SQL JOIN與2個條件在同一張表上

Question

我需要將2列合並為1行。 列之間的鍵是post_id 。

這是我的桌子（postmeta）：

post_id     meta_key    meta_value
36          cp_state    California
37          cp_state    Illinois
38          cp_state    California
39          cp_state    California
40          cp_state    California
36          cp_city     Los Angeles
37          cp_city     Chicago
38          cp_city     San Diego
39          cp_city     San Diego
40          cp_city     Los Angeles

這是即時通訊試圖輸出的內容：

post_id     state       city
36          California  Los Angeles
37          Illinois    Chicago
38          California  San Diego
39          California  San Diego
40          California  Los Angeles

這是我使用的SQL，但得到了瘋狂的結果：

SELECT a.post_id AS pID, a.meta_value AS state, b.meta_value AS city
FROM $wpdb->prefix"."postmeta AS a
LEFT JOIN $wpdb->prefix"."postmeta AS b
ON b.post_id = a.post_id
WHERE a.meta_key = 'cp_state'
AND b.meta_key = 'cp_city'

在使用的實時Wordpress表中，顯然還有數百個其他meta_key ，因此我需要它僅針對這2個meta_key （ cp_city ， cp_state ）進行過濾

Answer 1

您應該使用INNER JOIN而不是LEFT JOIN

SELECT a.post_id AS pID, a.meta_value AS state, b.meta_value AS city
FROM       $wpdb->prefix"."postmeta AS a
INNER JOIN $wpdb->prefix"."postmeta AS b
ON b.post_id = a.post_id
WHERE a.meta_key = 'cp_state'
AND b.meta_key = 'cp_city'