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C# LINQ INNER JOIN is returning wrong count

I have the following sql with JOIN but which returns a record count for 50 but when I convert it to LINQ, I am not getting the matching count. I noticed that when I add the ON clause, the visual studio intellisense dropdown does not show the ID property for the 2nd table. I am wondering if that is an issue.
Here is the simple SQL
SELECT * FROM Table1 T1 JOIN Table2 T2 ON T1.MyId = T2.MyId WHERE T1.IsCompleted
Here is my lamba LINQ with the comment where the VC Intellisense is not working right. For table2, the intellisense dropdown, it is only showing Equals, GetHashCode, GetType, and ToString. Just manually type the MyId and everything successfully builds but the count is too high. Thanks
var test = this.myDbContent.Table1
.Join (this.myDbContent.Table2
table1 => table1.MyId,
table2 => table2.MyId,
(table1, table2) => new { table1, table2}
)
.Where (joinedTable => joinedTable.Table1.IsCompleted == 1)

Join is the simple LINQ operation, but better understandable via LINQ Query.
var query =
from t1 in myDbContent.Table1
join t2 in myDbContent.table2 on t1.MyId equals t2.MyId
where t1.IsCompleted
select new
{
t1,
t2
};
If you still have different results, probably MyId is nullable field and EF also matches nulls.
It can be disabled via options configuration:
builder.UseSqlServer(ceConnection, x => x.UseRelationalNulls(true));

Ef core not ignoring columns in generated SQL

I am using EF for .NET Core
I have a table which I dont want to bring back all the columns for in a query
(from ae in Table1.Select(x => new {x.CreatedDate, x.EntryId, x.Amount})
join si in Table2.Select(x => new {x.SessionId, x.IdToFind})
.Where(y => y.SessionId == Guid.Parse("52F0C862-15D0-4C7A-975D-285C618342B0")) on ae.EntryId equals si.IdToFind
select new
{
create output
}).ToList();
As you can see, I am specifically only including 2 columns from Table2
SELECT [a].[EntryId], [a].[Amount]
FROM [Table1] AS [a]
INNER JOIN (
SELECT [s].[Id], [s].[CreatedBy], [s].[CreatedDate], [s].[IdToFind], [s].[SessionId], [s].[UpdatedDate], [s].[UpdateddBy]
FROM [Table2] AS [s]
WHERE [s].[SessionId] = '52f0c862-15d0-4c7a-975d-285c618342b0'
) AS [t] ON [a].[EntryId] = [t].[IdToFind]
When I use LinqPad to evaluate the generated SQL, I can see that all the columns from Table 2 have been include
How can I prevent this? I dont want to use the Ignore option on the model loading because in other situations I may want the other columns

You have to use this syntax:
ResultClass result =(from ae in Table1
join si in Table2
on ae.EntryId equals si.IdToFind
where(si.SessionId == Guid.Parse("52F0C862-15D0-4C7A-975D-285C618342B0"))
select new ResultClass
{
ae.CreatedDate,
ae.EntryId,
ae.Amount,
si.SessionId,
si.IdToFind
}).ToList();
But in this case you will need to create a new class that contains properties of both classes or you get an anonymos object that you have to use immediately.
When I like to avoid creating a new class I usually add some [NotMapped] properties to one of Table1 or Table2 classes in a partial class code and use this class instead of the ResultClass

