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Learn T-SQL Querying

You're reading from   Learn T-SQL Querying A guide to developing efficient and elegant T-SQL code

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Product type Paperback
Published in Feb 2024
Publisher Packt
ISBN-13 9781837638994
Length 456 pages
Edition 2nd Edition
Languages
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Authors (2):
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Pedro Lopes Pedro Lopes
Author Profile Icon Pedro Lopes
Pedro Lopes
Pam Lahoud Pam Lahoud
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Pam Lahoud
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Toc

Table of Contents (18) Chapters Close

Preface 1. Part 1: Query Processing Fundamentals
2. Chapter 1: Understanding Query Processing FREE CHAPTER 3. Chapter 2: Mechanics of the Query Optimizer 4. Part 2: Dos and Don’ts of T-SQL
5. Chapter 3: Exploring Query Execution Plans 6. Chapter 4: Indexing for T-SQL Performance 7. Chapter 5: Writing Elegant T-SQL Queries 8. Chapter 6: Discovering T-SQL Anti- Patterns in Depth 9. Part 3: Assembling Our Query Troubleshooting Toolbox
10. Chapter 7: Building Diagnostic Queries Using DMVs and DMFs 11. Chapter 8: Building XEvent Profiler Traces 12. Chapter 9: Comparative Analysis of Query Plans 13. Chapter 10: Tracking Performance History with Query Store 14. Chapter 11: Troubleshooting Live Queries 15. Chapter 12: Managing Optimizer Changes 16. Index 17. Other Books You May Enjoy

Implicit conversions

We introduced the concept of implicit conversions in the chapter Exploring Query Execution Plans, particularly in the context of PlanAffectingConvert warnings. An implicit conversion happens when the SQL Database Engine needs to compare two values that are not of the same data type. At this point we should understand how to recognize an implicit conversion in our query plans, but what may not always be obvious is how they got there in the first place and how to correct them.

The most obvious cause of implicit conversions is to compare two columns that are not of the same data type. We can easily avoid this by making sure that columns that are related in our database, and thus may be joined, are of the same data type. A common mistake that can cause this situation is where we have some tables that have been created with NVARCHAR strings and some tables that have VARCHAR strings. This may happen because a database was upgraded at some point to support Unicode...

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