Question

如何在同一张桌子上进行双联接？

基本上，我想这样做：

1 - Sort all records in descending order by a order_date_last_revised column. 2 - Group those records by order_base_id column to eliminate duplicates. 3 - Match the id of those records and return all columns for them.

这是我的SQL，它不起作用：

SELECT * FROM sys_quote_master q1 LEFT JOIN sys_quote_master q2 ON q1.order_id = q2.order_id group by q2.order_base_id LEFT JOIN sys_quote_master q3 ON q2.order_id = q3.order_id WHERE order_base_id="010313-6063" order by q3.order_date_last_revised desc;

基本上，当我尝试执行它时，我得到了这个：

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 sys_quote_master q2 ON q1.order_id = q2.order_id order by q3.order_dat' at line 4

Answer 1

如果放置了group by，它就会出现：它必须出现在表之后和where子句之后（如果有的话）：

SELECT *
FROM sys_quote_master q1 
LEFT JOIN sys_quote_master q2 ON q1.order_id = q2.order_id 
LEFT JOIN sys_quote_master q3 ON q2.order_id = q3.order_id
WHERE order_base_id="010313-6063"
group by q2.order_base_id 
order by q3.order_date_last_revised desc;

但请注意，按现状划分的小组不会有用;您必须列出非聚合列的所有列才能按预期运行。

也许分组不是你想要的 - 考虑使用distinct模仿重复：

select distinct *
from ...
-- and remove group by clause

Answer 2

这个怎么样？

SELECT * FROM sys_quote_master AS g1 JOIN (SELECT order_id, order_base_id, max(order_date_last_revised) as mostrecent FROM sys_quote_master WHERE order_base_id="010313-6063" group by order_base_id) AS g2 ON g2.mostrecent = g1.order_date_last_revised ORDER BY g1.order_id;

如何在同一个表上执行此双连接

2 个答案: