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Returning data using cursors

From: SQL Server: Triggers, Stored Procedures, and Functions

Video: Returning data using cursors

Next, I'd like to talk about returning data from a stored procedure using a cursor. A cursor is a data structure that contains multiple rows of data, and it allows us to cycle through each row one at a time and perform an action on each row. Returning a cursor from a stored procedure is pretty similar to returning data from a stored procedure using output parameters. And in a lot of ways you can think of a cursor as a special type of output parameter. I have some code on the screen. This code is available in your exercise files.

Returning data using cursors

Next, I'd like to talk about returning data from a stored procedure using a cursor. A cursor is a data structure that contains multiple rows of data, and it allows us to cycle through each row one at a time and perform an action on each row. Returning a cursor from a stored procedure is pretty similar to returning data from a stored procedure using output parameters. And in a lot of ways you can think of a cursor as a special type of output parameter. I have some code on the screen. This code is available in your exercise files.

The top-half of this code is going to create a stored procedure called procedure cursor that accepts one parameter that's called Authors, and that parameter has a data type that is defined by two words; one word is CURSOR and the second word is VARYING. Meaning, this cursor isn't always going to be exactly the same. It could be a different size based on the data that's currently in the database. This cursor will sometimes return a different number of records than other times. And then the last word is OUTPUT. That's the same as we've had before because this cursor will be used as an output parameter.

We still need the keyword OUTPUT. In the body of the stored procedure on line 4, we set the author's cursor equal to a cursor, and then we put a SELECT statement. So line 6, 7, and 8, I wrote a SELECT statement that's just going to select the first name from the author's table. Just keeping it simple right now. Line 10 is very important where we open the author's cursor. Notice in this stored procedure we never close the author's cursor. We will close it; we're just going to close it outside of the stored procedure.

So in the stored procedure, we only open it. On line 13, we have the keyword GO, and I'm going to scroll down so we can see what happens after this. This is all the code to execute the stored procedure, and get back that cursor. So the top two lines are declaring variables, then line 18 executes the stored procedure, and takes the output and puts it into my cursor. Line 20 and 21, cycle through the first record in my cursor, and then very important, 23 and 24 close the cursor, and de-allocate the cursor.

Close the cursor means it cannot be used anymore, de-allocate means it is removed from memory. It's important to do these things because the machine will not do them for you automatically. If you do not specifically say de- allocate the cursor, it will remain in memory for quite a long time; probably will remain there until you reboot your SQL server. So for the sake of efficiency, you always want to de-allocate. Let's go ahead and run this, and we see the stored procedure is created and it returns one record; the first name of the first author in the table.

We could if we wanted to modify this code at the bottom to do another fetch to get the next record, and another fetch to get the next record, so on, and so forth. Or most likely, you would put it in a loop to walk through all of the items in the cursor. Some developers are very much against cursors. Cursors can take up a lot of memory space, obviously the amount of memory they use is always proportional to the size of the table. So if you're working with a small amount of data, cursors are a good idea.

If you're working with a very large table, you might want to look at an alternative. As mentioned earlier, there are particular performance problems if you do not remove the cursor from memory. So again, the DEALLOCATE statement is important and the DEALLOCATE statement has to come after the CLOSE statement. So when working with cursors, remember, keep it small and always destroy what you create.

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This video is part of

Image for SQL Server: Triggers, Stored Procedures, and Functions
 
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  1. 2m 15s
    1. Welcome
      51s
    2. What you should know
      51s
    3. Using the exercise files
      33s
  2. 11m 1s
    1. Comparing triggers, functions, and procedures
      3m 25s
    2. Why use a stored procedure?
      4m 59s
    3. Why use functions?
      1m 27s
    4. Why use triggers?
      1m 10s
  3. 6m 2s
    1. Configuring your environment
      4m 53s
    2. Downloading and installing a sample database
      1m 9s
  4. 26m 25s
    1. Creating a stored procedure
      2m 46s
    2. Modifying a stored procedure
      2m 34s
    3. Returning data using data sets
      3m 45s
    4. Returning data using cursors
      3m 45s
    5. Using input and output parameters
      5m 24s
    6. Using security and permissions
      5m 24s
    7. Using transactions
      2m 47s
  5. 11m 56s
    1. Creating a user-defined function
      4m 59s
    2. Exploring single-value functions
      4m 18s
    3. Exploring table value functions
      2m 39s
  6. 9m 31s
    1. Using "after" triggers
      3m 47s
    2. Using "instead of" triggers
      2m 9s
    3. Using nested triggers
      1m 38s
    4. Using database-level triggers
      1m 57s
  7. 12m 43s
    1. Exploring a real-world INSERT procedure
      5m 32s
    2. Exploring a real-world UPDATE procedure
      3m 13s
    3. Implementing logging on DELETE
      3m 58s
  8. 19m 38s
    1. Understanding the Common Language Runtime (CLR) and the .NET framework
      1m 52s
    2. Using CLR with SQL Server 2012
      4m 11s
    3. Writing stored procedures with C# .NET
      5m 51s
    4. Writing functions with .NET
      5m 7s
    5. Choosing between T-SQL vs. CLR
      2m 37s
  9. 11m 34s
    1. Creating a basic web form and connecting to a database
      2m 56s
    2. Executing a stored procedure
      2m 4s
    3. Passing parameters
      3m 41s
    4. Getting return values
      2m 53s
  10. 1m 43s
    1. Next steps
      1m 43s

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