>LINQ Query Operators – Part I

>

Orders.OrderBy(o => o.OrderDate).Skip(5)

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, “Courier New”, courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

Before checking the operators lets remind some points.

  • Query operators are extension methods
  • Some Linq operations are only available with operators (or to say it otherwise "method syntax")
  • Syntax includes the source itself which need not be specified (because implemented as static extension method on IEnumerable, IOW source). Typical syntax looks like:

    public static IEnumerable<TSource> Where<TSource>( this IEnumerable<TSource> source, Func<TSource, Boolean> predicate)

      Looks like Chinese? It is very easy once you understand indeed. You can think of it as:

     IEnumerable<TSource> : It returns something that can be enumerated ( like an array, list, records …) – and its type is symbolically TSource.

     Where<TSource>( this IEnumerable<TSource> source :  First parameter is also something that can be enumerated and since it is an extension method of IEnumerable itself we don’t need to feed source parameter.

     Up to this point it is simple, gets something like a list and returns back another list. Just like an SQL select does.

     Func<TSource, Boolean> predicate) : Here comes the interesting part. It accepts a "function" that has the TSource type input parameter (you can think of TSource as a row of customer when the source is customers table) and returns back true or false. If true is returned "record" is selected.

     Now lets check this "Where" using in a Linq query and compare to its counterpart SQL select and Linq query syntax:

SQL select:

select * from Customers where Customers.Country = "USA" into cursor usaCustomers

LINQ Query Syntax:

IEnumerable<Customers> usaCustomers = from c in Customers where c.Country == "USA" select c;

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, “Courier New”, courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }LINQ Method Syntax:

IEnumerable<Customers> usaCustomers = Customers.Where( c => c.Country == "USA" );

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, “Courier New”, courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

 

Lets dissect the method syntax and see different ways of writing the same thing:

Customers.Where( c => c.Country == "USA" );

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, “Courier New”, courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

TSource is Customers. In method Where(), c is the input parameter to function (remember it represents something like a Customers row, so it has columns as properties). it returns the result of check: c.Country == "USA" which is a true or false. We could write the same thing like this:

Customers.Where( ( c ) => c.Country == "USA" );

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, “Courier New”, courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

When parameter count is one, we wouldn’t want to do that however and simply use the previous syntax. When parameter count is multiple we put them within parentheses.

For readability we are allowed to write the exact same thing like this (the newline and extra whitespace between Customers and .Where is insignificant):

IEnumerable<Customers> usaCustomers = Customers
                                      .Where( c => c.Country == "USA" );

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, “Courier New”, courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

 

Now that I think we are armed with basic syntax we can move on. LINQ operators are divided into categories. Members of categories may be different  from source to source but in general all sources have same categories (and some operators are listed in more than one category).

Filtering and Partitioning Operators:

Where:  This is the filtering operator which any VFP developer would be most familiar with. It has two forms

  1. Typical function getting a source and returning a boolean (filters out for the ones returning false)

    var usaCustomers = Customers.Where( c => c.Country == "USA" );

    .csharpcode, .csharpcode pre
    {
    font-size: small;
    color: black;
    font-family: consolas, “Courier New”, courier, monospace;
    background-color: #ffffff;
    /*white-space: pre;*/
    }
    .csharpcode pre { margin: 0em; }
    .csharpcode .rem { color: #008000; }
    .csharpcode .kwrd { color: #0000ff; }
    .csharpcode .str { color: #006080; }
    .csharpcode .op { color: #0000c0; }
    .csharpcode .preproc { color: #cc6633; }
    .csharpcode .asp { background-color: #ffff00; }
    .csharpcode .html { color: #800000; }
    .csharpcode .attr { color: #ff0000; }
    .csharpcode .alt
    {
    background-color: #f4f4f4;
    width: 100%;
    margin: 0em;
    }
    .csharpcode .lnum { color: #606060; }

  2. A function getting an index parameter besides source (this one is not supported with Linq To SQL or Linq to Entity Framework because there isn’t a supporting SQL for this. Practically speaking methods that do not a SQL counterpart do not work with L2S).

    int[] numbers = {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17};
     
    var some = numbers
        .Where((c, index) => ( 
                             (c%2 == 0) &&  
                             (index >= 8) && (index <= 15)));
     
    some.Dump("Even numbers between positions 8-15");

    .csharpcode, .csharpcode pre
    {
    font-size: small;
    color: black;
    font-family: consolas, “Courier New”, courier, monospace;
    background-color: #ffffff;
    /*white-space: pre;*/
    }
    .csharpcode pre { margin: 0em; }
    .csharpcode .rem { color: #008000; }
    .csharpcode .kwrd { color: #0000ff; }
    .csharpcode .str { color: #006080; }
    .csharpcode .op { color: #0000c0; }
    .csharpcode .preproc { color: #cc6633; }
    .csharpcode .asp { background-color: #ffff00; }
    .csharpcode .html { color: #800000; }
    .csharpcode .attr { color: #ff0000; }
    .csharpcode .alt
    {
    background-color: #f4f4f4;
    width: 100%;
    margin: 0em;
    }
    .csharpcode .lnum { color: #606060; }

1st syntax simply select customers from USA. 2nd syntax limits the "records" to position (sort of recno()) to 8-15 and within that range select numbers that are even.

