如何在一个 sql 表中加入所有内容，并从另一个表中第一次出现，保留所有来自第一个表

Question

以下 SQLite (android Room) 查询：

（格林威治标准时间 2021.01.03 07:43 更新：我增强了 Q）

select  o.*, gi.* 
from orders as o
left join galleryitems as gi on gi.siteId = o.siteId 
order by orderId asc

这些给定的表格：

orders: unique key is orderid

o.orderId | siteId      | col1      | col2
----------+-------------+-----------+----------
456       | 1001        | o.456c1r1 | o.456c2r1 
457       | 1002        | o.457c1r1 | o.457c2r1
458       | 1003        | o.458c1r1 | o.458c2r1
459       | 1004        | o.459c1r1 | o.459c2r1
460       | 1005        | o.460c1r1 | o.460c2r1
461       | 1006        | o.461c1r1 | o.461c2r1
462       | 1007        | o.462c1r1 | o.462c2r1

galleryitems: unique key is galleryId, reference to ordertable is gi.siteId

gi.galleryID | gi.col1     | gi.col2     | gi.siteId 
-------------+-------------+-------------+----------
0001         | gi1001.c1r1 | gi1001.c2r1 | 1001      
0002         | gi1001.c1r2 | gi1001.c2r2 | 1001      
0003         | gi1002.c1r1 | gi1002.c1r1 | 1002
0004         | gi1003.c1r1 | gi1003.c1r1 | 1005
0005         | gi1003.c1r2 | gi1003.c1r2 | 1005

给出这个结果：

o.orderId | o.siteId | col1      | col2      | gi.galleryID | gi.col1     | gi.col2     | gi.siteId 
----------+----------+-----------+-----------+--------------+-------------+-------------+----------
456       | 1001     | o.456c1r1 | o.456c2r1 | 0001         | gi1001.c1r1 | gi1001.c2r1 | 1001   <-- "duplicates"   
456       | 1001     | o.456c1r1 | o.456c2r1 | 0002         | gi1001.c1r2 | gi1001.c2r2 | 1001   <-- "duplicates"
457       | 1002     | o.457c1r1 | o.457c2r1 | 0003         | gi1002.c1r1 | gi1002.c1r1 | 1002
458       | 1003     | o.458c1r1 | o.458c2r1 | <null>       | <null>      | <null>      | <null>
459       | 1004     | o.459c1r1 | o.459c2r1 | <null>       | <null>      | <null>      | <null>
460       | 1005     | o.460c1r1 | o.460c2r1 | 0004         | gi1003.c1r1 | gi1003.c1r1 | 1005   <-- "duplicates"
460       | 1005     | o.460c1r1 | o.460c2r1 | 0005         | gi1003.c1r2 | gi1003.c1r2 | 1005   <-- "duplicates"
461       | 1006     | o.461c1r1 | o.461c2r1 | <null>       | <null>      | <null>      | <null>
462       | 1007     | o.462c1r1 | o.462c2r1 | <null>       | <null>      | <null>      | <null>

正如您在下面看到的，我希望来自gi 的数据仅在它们出现时出现，但只在第一次出现时出现，因此它们不会“重复”（如果找到 gi.siteid 只需填写 inn 数据（首先出现是可以的，甚至可以计算，如果在加入期间可能的话），否则 gi 字段应该是 <null> ）：

o.orderId | o.siteId | col1      | col2      | gi.galleryID | gi.col1     | gi.col2     | gi.siteId 
----------+----------+-----------+-----------+--------------+-------------+-------------+----------
456       | 1001     | o.456c1r1 | o.456c2r1 | 0001         | gi1001.c1r1 | gi1001.c2r1 | 1001   <-- no "duplicates"   
457       | 1002     | o.457c1r1 | o.457c2r1 | 0003         | gi1002.c1r1 | gi1002.c1r1 | 1002
458       | 1003     | o.458c1r1 | o.458c2r1 | <null>       | <null>      | <null>      | <null>
459       | 1004     | o.459c1r1 | o.459c2r1 | <null>       | <null>      | <null>      | <null>
460       | 1005     | o.460c1r1 | o.460c2r1 | 0004         | gi1003.c1r1 | gi1003.c1r1 | 1005   <-- no "duplicates"
461       | 1006     | o.461c1r1 | o.461c2r1 | <null>       | <null>      | <null>      | <null>
462       | 1007     | o.462c1r1 | o.462c2r1 | <null>       | <null>      | <null>      | <null>

我看到有很多建议，但它们无法处理 SQLite 受限库。

有什么建议吗？

使用 androidx 房间。

Answer 1

您可以通过聚合来获取最小值为 galleryitems.row1 的行，方法是使用 SQLite 的一个不是标准 SQL 但它是 documented 的功能：

select  min(gi.row1) as row1, gi.row2, gi.siteId, o.* 
from orders as o left join galleryitems as gi 
on gi.siteId = o.siteId 
group by o.siteId
order by orderId asc 

或者使用galleryitems的rowid来定义第一次出现：

select  gi.*, o.* 
from orders as o left join galleryitems as gi 
on gi.siteId = o.siteId 
group by o.siteId
having min(gi.rowid) or min(gi.rowid) is null
order by orderId asc

请参阅demo。

Answer 2

@forpas 将我纠正为宽阔的道路，我什至可以对其进行改进。谢谢！

select count(gi.siteId) as cc, gi.*, o.*
from orders as o
left join galleryitems as gi on gi.siteId = o.siteId
group by o.siteId 

这个结果现在变成这个结果：（注意!! group by o.siteId not gi.siteId）

cc |o.orderId | o.siteId | col1      | col2      | gi.galleryID | gi.col1     | gi.col2     | gi.siteId 
---+----------+----------+-----------+-----------+--------------+-------------+-------------+----------
2  |456       | 1001     | o.456c1r1 | o.456c2r1 | 0001         | gi1001.c1r1 | gi1001.c2r1 | 1001   <-- no "duplicates"   
1  |457       | 1002     | o.457c1r1 | o.457c2r1 | 0003         | gi1002.c1r1 | gi1002.c1r1 | 1002
0  |458       | 1003     | o.458c1r1 | o.458c2r1 | <null>       | <null>      | <null>      | <null>
0  |459       | 1004     | o.459c1r1 | o.459c2r1 | <null>       | <null>      | <null>      | <null>
2  |460       | 1005     | o.460c1r1 | o.460c2r1 | 0004         | gi1003.c1r1 | gi1003.c1r1 | 1005   <-- no "duplicates"
0  |461       | 1006     | o.461c1r1 | o.461c2r1 | <null>       | <null>      | <null>      | <null>
0  |462       | 1007     | o.462c1r1 | o.462c2r1 | <null>       | <null>      | <null>      | <null>