如何使用实体框架选择单个列？

有没有一种方法可以使用实体框架4获得一个列的全部内容？与这个 SQL 查询相同:

SELECT Name FROM MyTable WHERE UserId = 1;
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You could use the LINQ select clause and reference the property that relates to your Name column.

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最佳答案

You can use LINQ's .Select() to do that. In your case it would go something like:

string Name = yourDbContext
.MyTable
.Where(u => u.UserId == 1)
.Select(u => u.Name)
.SingleOrDefault(); // This is what actually executes the request and return a response

If you are expecting more than one entry in response, you can use .ToList() instead, to execute the request. Something like this, to get the Name of everyone with age 30:

string[] Names = yourDbContext
.MyTable
.Where(u => u.Age == 30)
.Select(u => u.Name)
.ToList();
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Using LINQ your query should look something like this:

public User GetUser(int userID){


return
(
from p in "MyTable" //(Your Entity Model)
where p.UserID == userID
select p.Name
).SingleOrDefault();


}

Of course to do this you need to have an ADO.Net Entity Model in your solution.

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I'm a complete noob on Entity but this is how I would do it in theory...

var name = yourDbContext.MyTable.Find(1).Name;

If It's A Primary Key.

-- OR --

var name = yourDbContext.MyTable.SingleOrDefault(mytable => mytable.UserId == 1).Name;

-- OR --

For whole Column:

var names = yourDbContext.MyTable
.Where(mytable => mytable.UserId == 1)
.Select(column => column.Name); //You can '.ToList();' this....

But "oh Geez Rick, What do I know..."

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If you're fetching a single item only then, you need use select before your FirstOrDefault()/SingleOrDefault(). And you can use anonymous object of the required properties.

var name = dbContext.MyTable.Select(x => new { x.UserId, x.Name }).FirstOrDefault(x => x.UserId == 1)?.Name;

Above query will be converted to this:

Select Top (1) UserId, Name from MyTable where UserId = 1;

For multiple items you can simply chain Select after Where:

var names = dbContext.MyTable.Where(x => x.UserId > 10).Select(x => x.Name);

Use anonymous object inside Select if you need more than one properties.


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