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How do I mimic a SQL Outer Apply in Linq using Entity Framework?

I would like to mimic a SQL OUTER APPLY using linq.
I have 2 tables: Main and Sub
The SQL looks something like this:
select
M.Id, M.Number, M.Stuff, SD.SubName
from
Main as M
outer apply (
select top 1 SubName
from Sub S
where M.Id = S.Id and M.Number = S.Number
) as SD
Based answers here and elsewhere like this one,I've tried too many iterations of Linq to put in here, but here's one:
var query1 =
from m in dbContext.Main
join s in dbContext.Sub on new {m.Id, m.Number} equals new {s.Id, s.Number} into subs
select new
{
m,
SubName = subs.FirstOrDefault().SubName
}
This compiles fine, but when I run it I get this exception:
Processing of the LINQ expression 'DbSet<Main>
// EF's attempt to translate my query
'NavigationExpandingExpressionVisitor' failed. This may indicate either a bug or a limitation in EF Core. See https://go.microsoft.com/fwlink/?linkid=2101433 for more detailed information.
and a stack trace.
Does anyone have any suggestions on how to go about coding this the correct way?
I'm running .NET core 3.1 against SQL Server 2017.

Try the following queries. EF Core 3.1, should translate this to Outer Appply, but higher versions may use JOIN and ROW_NUMBER
var query1 =
from m in dbContext.Main
from s in dbContext.Sub
.Where(s => m.Id == s.Id && m.Number == s.Number)
.Take(1)
.DefaultIfEmpty()
select new
{
m,
SubName = s.SubName
}
Or this variant:
var query1 =
from m in dbContext.Main
select new
{
m,
SubName = dbContext.Sub
.Where(s => m.Id == s.Id && m.Number == s.Number)
.Select(s => s.SubName)
.FirstOrDefaut()
}

Convert C# Entity Framework linq query to SQL

I am converting the C# linq query to SQL, LINQ never return the values.
But the same query I wrote in SQL returns some values. Can anyone help me find out what the issue is for my SQL query?
LINQ query:
var query = from l in _DbContext.Licenses
join lp in _DbContext.LicenseParts on l.PartNumber equals lp.PartNumber
join lpc in _DbContext.LicensePartConfigurations on lp.Id equals lpc.LicensePartId
join p in _DbContext.Products on lp.ProductId equals p.Id
join lsn in _DbContext.LicenseSerialNumbers on l.Id equals lsn.LicenseId
join lact in _DbContext.LicenseActivations on new { a = lsn.Id, b = lp.ProductId } equals new { a = lact.LicenseSerialNumberId, b = lact.ProductId }
where lact.AccountId == AccountId && JsonExtensions.JsonValue(lact.ActivationInfoJSON, "$.SubscriptionKey") !=
" " && (JsonExtensions.JsonValue(lact.ActivationInfoJSON, "$.IsConverted") == null || JsonExtensions.JsonValue(lact.ActivationInfoJSON, "$.IsConverted") == "0" || JsonExtensions.JsonValue(lact.ActivationInfoJSON, "$.IsConverted") == "false") && p.Name == "ClearPass Legacy"
select new SubscriptionKeys { SubscriptionKey = JsonExtensions.JsonValue(lact.ActivationInfoJSON, "$.SubscriptionKey"), CustomerMail = JsonExtensions.JsonValue(lact.ActivationInfoJSON, "$.CustomerMail"), CustomerName = JsonExtensions.JsonValue(lact.ActivationInfoJSON, "$.CustomerName") };
response.PageSize = pageSize;
response.PageNumber = pageNumber;
response.Model = await query.Distinct().ToListAsync();
response.ItemsCount = response.Model.Count();
SQL query:
SELECT
l.AccountId,CustomerMail,
JSON_VALUE(ActivationInfo, '$.SubscriptionKey')
FROM
Licenses l
JOIN
LicenseParts lp ON l.PartNumber = lp.PartNumber
JOIN
LicensePartConfigurations lpc ON lp.Id = lpc.LicensePartId
JOIN
Products p ON lp.ProductId = p.Id
JOIN
LicenseSerialNumbers lsn ON l.Id = lsn.LicenseId
JOIN
LicenseActivations lact ON lsn.Id = lact.LicenseSerialNumberId
AND lp.ProductId = lact.ProductId
WHERE
lact.AccountId = 'QWNjb3VudDoxNTMwNDAzMi00MWM2LTExZTktOWYzMy1kMzQxZjE5OWZlYjM='
AND JSON_VALUE(lact.ActivationInfo, '$.SubscriptionKey') != ' '
AND (JSON_VALUE(lact.ActivationInfo, '$.IsConverted') = NULL OR
JSON_VALUE(lact.ActivationInfo, '$.IsConverted') = 0 OR
JSON_VALUE(lact.ActivationInfo, '$.IsConverted') = 'false')
AND p.Name = 'ClearPass Legacy'

