November 19, 2003T-SQL Programming Part 3 - Processing Sequentially Through A Set of Recordsby Gregory A. Larsen
At some point you will have some business logic that will require you to process sequentially through a set of records one record at a time. For example you may have a list of databases, and for each database you may want to build a command that will perform some process against each database. Or you might have a set of records where you want to process through each record one at a time, so you can select additional information from another table based on the information contained in each record. This article will discuss TWO DiffERENT WAYS TO Process Through a set of records One Record At A Time.
Using a Cursor The first method I will discuss uses a cursor to process through a set of records one record at a time. A cursor is basically a set of rows that you define based on a record set returned from a query. A cursor allows applications a mechanism to process through a result set one row at a time. With a cursor an application is allowed to position itself to a specific row, scroll back and forth, and a number of other things. It would take a series of articles to describe all the functionality of a cursor. For the purpose of this article I'm only going to focus on how to use the default scrolling functionality of a cursor. This default functionality will only read from the first row to the last row in a cursor, one row at a time. I will leave additional cursor topics to another article series. to define a cursor the DECLARE CURSOR statement is used. Here is the basic format for the simple cursor topic I will be discussing in this article.DECLARE cursor_name CURSOR
FOR select_statement The cursor_name is the name you want to associate with the cursor. The select_statement is the query that will determine the rows that make up the cursor. Note there are other parameters / options associated with the DECLARE CURSOR statement that help define more complicated cursor processing than I will be covering in this article. For these additional options please read Microsoft SQL Server Books Online. Let's review a fairly simple cursor example. This example will define a cursor that contains the top 5 Customer_Id's in the Customer table in the Northwind database ... Is the code to do this. Here is the code to do this. Declare @custid nchar (5) DECLARE @Rownum Int
Declare Custlist Cursor for
Select Top 5 Customerid from Northwind.dbo.customers
Open Custlist
Fetch Next from CustList
INTO @custid
Set @Rownum = 0
While @@ fetch_status = 0
Begin
Set @Rownum = @Rownum 1
Print Cast (@Rownum as char (1)) '' @custid
Fetch Next from CustList
INTO @custid
End
Close Custlist
Deallocate Custlist
Here Are The Results That Are Generated from The Print Statement When I Run IT Against My Northwind Database. 1 Alfki
2 Anatr
3 Anton
4 AROUT
5 Bergs
Let's look at the above code in a little more detail. I first declared a cursor called "CustList". The "CustList" cursor is populated using a SELECT statement that uses the TOP clause to return only the top 5 CustomerId's. Next the cursor is opened. Each record in the "CustList" cursor is retrieved, one record at a time, using the "FETCH NEXT" next statement. The "FETCH NEXT" statement populates the local variable @CustID with the CustomerID of the current record being fetched. The @@ FETCH_STATUS variable controls whether the WHILE loop is executed. @@ FETCH_STATUS is set to zero when a record is successfully retrieved from the cursor "CustList". Inside the WHILE loop the @RowNum variable is incremented by 1 for each record processed. The calculated Row Number and @CustId are then printed out. lastly, a "FETCH NEXT" statement is used to retrieve the next row before the next cycle of the WHILE loop. This process continues one record at a time until all records in cursor " Custlist "Have B een processed. Using a Select Statement You can also use a SELECT statement to process through a set of records one record at a time. To do this I will issue an initial SELECT statement that will return the first row, then a series of follow on SELECT statements where each SELECT statement retrieves the next row. This is done by using the "TOP 1" clause of the SELECT statement, and a WHERE statement. I will use the same example as above and only return the top 5 CustomerID's from the Northwind Database Customers Table. in this code i will use two two "select top 1"
Statements and a while loop to return all 5 records. Each Record Will Be Processed One At A Time. Declare @custid nchar (5) Declare @Rownum Int
SELECT TOP 1 @ Custid = Customerid from Northwind.dbo.customers
Set @Rownum = 0
While @Rownum <5
Begin
Set @Rownum = @Rownum 1
Print Cast (@Rownum as char (1)) '' @custid
SELECT TOP 1 @ Custid = Customerid from Northwind.dbo.customers
Where customerid> @custid
End
Here you can see the first SELECT statement selects only the first CustomerID. This ID is placed in the local variable @CustID. The WHILE loop is controled by the local variable @RowNum. Each time through the WHILE loop, the Row Number and CustomerID are printed out. Prior to returning to the top of the WHILE loop I used another "SELECT TOP 1" statement to select the next CustomerID. This SELECT statement uses a WHERE clause on the SELECT statement to select the first CustomerID that is greater than the CustomerID that was just printed. The WHILE loop is process 5 times, allowing the SELECT TOP 1 method to retrieve the top 5 CustomerID's one records at a time. This example produces the same printed output as my prior CURSOR example. Conclusion Hopefully this article has given You some Ideas on how to use a cursor, and a select statement to process through a set of records, although I find uses a select statement to be a little simpler to code. you will n Eed to Decide Which Solution Makes The Most Sense in Your Environment.
