Python SQL - 两个左连接

Question

我对 Python 的 SQL 有一些问题，我希望你能帮助我 - 我正在尝试从 wordpress/woocommerce.[=14= 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

您的 SQL 语法不正确。试试这个：

  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 
    LEFT JOIN test_postmeta t3  ON t1.ID = t3.post_id 
    WHERE t3.meta_key = '_billing_last_name' and t2.meta_key = '_billing_first_name'
    GROUP BY t1.ID 
    ORDER BY t1.post_date DESC LIMIT 20")

关于 SQL Joins and WHERE 语句可能值得一读。

Answer 2

GROUP BY 前应该只有 1 个 WHERE 子句。
但是由于您使用 LEFT 联接，在 right table 上设置一个条件，例如 t2.meta_key = '_billing_first_name' 您会得到一个 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")

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

Python SQL - 两个左连接

Python SQL - two left joins

python

mysql

sql

woocommerce

mysql-connector-python