Cursors with SQL 2000 Part 2
November 5, 2004
This is the second article in the Cursors with SQL 2000 series. In the preceding article, cursor use and basic syntax were covered. A select cursor was created demonstrating the keywords DECLARE, OPEN, FETCH, @@FETCH_STATUS, and DEALLOCATE. These keywords guide basic cursor execution by assigning a TSQL statement to the cursor, moving records into the cursor, retrieving a specific record from the cursor, working with retrieved record, and then closing the cursor. The sample cursor demonstrated that SQL could act on one record at a time, as opposed to its usual method of acting on sets of data. In this article, the sample cursor will be expanded upon to include data modification and record positioning.
Following is the sample cursor from the first article. This example demonstrates the minimum statements required to create and use a cursor.
USE pubs GO DECLARE get_price CURSOR FOR SELECT price FROM titles OPEN get_price FETCH NEXT FROM get_price WHILE @@FETCH_STATUS = 0 FETCH NEXT FROM get_price CLOSE get_price DEALLOCATE get_price
The purpose of the this cursor was to lay the ground work for a statement that will change books priced under $20 to be raised by 10%, while books currently $20 or more, will be raised 5%. The above cursor will now be modified so prices returned from titles under twenty dollars will print "Under 20", while titles twenty dollars or more will print the price. Modify the cursors to:
DECLARE @price money DECLARE @get_price CURSOR SET @get_price = CURSOR FOR SELECT price FROM titles OPEN @get_price FETCH NEXT FROM @get_price INTO @price WHILE (@@FETCH_STATUS = 0) BEGIN IF @Price < 20 SELECT 'Under 20' ELSE SELECT @Price FETCH NEXT FROM @get_price INTO @price END CLOSE @get_price DEALLOCATE @get_price
One of the first differences to notice is the use of variables for the cursor and the price. Now that the cursor is a variable, it is SET to a SQL statement. The SQL statement itself has not changed. The cursor is opened like before, but the FETCH will move the field price into the variable @price. If more than one column were needed, say price and title, we would declare two variables, then SELECT INTO both as follows:
DECLARE @Price money, @Title varchar(50), @get_price CURSOR SET @get_price = CURSOR FOR SELECT Price, Title FROM titles OPEN @get_price FETCH NEXT FROM @get_price INTO @Price, @Title
The WHILE loop is still controlled with @@FETCH_STATUS, the use of variables does not alter this. Once fetched, @price can be used like a normal variable as demonstrated by "IF @Price < 20". Because the WHILE loop has more than one action line, it uses BEGIN and END tags. Once we are done working with this record, the next is fetched. At end of file, the loop exits and the cursor is closed and destroyed.
Running the cursor will produce the expected list of "Under," price, and nulls, depending on the value found in the titles table.