A cursor is a set of rows together with a pointer that identifies a current row.
In other word, Cursor is a database object used by applications to manipulate data in a set on a row-by-row basis, its like recordset in the ASP and visual basic.
SQL Server is very good at handling sets of data. For example, you can use a single UPDATE statement to update many rows of data. There are times when you want to loop through a series of rows a perform processing for each row. In this case you can use a cursor.
The basic syntax of a cursor is:
The READ_ONLY clause is important in the code sample above. That dramatically improves the performance of the cursor.
In this example, I just print the contents of the variable. You can execute any type of statement you wish here. In a recent script I wrote I used a cursor to move through the rows in a table and call a stored procedure for each row passing it the primary key. Given that cursors are not very fast and calling a stored procedure for each row in a table is also very slow, my script was a resource hog. However, the stored procedure I was calling was written by the software vendor and was a very easy solution to my problem. In this case, I might have something like this:
If you are going to update the rows as you go through them, you can use the UPDATE clause when you declare a cursor. You'll also have to remove the READ_ONLY clause from above.
You can code your UPDATE statement to update the current row in the cursor like this
In other word, Cursor is a database object used by applications to manipulate data in a set on a row-by-row basis, its like recordset in the ASP and visual basic.
SQL Server is very good at handling sets of data. For example, you can use a single UPDATE statement to update many rows of data. There are times when you want to loop through a series of rows a perform processing for each row. In this case you can use a cursor.
The basic syntax of a cursor is:
DECLARE @AuthorID char(11)
DECLARE c1 CURSOR READ_ONLY
FOR
SELECT au_id
FROM authors
OPEN c1
FETCH NEXT FROM c1
INTO @AuthorID
WHILE @@FETCH_STATUS = 0
BEGIN
PRINT @AuthorID
FETCH NEXT FROM c1
INTO @AuthorID
END
CLOSE c1
DEALLOCATE c1The DECLARE CURSOR statement defines the SELECT statement that forms the basis of the cursor. You can do just about anything here that you can do in a SELECT statement. The OPEN statement statement executes the SELECT statement and populates the result set. The FETCH statement returns a row from the result set into the variable. You can select multiple columns and return them into multiple variables. The variable @@FETCH_STATUS is used to determine if there are any more rows. It will contain 0 as long as there are more rows. We use a WHILE loop to move through each row of the result set.The READ_ONLY clause is important in the code sample above. That dramatically improves the performance of the cursor.
In this example, I just print the contents of the variable. You can execute any type of statement you wish here. In a recent script I wrote I used a cursor to move through the rows in a table and call a stored procedure for each row passing it the primary key. Given that cursors are not very fast and calling a stored procedure for each row in a table is also very slow, my script was a resource hog. However, the stored procedure I was calling was written by the software vendor and was a very easy solution to my problem. In this case, I might have something like this:
EXEC spUpdateAuthor (@AuthorID)
instead of my Print statement. The CLOSE statement releases the row set and the DEALLOCATE statement releases the resources associated with a cursor.If you are going to update the rows as you go through them, you can use the UPDATE clause when you declare a cursor. You'll also have to remove the READ_ONLY clause from above.
DECLARE c1 CURSOR FOR
SELECT au_id, au_lname
FROM authors
FOR UPDATE OF au_lnameYou can code your UPDATE statement to update the current row in the cursor like this
UPDATE authors
SET au_lname = UPPER(Smith)
WHERE CURRENT OF c1That covers the basics of cursors. You can check Books Online for more detailed information.
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