SQL Server – Selecting Top 1 in a Left Join

This scenario doesn’t come up too often, but the other day I had a requirement to do a left join through a one to many relationship but only get 1 result from the right side of the join. This kind of scenario can often be accomplished using sub selects, but in this case I needed access to multiple columns so a sub select would be out of the question.

The solution was to do an OUTER APPLY instead of a LEFT JOIN. An outer apply allows you to write a sub query referencing the ID from the original table just like a sub select. However as this is effectively a kind of join you have access to multiple columns.

Example Scenario

Imagine you have a table of customers and a table of addresses and each customer can have multiple addresses.

Our tables will look like this:

Customer (CustomerID, FirstName, LastName)

Address (AddressID, CustomerID, Line1, Line2, Town, County, Country, DateAdded)

The task is to get a list of customers and their most recent address.

Solution

By using an OUTER APPLY we can join the 2 tables and get the most recent address like this:

SELECT c.*, la.*
FROM Customer c
OUTER APPLY 
    (SELECT TOP 1 *
    FROM Address a
    WHERE a.CustomerID = c.CustomerID
    ORDER BY a.DateAdded DESC
    ) AS la

LINQ to SQL Inserts and Deletes

Inserting and Deleting records in a database using LINQ to SQL is just as easy as selecting information. What’s not so easy is actually finding out how to do it. There are lots of excellent blog posts around such as this one by Scott Guthrie http://weblogs.asp.net/scottgu/archive/2007/07/11/linq-to-sql-part-4-updating-our-database.aspx, however most of them we’re all written for the Beta version of LINQ to SQL which let you do a .Add() or .Remove() on your table, which was  changed on the final release. 

So to insert do something like this: 

DataClassesDataContext dataContext = new DataClassesDataContext(); 

//Create my new Movie record
Movie movie = new Movie();
movie.Name = "Tim's movie"; 

//Insert the movie into the data context
dataContext.Movies.InsertOnSubmit(movie); 

//Submit the change to the database
dataContext.SubmitChanges();

And to delete do something like this:

DataClassesDataContext dataContext = new DataClassesDataContext();

var movies = from m in dataContext.Movies
                  where m.Name == "Tim's movie"
                  select m;

dataContext.Movies.DeleteAllOnSubmit(movies);

dataContext.SubmitChanges();

How to search inside Stored Procedures?

A common problem faced by many developers when it comes to databases and SQL Server is how to search the text inside a stored procedure.

In many systems particularly older Classic ASP solutions, functional code has been moved from the actual application to stored procedures inside the database. This is usually because it will either run faster here, or because it was just a lot easier to perform the necessary task using TSQL. Following this though comes the problem of how you can search what’s in all those stored procedures, especially when you’re getting into the hundreds of them. Let’s say there was a Users table that contained fields for an address, but that now needs to be moved to a table of its own, you would need to search all the code for things accessing those table columns but SQL Server Management Studio certainly doesn’t provide any search box’s with the power to do this.

Never fear though syscomments is here. Syscomments contains the original text from amongst other things all the Stored Procedures in the DB s all you need to do is search that for what you’re looking for:

Select OBJECT_NAME(id), [text]
From syscomments
Where [text] like ‘%Create%’

The function OBJECT_NAME will also help you by converting the id number in the result set into the actual name of the stored procedure (or view, function etc). If you wanted to limit the result to just stored procedures you can add the following line to the where clause:

AND OBJECTPROPERTY(id, ‘IsProcedure’) = 1