Python SQL - 两个左连接

Question

我在 Python 的 SQL 方面遇到了一些问题，我希望你能帮助我 - 我正在尝试从 wordpress/woocommerce 检索一些数据。

我的代码:

    cursor.execute("
    SELECT t1.ID, t1.post_date, t2.meta_value AS first_name, t3.meta_value AS last_name
    FROM test_posts t1 
    LEFT JOIN test_postmeta t2 
    ON t1.ID = t2.post_id 
    WHERE t2.meta_key = '_billing_first_name' and t2.post_id = t1.ID 
    LEFT JOIN test_postmeta t3 
    ON t1.ID = t3.post_id 
    WHERE t3.meta_key = '_billing_last_name' and t3.post_id = t1.ID 
    GROUP BY t1.ID 
    ORDER BY t1.post_date DESC LIMIT 20")

我收到以下错误:

    mysql.connector.errors.ProgrammingError: 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'LEFT JOIN test_postmeta t3 ON t1.ID = t3.post_id WHERE t3.meta_key = '_billing' at line 1

我做错了什么？

提前致谢。

Answer 1

在 GROUP BY 之前应该只有 1 个 WHERE 子句。
但是由于您使用 LEFT 联接，在 right 表上设置一个条件，例如 t2.meta_key = '_billing_first_name'，您得到的是 INNER 联接，因为您拒绝了不匹配的行。
所以在 ON 子句中设置所有条件:

cursor.execute("
SELECT t1.ID, t1.post_date, t2.meta_value AS first_name, t3.meta_value AS last_name
FROM test_posts t1 
LEFT JOIN test_postmeta t2 
ON t1.ID = t2.post_id AND t2.meta_key = '_billing_first_name'
LEFT JOIN test_postmeta t3 
ON t1.ID = t3.post_id AND t3.meta_key = '_billing_last_name'
GROUP BY t1.ID 
ORDER BY t1.post_date DESC LIMIT 20")

尽管此查询对于 MySql 在语法上可能是正确的，但使用 GROUP BY 没有意义，因为您没有进行任何聚合。