp_postmeta post_meta
        WHERE post_meta.post_id = post.ID 
            && post_meta.meta_key = "WooCommerceEventsProductID") product_id,
    (SELECT meta_value 
        FROM wp_postmeta post_meta
        WHERE post_meta.post_id = post.ID 
            && post_meta.meta_key = "WooCommerceEventsVariations") ticket_type,
    (SELECT meta_value 
        FROM wp_postmeta 
        WHERE post_id = product_id
            && meta_key = "_sku") SKU
FROM wp_posts post 
WHERE post_type = "event_magic_tickets"

因为我做的（select）查询较少，但我做的连接比select重？

SELECT post.ID ID, 
         post.post_title ticket_id,
         customers.*,
         (SELECT meta_value 
            FROM wp_postmeta 
            WHERE post_id = product_id
            && meta_key = "_sku") SKU
FROM wp_posts post 
LEFT JOIN (SELECT post_id id,
    MAX(CASE WHEN meta_key = 'WooCommerceEventsPurchaserFirstName' THEN meta_value END) first_name,
    MAX(CASE WHEN meta_key = 'WooCommerceEventsPurchaserLastName' THEN meta_value END) last_name,
    MAX(CASE WHEN meta_key = 'WooCommerceEventsPurchaserEmail' THEN meta_value END) email,
    MAX(CASE WHEN meta_key = 'WooCommerceEventsPurchaserPhone' THEN meta_value END) phone,
    MAX(CASE WHEN meta_key = 'WooCommerceEventsProductID' THEN meta_value END) product_id,
    MAX(CASE WHEN meta_key = 'WooCommerceEventsVariations' THEN meta_value END) ticket_type
    FROM wp_postmeta post_meta
    GROUP BY post_id) customers
    ON customers.id = post.id
WHERE post_type = "event_magic_tickets";

有人能告诉我哪一个更有效，为什么(也许我该如何衡量这一点，这样我将来就能自己做了）。我用mariadb以防万一。。