Chui’s counterpoint

Python developers are probably already familiar with List Comprehensions, and lambda expressions. The interesting twist with LINQ is these expressions are translated into SQL and executed on the RDBMS instead of being done on the client side. There was a pretty clever Python project that achieved this in Python through dissassembly of python bytecodes, but I can’t recall it’s name [Update 23 Apr 2008: see http://svn.aminus.net/dejavu/branches/ldap/logic.py ] . There are some interesting ideas raised in LINQ that even Python developers ought to explore and consider adopting in a future Python. [Edit: 23 Apr 2008: Meaning I'd prefer to see this in Python core through AST transformation rather than bytecode hacks, C# has just got macros.]

1. Setting up a Linq DataContext

   MyDatabaseContext db = new MyDatabaseContext(MyConnectionString); // or MyDatabaseContext db = new MyDatabaseContext();
 

2. Basic LINQ Expression

   var AnIQueryable = from Customer in db.Customers
where Customer.FirstName.StartsWith("m")
select Customer; 


3. Logging the generated queries

   db.Log = Console.out

   The previous example translates into a LIKE operator:

   SELECT [t0].[CustomerID], [t0].[FirstName] AS [FirstName]
FROM [dbo].[Customer] AS [t0]
WHERE [t0].[FirstName] LIKE @p0

4. LINQ with Aggregates

   var AverageRuns =(from Master in this.db.Masters select Master.Runs).Average()
   

   The SQL generated isn’t too shabby either:

   SELECT AVG([t0].[Runs]) AS [value]
   FROM [dbo].[Master] AS [t0]

5. LINQ grouped aggregates

   
      var categories =
         from p in products
         group p by p.Category into g
         select new
                 {Category = g.Key,
                  AveragePrice = g.Group.Average(p => p.UnitPrice)
                  };
   

6. Lambda Expressions

   • Map

     db.Customers.Select(customer => master.Customer.LastName.length)

   • Filter

     db.Customers.Where(customer => customer.LastName.StartsWith("M")

   • Reduce / Fold

     
double[] doubles = { 1.7, 2.4, 3.5, 8.9 };
double result = doubles.Aggregate(0.0, (d1, d2) => d1+d2);


     Note: You can’t use Aggregate directly on IQueryable from database contexts. Instead,
     
var arr_masters = this.db.Masters.ToArray();
var totalRuns = arr_masters.Aggregate
(0, // seed
(int accum, MyDB.Master master) => accum + master.Runs
);


7. Updating Database

   
   this.Validate();
   this.masterBindingSource.EndEdit();
   db.SubmitChanges();
   

8. How to cheat

   Use Microsoft’s 101 LINQ samples

This entry was posted on Sunday, April 20th, 2008 at 2:30 pm and is filed under .Net, Python. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.