Understand when to use user defined functions in sql server tech-republic


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Understand when to use user defined functions in sql server tech-republic

  1. 1. Understand when to use user-defined functions in SQL Server | TechRepublic ZDNet Asia SmartPlanet TechRepublic Log In Join TechRepublic FAQ Go Pro! Blogs Downloads Newsletters Galleries Q&A Discussions News Research Library IT Management Development IT Support Data Center Networks Security Home / Blogs / The Enterprise Cloud Follow this blog: The Enterprise Cloud Understand when to use user- defined functions in SQL Server By Tim Chapman September 3, 2007, 11:49 PM PDT In the simplest terms, a user-defined function (UDF) in SQL Server is a programming construct that accepts parameters, does work that typically makes use of the accepted parameters, and returns a type of result. This article will cover two types of UDFs: table-valued and scalar-valued. (I will not be covering aggregate functions.) Types of UDFs Table-valued functions A table-valued UDF is a function that accepts parameters and returns the results in the form of a table. This type of function is special because it returns a table that you can query the results of and join with other tables. In SQL Server 2005, field values from other tables may be passed into the function during a join operation to return a record based result. To accomplish this, you must use SQL Server 2005’s APPLY operator. It can be difficult to know when it is appropriate to use a VIEW vs. when it is appropriate to use a table-valued UDF. VIEWs are a great tool for data abstraction, combining data, and logically using subsets of data. I like to use table-valued UDFs when I need to use one or more values from different tables in a join operation where some type of calculation needs to be done and an aggregation returned. Scalar-valued functions A scalar-valued UDF accepts parameters and, ultimately, returns a single, atomic value. There are seven reasons why these types of functions are different than stored procedures in the database engine. You cannot modify data inside of a UDF. A scalar-valued UDF returns only one value, where a stored procedure can have numerous OUTPUT parameters. You can use scalar-valued UDFs as the default value for a column in a table. Scalar-valued UDFs are an easy way to define constant values to use in your database environment. You can pass field values as parameters into UDFs.http://www.techrepublic.com/blog/datacenter/understand-when-to-use-user-defined-functions-in-sql-server/171[08/29/2012 3:08:18 PM]
  2. 2. Understand when to use user-defined functions in SQL Server | TechRepublic You can nest scalar function calls. This means that you can pass a call to a scalar-valued function to another function or stored procedure. You can use the results from scalar-valued UDFs as criteria in a WHERE statement. Although you can do it, this is typically not a good idea. (Later in the article, I’ll explain why I try to avoid this common practice.) There are two types of scalar-valued UDFs: deterministic and non-deterministic. Recognizing the determinism of the functions that are created is important. An example of the importance is the creation of indexed views. One of the many restrictions of creating an index on a view is that the view definition cannot use a non-deterministic function. Deterministic A deterministic UDF always returns the same result with the same set of input parameters. Some examples of deterministic functions are the system functions MONTH(), YEAR(), and ISNULL(). Non-deterministic A non-deterministic UDF is can potentially return a different value each time it is called with the same set of input parameters. Some examples of non-deterministic functions are the system functions GETDATE(), NEWID(), and @@CONNECTIONS. Two examples of UDFs Before presenting the examples, I will set up my SalesHistory table and load data into it: IF OBJECT_ID(SalesHistory)>0 DROP TABLE SalesHistory; CREATE TABLE [dbo].[SalesHistory] ( [SaleID] [int] IDENTITY(1,1) NOT NULL PRIMARY KEY, [Product] [varchar](10) NULL, [SaleDate] [datetime] NULL, [SalePrice] [money] NULL ) DECLARE @i SMALLINT SET @i = 1 WHILE (@i <=1000) BEGIN INSERT INTO SalesHistory(Product, SaleDate, SalePrice) VALUES (Computer, DATEADD(mm, @i, 3/11/1919), DATEPART(ms, GETDATE()) + (@i + 57)) INSERT INTO SalesHistory(Product, SaleDate, SalePrice) VALUES(BigScreen, DATEADD(mm, @i, 3/11/1927), DATEPART(ms, GETDATE()) + (@i + 13)) INSERT INTO SalesHistory(Product, SaleDate, SalePrice) VALUES(PoolTable, DATEADD(mm, @i, 3/11/1908), DATEPART(ms, GETDATE()) + (@i + 29)) SET @i = @i + 1 END GO The first UDF I will look at is the scalar-valued UDF. The script below defines a function named dbo.udf_GetProductSales that accepts three parameters and returns a MONEY value. The function uses the three input parameters as criteria in calculating the total sales from the SalesHistory table. CREATE FUNCTION dbo.udf_GetProductSales ( @Product VARCHAR(10), @BeginDate DATETIME, @EndDate DATETIME ) RETURNS MONEY AS BEGIN DECLARE @Sales MONEYhttp://www.techrepublic.com/blog/datacenter/understand-when-to-use-user-defined-functions-in-sql-server/171[08/29/2012 3:08:18 PM]
