Mysql cursor loop

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Mysql cursor loop

MySQL supports cursors inside stored programs. The syntax is as in embedded SQL. Cursors have these properties:. Cursor declarations must appear before handler declarations and after variable and condition declarations. Data Definition Statements. LIKE Statement.

mysql cursor loop

Silent Column Specification Changes. Secondary Indexes and Generated Columns.

Cursor in MySQL

Data Manipulation Statements. The Subquery as Scalar Operand. Comparisons Using Subqueries. Rewriting Subqueries as Joins. Restrictions on Subqueries. Transactional and Locking Statements. Statements That Cause an Implicit Commit. Restrictions on XA Transactions. Replication Statements. Variables in Stored Programs.

Local Variable Scope and Resolution. Flow Control Statements.Ladies and Gentlemen, welcome to the main event of the evening. Welcome along to our new series of SQL Smackdowns where we pitch two methods of achieving something against each other to find out which is the most efficient.

In those cases we need a way of looping through the data to perform an action. We all know that cursors are bad and many people will avoid using them at all costs. We decided to find out. Well, I gave up after the query had run for two minutes and in that time it had managed to get through only rows out of the million. Well the difference looks to be that sort. Ok, the loop is having to sort the data before it can do anything. Let us level things out a bit and sort the cursor to see what happens then.

With that huge right hook, Loops goes down. He looks to be out but somehow manages to climb to his feet as bell goes.

PL/SQL Cursor FOR LOOP

He staggers to his corner, bloodied. With the clustered index, the loop was finished in 59 seconds with 3 reads per iteration. Of course you do need to think about your particular use case and if a cursor is the best way to go or if the same could be accomplished using a more efficient set based approach. Wow, thanks for doing all this work to get data. I remember being advised against cursors once SQL 6.

Hopefully the objectivity will dispel bad dogma. Like Like. Notice the timings?

mysql cursor loop

What surprised me is how close they are in terms of performance, given how much work goes on under the covers with the cursor. It was working, although I was tweaking it quite a bit so might have managed to post up a bugged version by mistake.

I have done similar tests and gotten similar results. Like Liked by 1 person. Another reason they were avoided — and we advised against the — was that people would forget to de-allocate them, or otherwise abuse them. The beauty of looping with a table variable is that it went out of scope. Those are issues with bad coding practices rather than cursors themselves, used inappropriately they can be nasty things. It makes sense to me.I have been told these types of operations are what SQL Server is designed to process and it should be quicker than serial processing.

I know cursors exist, but I am not sure how to use them. Can you provide some cursor examples? Can you give any guidance on when to use cursors?

mysql cursor loop

I assume Microsoft included them in SQL Server for a reason so they must have a place where they can be used in an efficient manner. In some circles, cursors are never used. In others, they are a last resort. And in other groups they are used regularly. In each of these camps, they have different reasons for their stand on cursor usage. Regardless, they probably have a place in particular circumstances and not in others.

It boils down to your understanding of the coding technique then your understanding of the problem at hand to make a decision on whether or not cursor-based processing is appropriate or not. To get started let's do the following:. The purpose for the cursor may be to update one row at a time or perform an administrative process such as SQL Server database backups in a sequential manner. Creating a SQL Server cursor is a consistent process. Once you learn the steps you are easily able to duplicate them with various sets of logic to loop through data.

Let's walk through the steps:. From here, check out the examples below to get started on knowing when to use SQL Server cursors and how to do so. Based on the code and explanations above, let's break down the SQL Server cursor example and notate which sections would need to be updated when using this code. The analysis below is intended to serve as insight into various scenarios where cursor-based logic may or may not be beneficial:.

This tip provides sample code that can be used to expand your SQL Server cursor options beyond the syntax in this tip. He covers the following:. The code samples in this tip are valuable to illustrate the differences between cursors in SQL Server and the While Loop. In the example above, backups are issued via a cursor.

Check out these other tips that leverage cursor-based logic:. One of the blessing and curses of SQL Server is that there are numerous ways to solve a problem.

Knowing all of your options is important to select the best solution. There's no user input here so injection is mitigated.Write powerful, clean and maintainable JavaScript.

After my previous article on Stored Procedures was published on SitePoint, I received quite a number of comments. As cursors are a part of a Stored Procedure, we will elaborate a bit more on SP in this article as well. In particular, we will see how to return a dataset from an SP.

It is an essential component in stored procedures. With cursors, we can traverse a dataset and manipulate each record to accomplish certain tasks.

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Since a cursor is implemented in a stored procedure, it has all the benefits and limitations of an SP access control, pre-compiled, hard to debug, etc.

The official documentation on cursors is located here. It contains only four commands that are related to cursor declaration, opening, closing, and fetching. As mentioned above, we will also touch on some other stored procedure statements. The table structure behind it is straightforward:.

