SQL Server Cursor Example
My example - can I avoid cursors?: Let's have an update procedure X taking an account id and a transaction id - this is the unique key of the row I want to update. But there are more transactions for an account. SELECT accID, transID from table Now I use a cursor to loop on the table, always taking the accID and transID to get the row and update it. Nov 17, · Problems with cursors include extending locks, their inability to cache execution plans and CPU/RAM overhead. Many T-SQL programmers and DBAs do not know how to successfully loop over records without the need for cursors. In this tip, I'll share some alternatives to cursors that provide looping functionality. Method 1: Temp table with identity column.
Cursors can look like shortcuts to a developer. When you have a complex job to perform and you need to manipulate the rows in a table, the quickest way may seem to iterate through the rows one by one using a Transact-SQL cursor.
They designed the script to pivot a very large table and store the relevant data in new table horizontally, as concatenated strings. The vendor wanted to improve performance by making the table smaller, so they decided to store the detail data horizontally, as comma-delimited strings for each parent id.
By using a set-based approach, we were able to reduce the processing time from plus hours down to less than five minutes. Many accesses to database data must be iterative in some fashion in ucrsors to prepare the data for further manipulation.
Even the Cureors Server engine iterates through data when it scans or joins data using the various types of joins available to it. You can how to plant chili pequin seeds this when you examine the SQL Server query plan for a query that returns many rows from a how to get replacement 1099 data set.
For simpler queries, you may see a clustered or non-clustered index scan. Think about it: Microsoft has optimized and afoid the SQL Server engine for years to iterate through its available data as efficiently as possible.
Imagine, if you had the time and were willing to spend the energy, you could probably write low-level accesses to database data files that would be pretty efficient. Second, Transact-SQL cursors are just one way to iterate through a table row by row. If you curors uniquely identify every row of a table that you must iterate, you can use a WHILE loop rather than how to avoid cursors in sql server cursor, and potentially gain better performance. Let serve walk you through an example to show you why.
Assume you can uniquely identify each row of a table because the table has a unique key or unique group qsl columns. In a WHILE loop, all you need to do is find the lowest value of the unique condition, and then find the next highest value each time you iterate.
TransactionHistory table. On my laptop, after I ran it a few times to make sure the data is all in cache, the WHILE loop takes nine seconds and the cursor takes 17 seconds. Your own durations may vary. The cursor evidently adds more overhead. The cursor also requires additional commands, which make the code look cluttered.
Without getting into the details of how cursors work, which Microsoft explains fully in Microsoft SQL Server Books Online, notice that when you use a WHILE loop, there is no requirement to declare, open, close, and deallocate anything.
The logic is simpler, and you can even update rows freely along the way. To update the rows using the cursor, you will have to change the cursor type.
Sometimes cursors might seem to be necessary. When you simply must iterate through database data, row by row, in their physical order, sometimes only a cursor will work. This most commonly happens when you have duplicate rows and there is no way to uniquely identify a given row in the table. Such bags of data usually occur when you import data from an external source and you cannot completely trust the data.
However, you can always turn a bag of rows into a normalized table. Even if you have duplicate rows in a table, or no set of columns you can rely on for uniqueness, you can add an identity column to the table and seed the identity to start numbering with 1. Instead, you are peppering what does the bible say about organization with commands to retrieve just a single row at a time.
If you iterated yourself, you also are relying on extraneous physical information about the table, namely the order of the rows, when processing the data.
Second, set-based commands are more logical because thinking about data in sets abstracts you away from extraneous details that are more concerned with how the data is actually ordered. Stored procedures decouple your application from database internals, and they are more efficient than ad hoc queries.
Similarly, the set-base SQL commands inside Transact-SQL give you a logical interface to your relational data, and they are more efficient because you rely on the SQL Server storage engine for iterating through data. The ohw line is not that iterating through data is bad. How to avoid cursors in sql server, the point is, let the storage engine do it for you and rely instead on the logical interface of the set-based Transact-SQL commands. My Subscriber Account Advertise Write. Training Home State of.
Staffing Home Looking for Staff? Looking for Work? Contact Us. Dark Kimbie. Published in:. The Inevitability of Iteration Many accesses to database data must be iterative in some fashion in order to prepare the data for further eerver.
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SQL Server - ways to avoid using cursors
Feb 20, · How Developers Can Avoid Transact-SQL Cursors. Cursors can look like shortcuts to a developer. When you have a complex job to perform and you need to manipulate the rows in a table, the quickest way may seem to iterate through the rows one by one using a Transact-SQL cursor. After all, since you have to iterate through data structures in your. Ways to avoid cursors: Write SQL Statements instead of cursors if action applied to rows is same. Rewrite cursors as a derived query. Use temporary tables instead of cursors. Cursors have poor performance, thus, rewrite them as complex SQL Statements. Mar 22, · Table Variable Strategy: Declare a table variable. Store tables names and ids in the table variable we declared. Set the counter to 1 and get the total number of records from the table variable. Use a ‘while’ loop as long as the counter is less than or equal to the total number of records. Inside Author: Haroon Ashraf.
There may be times when you need to loop through a resultset a row at a time and perform a certain action for each row. The most obvious way to solve this task is to use a SQL Cursor.
Whilst cursors may seem like a good idea they can often cause your database application problems as they can lock the tables that are used to populate the cursor whilst the rows in the cursor are looped through. Depending on the action that you are performing on each row this can take a considerable time. The effect of this is that tables cannot be updated or accessed by other users whilst the cursor is open.
I know one SQL specialist who when interviewing for DBA roles asks the candidate to write the syntax for using a cursor. If the candidate knows it then it is a negative point against them as they shouldn't use cursors frequently enough so that they can remember the syntax. This article shows how alternatives to cursors can be implemented. This can be utilised with a temporary table that enables us to avoid using a cursor:. We can see the above code gives the same functionality as the first code example but without using a cursor.
This gives us the benefits that the Customer table is not locked as we are looping through our resultset so other queries on the Customer table that are submitted by other users will execute much faster. We will also have a faster operating SQL script by avoiding cursors which are slow in themselves. Cursors are sometimes used to perform a calculation on values that come from each row in its rowset.
This scenario can also be achieved by replacing a Cursor with a User Defined Function. An example of a User Defined Function performing a calculation is given below:.
In this article we have seen how SQL Cursors can cause performance problems and affect other queries by locking tables. We have demonstrated two popular ways of avoiding the use of cursors.
When attempting to redesign your code to avoid cursors you should always check the execution time of your scripts. Very occassionally you may find a cursor gives better performance than an alternative method of performing the same task.
When doing this remember that the script execution time is not the only thing to check, impact on queries being run by other users at the same time is also a key factor. Subscribe to our free weekly newsletter and get exclusive SQL tips and articles straight to your inbox. Join our newsletter Subscribe to our free weekly newsletter and get exclusive SQL tips and articles straight to your inbox. Join Now! Your email address is only used to send you our newsletter.
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