Your approach should work, however mixing the Linq query syntax and Fluent methods isn't making it very readable at all and I suspect that may be responsible for the unexpected query generation. Particularly this:
(from ae in Table1.Select(x => new {x.CreatedDate, x.EntryId, x.Amount})
join si in Table2.Select(x => new {x.SessionId, x.IdToFind}).Where(y => y.SessionId == Guid.Parse("52F0C862-15D0-4C7A-975D-285C618342B0"))
Which has the Where condition on the Table2 Select.
From what I can deduce from your approach, you are being too eager with your optimization:
With EF queries, Select projections do not need to be done at a per-entity level, they are done at the end of the expression and EF will work out exactly what columns from what table need to be included. That said, you can pre-select columns and EF should work it all out, but it will do so from your final projection.
Provided you have an actual FK between Table1.EntryId and Table2.IdToFind, ideally you would want that relationship mapped out in the entities rather than relying on an explicit join. Navigation properties make it much easier to organize queries, but there can be more generic cases where Table2 can serve multiple other tables etc.
Assuming that you want to just build an output from the CreatedDate, EntryId, Amount, and SessionId from these two loosely related tables:
var results = context.Table1
.Join(context.Table2.Where(t2 => t2.SessionId == sessionId),
t1 => t1.EntryId,
t2 => t2.IdToFind,
(t1, t2) => new { Table1 = t1, Table2 = t2 })
.Select(x => new ResultViewModel
{
EntryId = x.Table1.EntryId,
CreatedDate = x.Table1.CreatedDate,
Amount = x.Table1.Amount,
// ...
}).ToList();
Assuming you don't even need anything from Table2 then:
// ...
.Join(context.Table2.Where(t2 => t2.SessionId == sessionId),
t1 => t1.EntryId,
t2 => t2.IdToFind,
(t1, t2) => t1)
.Select(x => new ResultViewModel
{
EntryId = x.EntryId,
CreatedDate = x.CreatedDate,
Amount = x.Amount
}).ToList();
When you read the Linq it may seem like EF would be selecting all columns from the related tables so you'd want to add Select projections there, but they are not needed. The final query should select columns strictly by the final Select projection. I would avoid mixing Linq query language (from Table1 join Table2 select ....) with the Fluent builder methods. IMO the Fluent methods are easier to read from a C# code perspective, but ultimately I'd use one or the other, not both.

Does Entity Framework query the database multiple times if I use different fields of the same Linq query at different times?

I tried the Internet and the SOF but couldn't locate a helpful resource. Perhaps I may not be using correct wording to search. If there are any previous questions I have missed due to this reason please let me know and I will take this question down.
I am dealing with a busy database so I am required to send less queries to the database.
If I access different columns of the same Linq query from different levels of the code then is Entity Framework smart enough to foresee the required columns and bring them all or does it call the db twice?
eg.
var query = from t1 in table_1
join t2 in table_2 on t1.col1 equals t2.col1
where t1.EmployeeId == EmployeeId
group new { t1, t2 } by t1.col2 into grouped
orderby grouped.Count() descending
select new { Column1 = grouped.Key, Column2 = grouped.Sum(g=>g.t2.col4) };
var records = query.Take(10);
// point x
var x = records.Select(a => a.Column1).ToArray();
var y = records.Select(a => a.Column2).ToArray();
Does EF generate query the database twice to faciliate x and y (send a query first to get Column1, and then send another to get Column2) or is it smart enough to know it needs both Columns to be materialised and bring them both at point x?
Added to clarify the intention of the question:
I understand I can simply add a greedy method to the end of query.Take(10) and get it done but I am trying to understand if the approach I try (and in my opinion, more elegant) does work of if not what makes EF to make two queries please.

