为什么我的实体框架创建了多个查询而不是单个UNION查询？(Why my Entity Framework creates several queries instead of a single UNION query?)

 我使用Entity Framework Core 2.0.1。 在我的EF查询中，我需要将几个简单查询组合到一个UNION查询中。 所以，对于一个非常基本的例子，我有几个类似的查询 
 
using (var dbc = new ItemsDbContext("some-connection-string"))
{
    var q1 = dbc.Items.Where(a => a.ItemType == "A");
    var q2 = dbc.Items.Where(a => a.ItemType == "B");

    var myList = q1.Union(q2).ToList();
}
 
 我希望这个SQL语句作为单个查询对我的SQL Server数据库执行： 
 
SELECT I.*
    FROM [dbo].[Items]
    WHERE I.ItemType = N'A'
UNION
SELECT I.*
    FROM [dbo].[Items]
    WHERE I.ItemType = N'B'
 
 但是，正如我在SQL Server Profiler中看到的那样，执行了两个单独的查询： 
 
 查询1： 
 
SELECT I.*
    FROM [dbo].[Items]
    WHERE I.ItemType = N'A'
 
 查询2： 
 
SELECT I.*
    FROM [dbo].[Items]
    WHERE I.ItemType = N'B'
 
 这两个单独查询的结果稍后由实体框架联合。 但它是在调用代码中完成的，而不是在SQL Server端。 
 
 如何让Entity Framework Core生成UNION而不是多个查询？ 

I use Entity Framework Core 2.0.1. In my EF query I need to combine several simple queries into a single UNION query. So, for a very basic example, I have a couple of similar queries
 
using (var dbc = new ItemsDbContext("some-connection-string"))
{
    var q1 = dbc.Items.Where(a => a.ItemType == "A");
    var q2 = dbc.Items.Where(a => a.ItemType == "B");

    var myList = q1.Union(q2).ToList();
}
 
I would like such SQL statement to be executed, as a single query, against my SQL Server database:
 
SELECT I.*
    FROM [dbo].[Items]
    WHERE I.ItemType = N'A'
UNION
SELECT I.*
    FROM [dbo].[Items]
    WHERE I.ItemType = N'B'
 
However, as much as I see in SQL Server Profiler, two separate queries are executed:
 
Query 1:
 
SELECT I.*
    FROM [dbo].[Items]
    WHERE I.ItemType = N'A'
 
Query 2:
 
SELECT I.*
    FROM [dbo].[Items]
    WHERE I.ItemType = N'B'
 
Result of this two separate queries is later union'ed by Entity Framework. But it is done in the calling code, not on SQL Server's side.
 
How can I make Entity Framework Core generate UNION instead of several queries?

原文：https://stackoverflow.com/questions/49597645