Tutorials
Practical T-SQL
Practical T-SQL
  • Practical T-SQL Pocket Guide For Beginners
  • Preface
    • Section I. A Note From The Author
    • Section II. Tutorial Overview
    • Section III. Running The Examples
    • Section IV. How To Report An Issue
    • Section V. Join The MSU Community On Discord
    • Section VI. Supplementary Material
  • Language Basics
    • Lesson 1. Obligatory Hello World
    • Lesson 2. Code Comments With T-SQL
    • Lesson 3. Basic Syntax
    • Lesson 4. Your First Query
    • Lesson 5. Filtering Data
    • Lesson 6. Sorting Data
    • Lesson 7. Complex Data Filtering
    • Lesson 8. Aliases
    • Lesson 9. String Functions
    • Lesson 10. Creating New Columns From Existing Data (Calculated Fields)
    • Lesson 11. Displaying Data Based On Conditions (Case Statement)
    • Lesson 12. Aggregate Functions
    • Lesson 13. Grouping And Summarizing Data
    • Lesson 14. Querying More Than One Table
    • Lesson 15. Combining Queries
    • Lesson 16. Subqueries
    • Lesson 17. Creating Data
    • Lesson 18. Updating Data
    • Lesson 19. Deleting Data
    • Lesson 20. Common Table Expressions (CTEs)
    • Lesson 21. Derived Tables
    • Lesson 22. Putting It All Together
  • Advanced Topics
    • Lesson 23. Selecting Unique Values
    • Lesson 24. Updating Data With A Join
    • Lesson 25. Data Types
    • Lesson 26. Casting Data Types
    • Lesson 27. Creating Tables
    • Lesson 28. Altering Tables
    • Lesson 29. Dropping Tables
    • Lesson 30. Variables
    • Lesson 31. Controlling Flow
    • Lesson 32. Looping
    • Lesson 33. Error Processing
    • Lesson 34. Temporary Tables
    • Lesson 35. Views
    • Lesson 36. Indexed Views
    • Lesson 37. User Defined Functions
    • Lesson 38. Stored Procedures
    • Lesson 39. BULK INSERT
    • Lesson 40. Loading Tables With MERGE
    • Lesson 41. Partitioning A Dataset
    • Lesson 42. Pivoting Data
    • Lesson 43. Dynamic SQL
    • Lesson 44. Cursors
  • Solutions To Real World Problems
    • Lesson 45. Listing All Tables In A SQL Server Database
    • Lesson 46. Listing All Columns In A SQL Server Database
    • Lesson 47. Pull Records From A Table At Random
    • Lesson 48. A Better Alternative To WITH (NOLOCK)
    • Lesson 49. Boost Performance When Calling A Stored Proc From SSIS
    • Lesson 50. Setting Up Queries For Ablation Testing
    • Lesson 51. Reduce Code And Save Time With Default Column Values
    • Lesson 52. Finding Duplicate Records In A Table
    • Lesson 53. Why You Cannot Have More Than One Clustered Index On A Table
    • Lesson 54. Converting Dates To YYYYMMDD
    • Lesson 55. Sending Notification Emails With T-SQL Without Using Hardcoded Email Addresses
    • Lesson 56. Troubleshooting Long Running Queries
    • Lesson 57. Loading Large CSVs Into Data Warehouse Staging Tables
    • Lesson 58. The Only Bloody Good Reason To Use Cursors
    • Lesson 59. Loading A Type II Slowly Changing Dimension With SQL Merge
    • Lesson 60. A Clearer Explanation Of The Parameters Of The Numeric Data Type
    • Lesson 61. Why You Cannot Join On Null Values
    • Lesson 62. A Deep Dive On How The Where Clause Functions
    • Lesson 63. Using HASHBYTES() To Compare Character Strings
    • Lesson 64. Using Pipe To Hash Multiple Columns For Matching
    • Lesson 65. Why People That Indent Code Drive Me Nuts
    • Lesson 66. How To Rapidly Stand Up A Data Warehouse From Scratch
    • Lesson 67. How To Pivot Data With T-SQL When Columns Are Not Predefined
    • Lesson 68. Prepopulating A Junk Dimension
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  1. Advanced Topics

Lesson 44. Cursors

PreviousLesson 43. Dynamic SQLNextSolutions To Real World Problems

Last updated 3 years ago

Cursors are the devil and they should only be used in one scenario which I talk about in the lesson titled .

But I do talk about them, so I am going to show you how to create one here.

Cursors are a really inefficient way to loop, so they should only be used in those cases where your process is so inefficient that a cursor is the only thing less efficient than your process. A good example of this is loading large amounts of data between tables.

When you have this scenario, you can split the load into batches that can be committed to the server in smaller amounts. This takes the load off several elements of SQL Server internal mechanisms, most notably the log. Every SQL Server operation is logged so, in the case of a rollback, things can be returned to the state they were in prior to your operation. When you commit smaller batches of data, the log does not get filled up so fast.

When you create a cursor, you need to pick something to partition your dataset by. I usually use dates. Just the year will do, but in the case of extra-large amounts of data, I may have to partition by month as well. However, you have to be careful with this because partitioning by year AND month may actually make the load last longer than just using the year.

Examples

Generic Cursor Example

The below example of creating a cursor is generic. We will do a specific example later.

In [ ]:

USE demo

DECLARE @Year INT
DECLARE @Month INT

DECLARE BatchingCursor CURSOR FOR
SELECT DISTINCT YEAR([SomeDateField]),MONTH([SomeDateField])
FROM [Sometable];


OPEN BatchingCursor;
FETCH NEXT FROM BatchingCursor INTO @Year, @Month;
WHILE @@FETCH_STATUS = 0
BEGIN

BEGIN TRANSACTION
--All logic goes in here
--Any select statements from [Sometable] need to be suffixed with:
--WHERE Year([SomeDateField])=@Year AND Month([SomeDateField])=@Month   
COMMIT TRANSACTION

FETCH NEXT FROM BatchingCursor INTO @Year, @Month;
END;
CLOSE BatchingCursor;
DEALLOCATE BatchingCursor;
GO
The Only Bloody Good Reason To Use Cursors (AKA Moving Large Amounts Of Data Between Tables)