To start from a valid point, execute the code where it fires this linq query and use sql profiler to catch it up. That is a good way to find the exact equivalent sql statement it finally produces and executes. You need to set up a trace to sql profiler prior to execute the linq. Get the statement and then you can compare with the sql you already have.
this sql:
(JSON_VALUE(lact.ActivationInfo, '$.IsConverted') = NULL
is not equal to:
(JsonExtensions.JsonValue(lact.ActivationInfoJSON, "$.IsConverted") == null
as in first case you compare to database NULL value using '=' and this requires ansi_nulls off to work properly and it is not a good practice.

ORMs like EF Core are meant to Map Objects to Relational constructs. Instead of trying to write SQL in C# through LINQ, you should try to Map the attributes you need to entity properties.
In this case the SubscriptionKey and IsConverted fields should appear in the table itself, either as proper fields or computed columns. If that's not possible, you could use computed columns to map the SubscriptionKey and IsConverted attributes to entity properties so you can use them in queries.
In your LicenseActivation class add these properties:
public bool? IsConverted {get;set;}
public string? SubscriptionKey {get;set;}
public string? CustomerEmail {get;set;}
In your DbContext, you can specify computed columns with HasComputedColumnSql:
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
modelBuilder.Entity<LicenseActivation>()
.Property(b => b.SubscriptionKey)
.HasComputedColumnSql("JSON_VALUE(ActivationInfo, '$.SubscriptionKey')");
modelBuilder.Entity< LicenseActivations >()
.Property(b => b.IsConverted)
.HasComputedColumnSql("JSON_VALUE(ActivationInfo, '$.IsConverted')");
modelBuilder.Entity< LicenseActivations >()
.Property(b => b.CustomerEmail)
.HasComputedColumnSql("JSON_VALUE(ActivationInfo, '$.CustomerEmail')");
}
This will allow you to use the properties in LINQ queries.
You shouldn't have to use all those JOINs either. It's the ORM's job to generate JOINs from the relations between objects. If you add proper relations between the entities the query could be simplified to :
var binValue='QWNjb3VudDoxNTMwNDAzMi00MWM2LTExZTktOWYzMy1kMzQxZjE5OWZlYjM=';
var query=_dbContext.LicenseActivations
.Where(lact=>
lact.AccountId == binValue
&& (lact.IsConverted==null || !lact.IsConverted))
.Select(lact=>new {
lact.AccountId,
lact.SubscriptionKey,
lact.CustomerEmail});
or, if the AccountId fields don't hold the same data :
.Select(lact=>new {
AccountId =lact.LicensePart.License.AccountId,
lact.SubscriptionKey,
lact.CustomerEmail
});
EF Core will generate the appropriate SQL and JOIN clauses to get from LicenseActivation to License based on the relations between the entities

Write query in Entity Framework

I have a query which I ran successfully in SQL Server Management Studio, which returns the table values shown in the screenshot
The query I used is:
SELECT tcoid, COUNT(*) ownleasetank
FROM TankProfile
WHERE ownleasetank = 3
GROUP BY tcoid
Now I'm using Entity Framework to make things easier in my sample project.
I used this method to return the table values as array object:
public async Task<Object> GetLeaseInformationPrincipal()
{
ISOTMSEntities context = new ISOTMSEntities();
var testleaseinfo = from d in context.TankProfiles
join f in context.TankOperators
on d.tcoid equals f.tcoId
where (d.ownleasetank == 3)
select new { f.tcoName, d.ownleasetank } into x
group x by new { x.tcoName } into g
select new
{
tconame = g.Key.tcoName,
ownleasetank = g.Select(x => x.ownleasetank).Count()
};
return testleaseinfo.ToList();
}
but it is not working properly. I also tried other ways, when I use where and groupby method in Entity Framework it doesn't working properly for me.
Does anybody know the solution for this?

