mysql - 具有多个连接的查询将仅返回我的左连接之一

Question

我在查询时遇到问题，问题是我从同一个表中连接了三次。

有问题的查询是这样的:

SELECT
      o.order_id, 
      n.title, 
      c.first_name,
      tdv5.tid,
      tdv6.name,
      tdv8.name AS settlement_month
 FROM orders o
      join products p           on o.product_id = p.nid
      join node n               on p.nid = n.nid
      join customers c          on o.customer_email = c.customer_email
      join term_node tn         on tn.nid = p.nid
      join term_data tdv6       on tn.tid = tdv6.tid AND tdv6.vid = 6
      left join term_data tdv5  on tn.tid = tdv5.tid AND tdv5.vid = 5
      left join term_data tdv8  on tn.tid = tdv8.tid AND tdv8.vid = 8

当我使用此查询时，只有使用 tn.tid 的第一个 JOIN 才有效。例如，如果最后三个的唯一连接是这样的:

join term_data tdv6       on tn.tid = tdv6.tid AND tdv6.vid = 6

它会工作得很好。与

相同

left join term_data tdv5  on tn.tid = tdv5.tid AND tdv5.vid = 5

和

left join term_data tdv8  on tn.tid = tdv8.tid AND tdv8.vid = 8

但是当我将它们组合起来时，它们根本不起作用。当我组合三个查询时，只有 tdv6 有效。

您对问题有什么建议吗？谢谢。

Answer 1

如果您尝试放置 WHERE 子句，如下所示:

SELECT
      o.order_id, 
      n.title, 
      c.first_name,
      tdv5.tid,
      tdv6.name,
      tdv8.name AS settlement_month
 FROM orders o
      join products p           on o.product_id = p.nid
      join node n               on p.nid = n.nid
      join customers c          on o.customer_email = c.customer_email
      join term_node tn         on tn.nid = p.nid
      join term_data tdv6       on tn.tid = tdv6.tid
      left join term_data tdv5  on tn.tid = tdv5.tid
      left join term_data tdv8  on tn.tid = tdv8.tid
WHERE
      tdv6.vid = 6 and
      tdv5.vid = 5 and
      tdv8.vid = 8