why does these queries generate different sql? - linq

Question

I have a problem with a linq query in Entity framework. I am querying on a field on a navigation property. The problem is that the generated sql is less than optimal. The example below is simplified, actually I am trying to pass an expression tree, and that is why the second query with the let binding is not a sufficient solution even though the produced sql is what I want.
So to summarize I have two questions:
Why is the generated sql different? And is there any way to produce a sql query which will not create a join for every criteria with expression trees?
Update: I realize that I had Include("Securities") on the first query and not the second when I first posted the question, but it does not change the way the criterias are applied, only which columns are selected.
var qry = db.Positions
.Where(criteria)
.ToList();
var qry1 = (from p in db.Positions
where p.Security.Country == "NO" || p.Security.Country == "US" || p.Security.Country == "GB"
select p).ToList();
var qry2 = (from p in db.Positions
let s = p.Security
where s.Country == "NO" || s.Country == "US" || s.Country == "GB"
select p).ToList();
--qry1
SELECT
[Extent1].* --All columns from tblPositions
FROM [dbo].[tblPositions] AS [Extent1]
LEFT OUTER JOIN [dbo].[tblSecurities] AS [Extent2] ON ([Extent2].[SecurityType] IN (1,2..)) AND ([Extent1].[Security] = [Extent2].[SecuritySeq])
LEFT OUTER JOIN [dbo].[tblSecurities] AS [Extent3] ON ([Extent3].[SecurityType] IN (1,2..)) AND ([Extent1].[Security] = [Extent3].[SecuritySeq])
LEFT OUTER JOIN [dbo].[tblSecurities] AS [Extent4] ON ([Extent4].[SecurityType] IN (1,2..)) AND ([Extent1].[Security] = [Extent4].[SecuritySeq])
LEFT OUTER JOIN [dbo].[tblSecurities] AS [Extent5] ON ([Extent5].[SecurityType] IN (1,2..)) AND ([Extent1].[Security] = [Extent5].[SecuritySeq])
WHERE [Extent2].[Country] = 'NO' OR [Extent3].[Country] = 'US' OR [Extent4].[Country] = 'GB'
--qry2
SELECT
[Extent1].*
FROM [dbo].[tblPositions] AS [Extent1]
LEFT OUTER JOIN [dbo].[tblSecurities] AS [Extent2] ON ([Extent2].[SecurityType] IN (1,2..)) AND ([Extent1].[Security] = [Extent2].[SecuritySeq])
WHERE [Extent2].[Country] IN ('NO','US','GB')

Answer 1

In your first query
var qry1 = (from p in db.Positions.Include("Security")
where p.Security.Country == "NO"
|| p.Security.Country == "US"
|| p.Security.Country == "GB"
select p).ToList();
It seems you assume that Entity Framework has magical powers and can deduce that each of the or statements in the generated expression tree are on the same object, and that it can combine them into a contains. It is, by far, not that smart.
Additionally, the rows returned by both queries are not the same. The second one would also need an include, to include the rows from security (if needed). Otherwise you could remove them from the first query (include only means return rows, it has nothing to do with the ability of the where clause to filter rows).
Or to make it more object oriented and readable.
var allowedCountries = new List<string>() { "NO, "US", "GB" };
var qry1 = (from p in db.Positions
// I'm not sure if this is exactly correct
where p.Security.Country in allowedCountries
select p).ToList();
or Lambda (I'm much more familiar with)
var qry1 = db.Positions
.Where(p => allowedCountries.Contains(p.Security.Country))
.ToList();