It's very simple with LINQ methods:
context.TankProfiles
.Where(t => t.ownleasetank = 3)
.GroupBy(t => t.tcoid)
.Select(g => new {g.Key, g.Count()})
.ToArray();
I have no idea why in your C# version of the query you have such opeartions such join, while your SQL query is very simple. You have to rethink that :)

var c = from t in context.TankProfile
where t.ownleasetank == 3
group t by t.tcoid into g
select new { tcoid=g.Key, ownleasetank=g.Select(x => x.ownleasetank).Count() };
return c.ToList();

Linq to Sql - How to get data from second level table

I'm newbie to linq to sql, just trying to understand what type of queries I can handle with linq.
Here is my database scheme,
I want to get all customers of a specific user and this is what I've done,
var userId = 4;
var companies = from c in db.Company
where c.UserId == userId
select c.Id;
var costumers = from c in db.Customers
where companies.Contains(c.CompanyId)
select c;
I'm just wondering whether it's a nice approach and is there any better method to handle this type of queries?

Use can also get customers by this way also:
var result = db.Customers.Join(
db.Company, customer => customer.CompanyId, comp => comp.Id, (customer, comp)
=> new { customer, comp }).Where(#t => #t.comp.UserId == 4)
.Select(#t => #t.customer);

You can also keep it simple like this.
select * from db.Customers Cus
inner join db.company Com on Com.Id = Cus.CompanyId
where Com.UserId= userId

Contains is the equivalent of IN in SQL and your Linq statement will be translated to a SQL statement. So I can't really see another way that will give you better performance with Linq. If you want to use less code you can maybe try the following instead:
var companies = db.Companies.Where(x=> x.UserId == userid).Select(x=>x.Id);
var customers = db.Customers.Where(x=> companies.Contains(x.CompanyId));

How do I create multiple joins using LINQ extension methods?

I'm having trouble using LINQ method calls with multiple joins. I'm trying to do something like this:
if (!isDepSelect)
{
query = (from Items in db.DEPARTMENTs
select Items);
}
else
{
query = (from Items in db.DEPARTMENTs
from gDept in db.DEPT_PROFILE
from wAccess in db.WEB_ACCESS
where Items.DEPT_CODE == gDept.DEPT_CODE && gDept.USER_ID == wAccess.USER_ID && wAccess.EMP_ID == id
select Items);
}
I had done this:
IQueryable<DEPARTMENT> query = db.DEPARTMENTs;
if (isDepSelect)
{
query = query.Join(db.DEPT_PROFILE,depts => depts.DEPT_CODE,prof => prof.DEPT_CODE,(depts, prof) => depts);
}
But now I don't know how to add the JOIN of DEPT_PROFILE table with the WEB_ACCESS table and the condition of the EMP_ID = id.
The reason I'm doing this is that the isDepSelect boolean is not the only condition that this query will change its relations and I need someway to add this relations without repeating my LINQ for each of my conditions.
Thank you for your time.

Try with,
List<DEPARTMENTs> list = db.DEPARTMENTs.Join(db.DEPT_PROFILE, dept => dept.DEPT_CODE, prof => prof.DEPT_CODE, (dept,prof) => new {dept, prof})
.Join(Wdb.WEB_ACCESS, depts => depts.prof.USER_ID,web => web.USER_ID,(depts,web) => new { depts, web})
.Where(result => result.web.EMP_ID== id).Select(s => s.depts.dept).ToList<DEPARTMENTs>();

If you have your associations setup, you can do this without any joins in your code at all:
query = db.DEPARTMENTs
.Any(item => item.DEPT_PROFILEs
.Any(gDept => gDept.WEB_ACCESSs
.Any(wAccess => wAccess.EMP_ID == id)));
Of course this is assuming a 1-m relationship between each of the objects in the graph. You can eliminate some of the Any methods if there are 1-0..1 relationships in the graph as necessary.

you should use the equals operator...
query = from Items in db.DEPARTMENTs
from gDept in db.DEPT_PROFILE
join wAccess in db.WEB_ACCESS on
gDept.DEPT_CODE equals Items.DEPT_CODE
select Items;
thats just a snippet of your example query, but you can see how i am using the join operator to introduce a 2nd table and the equals operator to declare the joining columns.

This should work:
query = (from Items in db.DEPARTMENTs
join gDept in db.DEPT_PROFILE
on Items.DEPT_CODE equals gDept.DEPT_CODE
join wAccess in db.WEB_ACCESS
on gDept.USER_ID equals wAccess.USER_ID
where wAccess.EMP_ID == id
select Items);