Select only a single column in LINQ
89,047
Solution 1
//var Country = Countries.FirstOrDefault(o => o.Id == 100000581);
var personnelIds = context.Personnels
.Where(p => p.Country.Id == 100000581)
.Select(p => p.Id)
.ToArray();
personnelIds.Dump();
Try this, it should be better.
Solution 2
Personnels collection will be populated via lazy loading when accessed, hence retrieving all of the fields from the DB. Here's what's happening...
// retrieves data and builds the single Country entity (if not null result)
var Country = Countries.FirstOrDefault(o => o.Id == 100000581);
// Country.Personnels accessor will lazy load and construct all Personnel entity objects related to this country entity object
// hence loading all of the fields
var personnelIds = Country.Personnels.Select(p => p.Id).ToArray();
You want something more like this:
// build base query projecting desired data
var personnelIdsQuery = dbContext.Countries
.Where( c => c.Id == 100000581 )
.Select( c => new
{
CountryId = c.Id,
PersonnelIds = c.Personnels.Select( p => p.Id )
}
// now do enumeration
// your example shows FirstOrDefault without OrderBy
// either use SingleOrDefault or specify an OrderBy prior to using FirstOrDefaul
var result = personnelIdsQuery.OrderBy( item => item.CountryId ).FirstOrDefault();
OR:
var result = personnelIdsQuery.SingleOrDefault();
Then get the array of IDs if not null
if( null != result )
{
var personnelIds = result.PersonnelIds;
}
Solution 3
Try can also try grouping personnel into a single query
var groups =
(from p in Personnel
group p by p.CountryId into g
select new
{
CountryId = g.Key
PersonnelIds = p.Select(x => x.Id)
});
var personnelIds = groups.FirstOrDefault(g => g.Key == 100000581);
Author by
Stef Heyenrath
Updated on March 29, 2020Comments
-
Stef Heyenrath about 4 years
The EntityModel is defined as: Personnel has a link to a Country
When executing this code in LinqPad, I see that the SQL which is generated is not optimized (all fields are returned) in the first query ? What am I missing here or doing wrong ?
Query 1 LINQ
var Country = Countries.FirstOrDefault(o => o.Id == 100000581); var personnelIds = Country.Personnels.Select(p => p.Id).ToArray(); personnelIds.Dump();
Query 1 SQL
exec sp_executesql N'SELECT [t0].[Id], [t0].[Version], [t0].[Identifier], [t0].[Name], , [t0].[UpdatedBy] FROM [Personnel] AS [t0] WHERE [t0].[Country_Id] = @p0',N'@p0 bigint',@p0=100000581
Query 2 LINQ
var Country = Countries.FirstOrDefault(o => o.Id == 100000581); var personnelIds2 = Personnels.Where(p => p.Country == Country).Select(p => p.Id).ToArray(); personnelIds2.Dump();
Query 2 SQL
exec sp_executesql N'SELECT [t0].[Id] FROM [Personnel] AS [t0] WHERE [t0].[Country_Id] = @p0',N'@p0 bigint',@p0=100000581
The database used is SQL Express 2008. And LinqPad version is 4.43.06