  3. 3. Understand when to use user-defined functions in SQL Server | TechRepublic SELECT @Sales = SUM(SalePrice) FROM SalesHistory WHERE Product = @Product AND SaleDate BETWEEN @BeginDate AND @EndDate RETURN(@Sales) END The script below calls the UDF created in the above script. Note: The schema the function belongs to must be used in the call. In this case, the function belongs to the dbo schema. SELECT dbo.udf_GetProductSales(PoolTable, 1/1/1990, 1/1/2000) I usually discourage using scalar-valued UDFs in a WHERE criteria statement because, for every record considered in the query, the scalar-valued function will be called. This means that a function used in the WHERE criteria will cause a scan of the values being searched, which is going to be slower than if an index is able to be used. (I will provide more details on this concept in a future article.) Although the use of a correlated sub-query is sometimes confusing and complicated, the use of them can help solve some of the more challenging query problems. While using these special queries is useful, they only return one column of data. You can use the upgraded table-valued UDFs in SQL Server 2005 to overcome this shortcoming. I’ll show you how to use the APPLY operator to accept column values from a table and return a table-result of correlated values. CREATE FUNCTION dbo.udf_GetProductSalesTable ( @Product VARCHAR(10), @SaleID INT ) RETURNS @SalesTable TABLE ( SalesTotal MONEY, SalesCount INT ) BEGIN INSERT INTO @SalesTable(SalesTotal, SalesCount) SELECT SUM(SalePrice), COUNT(SaleID) FROM SalesHistory WHERE Product = @Product AND SaleID <= @SaleID RETURN END GO The above function accepts the particular product for which we were searching, along with the SaleID from the SalesHistory table. From the function definition, you can see that the function returns a table named @SalesTable that contains two columns: SalesTotal and SalesCount. The body of the function inserts aggregate values into the @SalesTable table variable based upon the input parameters. The following code uses the APPLY operator to invoke the table-valued function with the values from the SalesHistory table. (Note: Logically, you may want to use a JOIN operator here, but it is not necessary. The APPLY operator essentially does the “JOIN” for us by applying the values from the SalesHistory table to the table-valued function. In a sense, this code works the same way a correlated sub-query does, except that it can return multiple correlated values.) SELECT * FROM SalesHistory sh CROSS APPLY dbo.udf_GetProductSalesTable(sh.Product, sh.SaleID) ORDER BY sh.SaleID ASC Additional TechRepublic resources on UDFs SQL Server 2000’s user-defined functions add flexibility, cut coding time Building and using table UDFs in SQL Server Incorporate SQL Server UDFs in your .NET applicationshttp://www.techrepublic.com/blog/datacenter/understand-when-to-use-user-defined-functions-in-sql-server/171[08/29/2012 3:08:18 PM]
  4. 4. Understand when to use user-defined functions in SQL Server | TechRepublic Tim Chapman is a SQL Server database administrator who works for a bank in Louisville, KY, and has more than 7 years of IT experience. He is also Microsoft certified in SQL Server 2000 and SQL Server 2005. If you would like to contact Tim, please e-mail him at chapman.tim@gmail.com. —————————————————————————————– Get SQL tips in your inbox TechRepublic’s free SQL Server newsletter, delivered each Tuesday, contains hands-on tips that will help you become more adept with this powerful relational database management system. Automatically subscribe today! Get IT Tips, news, and reviews delivered directly to your inbox by subscribing to TechRepublic’s free newsletters. About Tim Chapman Full Bio Contact Talking intelligently about Defragment your Windows virtualization Server 2003 hard drive on a set schedule 7 Join the conversation! Add Your Opinion Comments Follow via: Staff Picks Top Rated Most Recent My Contacts See All Comments UDF or View 0 pixelwiz 15th Jul 2011 Votes Can you please give some more details on when it is more appropriate to use a View vs a table-valued UDF? I have a few table-valued UDFs created that now I feel maybe should have been views, but I... Read Whole Comment + View in thread Scalar UDF in Where Clause 0 unclebiguns@... 13th Sep 2007 Votes So you are saying that if I use the a Scalar UDF ona variable or parameter in a where clause, it will not use an index? For example: Select product_id, product_name from products where... Read Whole Comment + View in thread RE: Understand when to use user-defined functions in 0 SQL Server Votes alaniane@... 7th Sep 2007 Thanks for the article on UDFs. I also like to use UDFs when designing queries. Ithttp://www.techrepublic.com/blog/datacenter/understand-when-to-use-user-defined-functions-in-sql-server/171[08/29/2012 3:08:18 PM]
  5. 5. Understand when to use user-defined functions in SQL Server | TechRepublic makes it easier to break complex queries into smaller chunks for debugging purposes. Later on, I will replace the... Read Whole Comment + View in thread See all comments Join the TechRepublic Community and join the conversation! Signing-up is free and quick, Do it now, we want to hear your opinion. Join Loginhttp://www.techrepublic.com/blog/datacenter/understand-when-to-use-user-defined-functions-in-sql-server/171[08/29/2012 3:08:18 PM]