Python SQL-两个左连接

letsdothis • 5 年前 • 1652 次点击

我有一些关于SQL for Python的问题,希望您能帮助我-我正在尝试从wordpress/woomerce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")

我得到以下错误:

    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

我做错什么了?

提前谢谢。

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1652 次点击

文章 [ 2 ] | 最新文章 5 年前

• 1 楼

Jerry M. 5 年前

你的 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")

也许值得一读 SQL语言 Joins 和 WHERE 声明。

• 2 楼

forpas 5 年前

在GROUPBY之前应该只有一个WHERE子句。
但是由于您使用左连接,因此在 正确的 桌子状 t2.meta_key = '_billing_first_name' 因为拒绝不匹配的行,所以会得到一个内部联接。
因此,请在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")

尽管这个查询在语法上可能对MySql是正确的,但是使用GROUP BY是没有意义的,因为您不做任何聚合。

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