The first is badly written cursors. Sadly, bad programming can be found in virtually any environment and only proper training can solve this problem. The second category of poor cursors consists of developers who use cursors when a better solution is available.
As we shall soon see in code examples, cursors are data structures that permit the T-SQL developer to execute a query and then step through the row results one row at a time. Sadly, some programmers use cursors because the step-by-step logic of a cursor seems more natural to them than the declarative UPDATE statement.
Consider a formidable calculation, however. Perhaps a tax calculation must be performed on every row. It is likely that there are many different conditions that affect the calculation, potentially involving lots of conditional logic in the code. It may simply not be possible to perform the calculation armed only with declarative SQL, and a row-oriented cursor solution may be the only practical approach. There are actually two distinct declarations for cursors in T-SQL.
In general, I will tend to prefer standard techniques over platform-specific methods, but here I make an exception. A trigger cannot be called or executed; the DBMS automatically fires the trigger as a result of a data modification to the associated table. Not really. In terms of what it is doing, a while loop and a cursor both do the same thing, they operate on one row at a time. A stored procedure is a set of Structured Query Language SQL statements with an assigned name, which are stored in a relational database management system as a group, so it can be reused and shared by multiple programs.
Oracle creates a memory area, known as the context area, for processing an SQL statement, which contains all the information needed for processing the statement; for example , the number of rows processed, etc.
A cursor is a pointer to this context area. A cursor holds the rows one or more returned by a SQL statement. Cursors in sql server allow you to fetch a set of data, loop through each record, and modify the values as necessary; then, you can easily assign these values to variables and perform processing on these values.
While loop also same as cursor to fetch set of data and process each row in sql server. Alternative 2: Temporary Tables We can also use temporary tables instead of SQL cursors to iterate the result set one row at a time. Temporary tables have been in use for a long time and provide an excellent way to replace cursors for large data sets.
The fields in a view are fields from one or more real tables in the database. In SQL procedures, a cursor makes it possible to define a result set a set of data rows and perform complex logic on a row by row basis. A cursor can be viewed as a pointer to one row in a set of rows. The cursor block is so ugly that it's difficult to use in a clear and effective way. All that having been said, there are some cases where a cursor really is best--they just aren't usually the cases that beginners want to use them for.
Sometimes the nature of the processing you need to perform requires cursors, though for performance reasons it's always better to write the operation s using set-based logic if possible. I wouldn't call it "bad practice" to use cursors, but they do consume more resources on the server than an equivalent set-based approach and more often than not they aren't necessary. Given that, my advice would be to consider other options before resorting to a cursor. There are several types of cursors forward-only, static, keyset, dynamic.
Each one has different performance characteristics and associated overhead. Make sure you use the correct cursor type for your operation. Forward-only is the default. One argument for using a cursor is when you need to process and update individual rows, especially for a dataset that doesn't have a good unique key.
You can easily use set based theory to do it. Eg: with Sql And you can do the same with Sql but the syntax of query would be different. Cursors are usually not the disease, but a symptom of it: not using the set-based approach as mentioned in the other answers. Not understanding this problem, and simply believing that avoiding the "evil" cursor will solve it, can make things worse.
For example, replacing cursor iteration by other iterative code, such as moving data to temporary tables or table variables, to loop over the rows in a way like:. Such an approach, as shown in the code of another answer, makes things much worse and doesn't fix the original problem. It's an anti-pattern called cargo cult programming : not knowing WHY something is bad and thus implementing something worse to avoid it!
Still lacking set-based approach being the lesser evil , but the best I could do that moment. Another symptom of this lack of understanding can be what I sometimes call "one object disease": database applications which handle single objects through data access layers or object-relational mappers. Typically code like:. There are very, very few cases where the use of a cursor is justified. There are almost no cases where it will outperform a relational, set-based query. Sometimes it is easier for a programmer to think in terms of loops, but the use of set logic, for example to update a large number of rows in a table, will result in a solution that is not only many less lines of SQL code, but that runs much faster, often several orders of magnitude faster.
Even the fast forward cursor in Sql Server can't compete with set-based queries. Cursors do have their place, however I think it's mainly because they are often used when a single select statement would suffice to provide aggregation and filtering of results. Avoiding cursors allows SQL Server to more fully optimize the performance of the query, very important in larger systems. The basic issue, I think, is that databases are designed and tuned for set-based operations -- selects, updates, and deletes of large amounts of data in a single quick step based on relations in the data.
In-memory software, on the other hand, is designed for individual operations, so looping over a set of data and potentially performing different operations on each item serially is what it is best at.
Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. Because of its nature, a cursor can get a lot closer to doing the latter than an ordinary while loop ever could.
As an aside, another terrible way to handle this example would be joining the table to itself with a less than condition and SUM aggregate. It would still be RBAR under the hood—just masked with the join. Thank you for taking the time to challenge dogma with data. Much appreciated! There are IIRC about six different set-based solutions to running totals, most of which have been extensively explored and compared over the years.
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Create a website or blog at WordPress. To test this out we created the following table and populated it with 1 million rows. Cursor body slams Loops into the canvas, we felt that from up here. Loop Execution Plan Well the difference looks to be that sort.
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