SQL Server Loop through Table Rows without Cursor

I have been updating this table since You can use your favorite tool. Well, I have to admit Lakers are not playing very well these days. This is certainly doable but if the requirement becomes more complicated in a larger table, it takes more time and is more error prone.

Can we do this with a single SQL statement? The input of gurus will be highly appreciated — leave it in the comments below. Can we do this in PHP?

Yes, of course. Can we do this via other means? Yes, it is possible. Every insertion of the record will update that table too. But this is way too cumbersome and too error prone. As the name of this article suggests, we will see a better alternative in my view to solve this problem: using cursor in a Stored Procedure.

In this SP, we have one input parameter and two output parameters.

mysql cursor loop

This defines the signature of the SP. The above line is the cursor declaration.Do you want to delete a " " lesson.

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This delete will remove all it's sub lessons and contents. Do you want to delete a " " sub lesson. This delete will remove all it's contents.

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Introduction to Database Cursors

Pune, Maharashtra, India. Mysql Indeep. How to get total milliseconds, seconds, minutes and hour by Date in Mysql. Smart Tricks. How to copy table with constraints in mysql? Procedure Recursion. Cool Topics. How to ignore constraints while insert,update or delete records in Mysql?

Methods [cont'd]. How to configure current or default time zone as GMT in Mysql using my. Difference between. Multiple cursors in mysql stored procedure with example. How to create multiple blocks of statement in MySQL. Next Topics.You have always heard that you should avoid cursors in your T-SQL code as a SQL Server best practice, because cursors are detrimental to performance and sometimes cause issues.

But sometimes there is a need to loop through the data one row at a time, so in this tip we will look at a comparison of how to do a loop without using cursor. We all know that SQL Server, like every relational database allows the user to perform set based operations.

It adds constructs found in procedural languages allowing a more straightforward coding to developers. These constructs were added for a reason and sometimes this is the only approach to the task at hand. If you have ever worked with cursorsyou may find this title a bit confusing because after all, cursors uses while constructs to iterate between rows. But besides that, I want to show you that in some circumstances when we use a cursor to iterate over a set of rows we can change it to a while loop.

In such cases, the only challenge will be to choose a proper exit condition. Not everything is wrong with cursors, they also have some advantages over other looping techniques.

There are also some negative aspects that you should be aware when using cursors instead of other looping options. The idea is to loop trough the table rows ordered by the CursorTestID column and update the RunningTotal column with the sum of the CursorTestID column value and the value of the RunningTotal column of the previous row.

On the script above you will notice that I only used a single insert statement and I took advantage of the batch separator the GO command as a shortcut to execute this insert statement times. In other words, we are instructing SQL Server to use a read only cursor that can only move forward and be scrolled from the first to the last row.

The next image is a screen capture showing the execution of the script above. As you can see, it took three minutes and five seconds to update therows of our test table. Now I will rewrite the previous script avoiding the use of a cursor. You will notice that it contains a While loop which is almost identical to the one in the cursor script.

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This is, as I previously said, because even when working with cursors you need to use an iterative control structure. The next image is a screen capture of the execution of the script above. It took less time to run the while loop than the cursor. A word of advice, in order to run this code, you should follow the steps in the tip to create the test environment. If we dissect this code, we can see that there is one cursor that goes through the table products which I copied below.

In our particular case we will use a sequential integer key starting from 1, so we can use it as an iterator. This key will be associated to a ProductID from the Products table.

Cursor in SQL Server

Additionally a row can be deleted while we are running our code and it could lead to execution errors. I will go into this further on.

For this matter I added two new integer variables named Iterator and MaxIterator. The MaxIterator variable is used to keep the number of items in the TallyTable table and we set its value only once before starting the loop. The Iterator variable is initialized to 1, as we defined it as the starting number on the sequence and we are going to increment its value at each iteration. ON xmo. Daniel Farina was born in Buenos Aires, Argentina. Self-educated, since childhood he showed a passion for learning.

View all my tips. Become a paid author. Recommended Reading. Rolling up multiple rows into a single row and column for SQL Server data. Cursor in SQL Server.MySQL supports cursors inside stored programs. The syntax is as in embedded SQL. Cursors have these properties:. Cursor declarations must appear before handler declarations and after variable and condition declarations. Data Definition Statements. Atomic Data Definition Statement Support.

LIKE Statement. Silent Column Specification Changes. Secondary Indexes and Generated Columns. Data Manipulation Statements. Parenthesized Query Expressions. The Subquery as Scalar Operand. Comparisons Using Subqueries. Restrictions on Subqueries. Transactional and Locking Statements. Statements That Cause an Implicit Commit. Restrictions on XA Transactions. Replication Statements. Function which Configures Group Replication Primary.

Functions which Configure the Group Replication Mode. Compound Statement Syntax. Variables in Stored Programs. Local Variable Scope and Resolution. Flow Control Statements.

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Restrictions on Server-Side Cursors.


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