Yes currently your code will generate 2 queries that will be executed to the database. Reason being is because you have 2 different sqls generated:
First is the top query, taking only 10 records and then only Column1
Second is the top query, taking only 10 records and then only Column2
The reason these are 2 queries is because you have a ToArray over different Select statements -> generating different sql. Most of linq queries are differed executed and will be executed only when you use something like ToArray()/ToList()/FirstOrDefault() and so on - those that actually give you the concrete data. In your original query you have 2 different ToArray on data that has not yet been retrieved - meaning 2 queries (once for the first field and then for the second).
The following code will result in a single query to the database
var records = (from t1 in table_1
join t2 in table_2 on t1.col1 equals t2.col1
where t1.EmployeeId == EmployeeId
group new { t1, t2 } by t1.col2 into grouped
orderby grouped.Count() descending
select new { Column1 = grouped.Key, Column2 = grouped.Sum(g=>g.t2.col4) })
.Take(10).ToList();
var x = records.Select(a => a.Column1).ToArray();
var y = records.Select(a => a.Column2).ToArray();
In my solution above I added a ToList() after filtering out only that data you need (Take(10)) and then at that point it will execute to the database. Then you have all the data in memory and you can do any other linq operation over it without it going again to the database.
Add to your code ToString() so you can check the generated sql at different points. Then you will understand when and what is being executed:
var query = from t1 in table_1
join t2 in table_2 on t1.col1 equals t2.col1
where t1.EmployeeId == EmployeeId
group new { t1, t2 } by t1.col2 into grouped
orderby grouped.Count() descending
select new { Column1 = grouped.Key, Column2 = grouped.Sum(g=>g.t2.col4) };
var generatedSql = query.ToString(); // Here you will see a query that brings all records
var records = query.Take(10);
generatedSql = query.ToString(); // Here you will see it taking only 10 records
// point x
var xQuery = records.Select(a => a.Column1);
generatedSql = xQuery.ToString(); // Here you will see only 1 column in query
// Still nothing has been executed to DB at this point
var x = xQuery.ToArray(); // And that is what will be executed here
// Now you are before second execution
var yQuery = records.Select(a => a.Column2);
generatedSql = yQuery.ToString(); // Here you will see only the second column in query
// Finally, second execution, now with the other column
var y = yQuery.ToArray();

When you are running linq statement on an entity in EF if only prepares the Select statement (thats why the type is IQueryable). The data is loaded lazily. When you try to use a value from that query then only the result gets evaluated using a enumerator.
So when you turn it to a collection (.toList() etc.) explicitly it tries to get data to populate the list and hence the sql command is fired.
It is designed so to enhance the performance. So if a particular property of an entity is to be used EF doesn't get the value for all the columns from that table

Linq To Entities, Select multiple sets of data with one query

I've found plenty of info on how to select multiple result sets with stored procedures but nothing substantial on how to do so with a linq query.
For example, I can do sub-queries that return mulitple sets of results with something like
var query = (from school in context.Schools
where school.id == someId
select new
{
subSetA = (from student in context.Students
select student).ToList(),
subSetB = (from building in context.Buildings
select building).ToList(),
}).First();
query.subSetA; //Access subSetA
query.subSetB; //Access subSetB
Which works fine, but what if I just want to select both subSetA and subSetB without querying against the school table? I want to select two separate sets of data that gets sent to the server in one query.
Any information as to how to do this with EF 6 would be great.

Well, I'm sure there are many ways to do this, but if you want to avoid introducing a third DbSet into the mix...
var query = (from s in context.Students.Take(1)
select new
{
subSetA = context.Students.ToList(),
subSetB = context.Buildings.ToList(),
})
Then, you can use query.ToList() or maybe using query.Load() and working with context.Students.Local, etc. would work.

Most efficient way to get MAX value of joined table in LINQ query using C#

I am trying to find the most efficient way to get the most recent record in a joined table in LINQ.
This query may handle thousands of records so I didn't want to perform a subquery.
I need everything from items, but only the most recent date from the "Notes" table, whose field name is SubmittedDate.
var items = (from t1 in db.Items
from t2
in db.Notes
.Where(o => (t1.Item_ID == o.Item_ID))
select new ItemModel
{
Name = t1.Name,
MostRecentUpdate = t2.SubmittedDate <== Need max value in model
});
Any help would be greatly appreciated.

It looks like you probably just want a group join:
var items = from t1 in db.Items
join t2 in db.Notes on t1.Item_ID equals t2.Item_ID into notes
select new ItemModel
{
Name = t1.Name,
MostRecentUpdate = notes.Max(x => (DateTime?) x.SubmittedDate)
};
Now MostRecentUpdate should be null if there are no non-null dates in the matching Notes rows. At least, that's what the LINQ to Objects behaviour would be, so fingers crossed the abstraction holds...