Skip: Skips N elements.

int[] numbers = {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17};
var some = numbers.Skip(5);
some.Dump("Skipped first 5 numbers");

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, “Courier New”, courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

 

In T-SQL it looks like:

select number from 
(select number,Row_Number() over (order by number) as row from numbers) numberList
where row > 5 

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, “Courier New”, courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

 

Actually if you run this against Northwind:

Orders.OrderBy(o => o.OrderDate).Skip(5)

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, “Courier New”, courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

This is the generated T-SQL code:

-- Region Parameters
DECLARE @p0 Int SET @p0 = 5
-- EndRegion
SELECT [t1].[OrderID], [t1].[CustomerID], [t1].[EmployeeID],
   [t1].[OrderDate], [t1].[RequiredDate], [t1].[ShippedDate], 
   [t1].[ShipVia], [t1].[Freight], [t1].[ShipName], [t1].[ShipAddress],
   [t1].[ShipCity], [t1].[ShipRegion], [t1].[ShipPostalCode], [t1].[ShipCountry]
FROM (
    SELECT ROW_NUMBER() OVER (ORDER BY [t0].[OrderDate]) AS [ROW_NUMBER], 
        [t0].[OrderID], [t0].[CustomerID], [t0].[EmployeeID], 
        [t0].[OrderDate], [t0].[RequiredDate], [t0].[ShippedDate], 
        [t0].[ShipVia], [t0].[Freight], [t0].[ShipName], [t0].[ShipAddress], 
        [t0].[ShipCity], [t0].[ShipRegion], [t0].[ShipPostalCode], [t0].[ShipCountry]
    FROM [Orders] AS [t0]
    ) AS [t1]
WHERE [t1].[ROW_NUMBER] > @p0
ORDER BY [t1].[ROW_NUMBER]

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, “Courier New”, courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

 

You can also achieve the same result with a query like:

SELECT * from Orders
WHERE OrderId not in 
   (select top 5 OrderID from orders ORDER BY OrderDate)

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, “Courier New”, courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

 

Take: Takes top N – IOW typical TOP N query operator.

Orders.OrderBy(o => o.OrderDate).Take(5)

 
T-SQL counterpart:

SELECT TOP 5 * FROM [Orders] order by OrderDate

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, “Courier New”, courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

 
Both Skip and Take have another version which is not supported by L2S:
 
SkipWhile: Skips over elements as long as a condition is true. It has 2 overloads:

  1. First overload skips while a condition is true.

    int[] numbers = {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17};
    var some = numbers.SkipWhile( n => n < 5 );
    some.Dump("Skipped while number is less than 5");

    .csharpcode, .csharpcode pre
    {
    font-size: small;
    color: black;
    font-family: consolas, “Courier New”, courier, monospace;
    background-color: #ffffff;
    /*white-space: pre;*/
    }
    .csharpcode pre { margin: 0em; }
    .csharpcode .rem { color: #008000; }
    .csharpcode .kwrd { color: #0000ff; }
    .csharpcode .str { color: #006080; }
    .csharpcode .op { color: #0000c0; }
    .csharpcode .preproc { color: #cc6633; }
    .csharpcode .asp { background-color: #ffff00; }
    .csharpcode .html { color: #800000; }
    .csharpcode .attr { color: #ff0000; }
    .csharpcode .alt
    {
    background-color: #f4f4f4;
    width: 100%;
    margin: 0em;
    }
    .csharpcode .lnum { color: #606060; }

  2. Second overload skips while a condition is true and also gets an index parameter as input.

    int[] numbers = {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17};
    var some = numbers.SkipWhile( (n, index) => index < 5);
    some.Dump("Skipped while index position is less than 5");

    .csharpcode, .csharpcode pre
    {
    font-size: small;
    color: black;
    font-family: consolas, “Courier New”, courier, monospace;
    background-color: #ffffff;
    /*white-space: pre;*/
    }
    .csharpcode pre { margin: 0em; }
    .csharpcode .rem { color: #008000; }
    .csharpcode .kwrd { color: #0000ff; }
    .csharpcode .str { color: #006080; }
    .csharpcode .op { color: #0000c0; }
    .csharpcode .preproc { color: #cc6633; }
    .csharpcode .asp { background-color: #ffff00; }
    .csharpcode .html { color: #800000; }
    .csharpcode .attr { color: #ff0000; }
    .csharpcode .alt
    {
    background-color: #f4f4f4;
    width: 100%;
    margin: 0em;
    }
    .csharpcode .lnum { color: #606060; }

TakeWhile: Takes elements as long as the condition is true. Similar to SkipWhile it has same 2 overloads.
 

int[] numbers = {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17};
var some = numbers.TakeWhile( n => n < 5);
some.Dump("Take while number is less than 5");

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, “Courier New”, courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

int[] numbers = {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17};
var some = numbers.TakeWhile( (n, index) => index < 5);
some.Dump("Take while index position is less than 5");

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, “Courier New”, courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

 

You can combine or in other words chain these operators. Here is a sample (and already we did one before using OrderBy() above):

Orders
 .OrderBy( o => o.CustomerID)
 .Where( o => o.Customer.Country == "USA" )
 .Skip(5)
 .Take(3)

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, “Courier New”, courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

Note that using Skip() and Take() combination you can easily create a "paging query" for a web application. For example this gets the records 21-30 (Page 3 where pages are 10 records high) in a particular order of a paged query:

Customers.OrderBy(c => c.CustomerID).Skip( 2*10 ).Take(10)

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, “Courier New”, courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

About cetinbasoz

A developer working primarily with C#,F#,VFP,SQL server, Silverlight,Linq ...
This entry was posted in Uncategorized. Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s