T-SQL Querying

Microsoft Press
Itzik Ben-Gan / Adam Machanic / Dejan Sarka / Kevin Farlee  
Microsoft Press
Total pages
March 2015
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T-SQL Querying
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Squeeze maximum performance and efficiency from every T-SQL query you write or tune. Four leading experts take an in-depth look at T-SQL’s internal architecture and offer advanced practical techniques for optimising response time and resource usage. Emphasising a correct understanding of the language and its foundations, the authors present unique solutions they have spent years developing and refining. All code and techniques are fully updated to reflect new T-SQL enhancements in Microsoft SQL Server 2014 and SQL Server 2012.

Write faster, more efficient T-SQL code:

  • Move from procedural programming to the language of sets and logic
  • Master an efficient top-down tuning methodology
  • Assess algorithmic complexity to predict performance
  • Compare data aggregation techniques, including new grouping sets
  • Efficiently perform data-analysis calculations
  • Make the most of T-SQL’s optimised bulk import tools
  • Avoid date/time pitfalls that lead to buggy, poorly performing code
  • Create optimised BI statistical queries without additional software
  • Use programmable objects to accelerate queries
  • Unlock major performance improvements with In-Memory OLTP
  • Master useful and elegant approaches to manipulating graphs


  • Covers moving from procedural programming to the language of sets and logic
  • Shows how to optimize query tuning with a top-down methodology
  • Assesses algorithmic complexity to predict performance
  • Compares data-aggregation techniques, including new grouping sets

Table of Contents

Foreword     xv
Introduction     xvii
Chapter 1: Logical query processing     1

Logical query-processing phases     3
Logical query-processing phases in brief     4
Sample query based on customers/orders scenario     6
Logical query-processing phase details     8
Step 1: The FROM phase     8
Step 2: The WHERE phase     14
Step 3: The GROUP BY phase     15
Step 4: The HAVING phase     16
Step 5: The SELECT phase     17
Step 6: The ORDER BY phase     20
Step 7: Apply the TOP or OFFSET-FETCH filter     22
Further aspects of logical query processing     26
Table operators     26
Window functions     35
The UNION, EXCEPT, and INTERSECT operators     38
Conclusion     39
Chapter 2: Query tuning     41
Internals     41
Pages and extents     42
Table organization     43
Tools to measure query performance     53
Access methods     57
Table scan/unordered clustered index scan     57
Unordered covering nonclustered index scan     60
Ordered clustered index scan     62
Ordered covering nonclustered index scan     63
The storage engine’s treatment of scans     65
Nonclustered index seek + range scan + lookups     81
Unordered nonclustered index scan + lookups     91
Clustered index seek + range scan     93
Covering nonclustered index seek + range scan     94
Cardinality estimates     97
Legacy estimator vs. 2014 cardinality estimator     98
Implications of underestimations and overestimations     99
Statistics     101
Estimates for multiple predicates     104
Ascending key problem     107
Unknowns     110
Indexing features     115
Descending indexes     115
Included non-key columns     119
Filtered indexes and statistics     120
Columnstore indexes     123
Inline index definition     130
Prioritizing queries for tuning with extended events     131
Index and query information and statistics     134
Temporary objects     139
Set-based vs. iterative solutions     149
Query tuning with query revisions     153
Parallel query execution     158
How intraquery parallelism works     158
Parallelism and query optimization     175
The parallel APPLY query pattern     181
Conclusion     186
Chapter 3: Multi-table queries     187
Subqueries     187
Self-contained subqueries     187
Correlated subqueries     189
The EXISTS predicate     194
Misbehaving subqueries     201
Table expressions     204
Derived tables     205
CTEs     207
Views     211
Inline table-valued functions     215
Generating numbers     215
The APPLY operator     218
The CROSS APPLY operator     219
The OUTER APPLY operator     221
Implicit APPLY     221
Reuse of column aliases     222
Joins          224
Cross join     224
Inner join     228
Outer join     229
Self join     230
Equi and non-equi joins     230
Multi-join queries     231
Semi and anti semi joins     237
Join algorithms     239
Separating elements     245
The UNION, EXCEPT, and INTERSECT operators     249
The UNION ALL and UNION operators     250
The INTERSECT operator     253
The EXCEPT operator     255
Conclusion     257
Chapter 4: Grouping, pivoting, and windowing     259
Window functions     259
Aggregate window functions     260
Ranking window functions     281
Offset window functions     285
Statistical window functions     288
Gaps and islands     291
Pivoting     299
One-to-one pivot     300
Many-to-one pivot     304
Unpivoting     307
Unpivoting with CROSS JOIN and VALUES     308
Unpivoting with CROSS APPLY and VALUES     310
Using the UNPIVOT operator     312
Custom aggregations     313
Using a cursor     314
Using pivoting     315
Specialized solutions     316
Grouping sets     327
GROUPING SETS subclause     328
CUBE and ROLLUP clauses     331
Grouping sets algebra     333
Materializing grouping sets     334
Sorting     337
Conclusion     339
Chapter 5: TOP and OFFSET-FETCH     341
The TOP and OFFSET-FETCH filters     341
The TOP filter     341
The OFFSET-FETCH filter     345
Optimization of filters demonstrated through paging     346
Optimization of TOP     346
Optimization of OFFSET-FETCH     354
Optimization of ROW_NUMBER     358
Using the TOP option with modifications     360
TOP with modifications     360
Modifying in chunks     361
Top N per group     363
Solution using ROW_NUMBER     364
Solution using TOP and APPLY     365
Solution using concatenation (a carry-along sort)     366
Median     368
Solution using PERCENTILE_CONT     369
Solution using ROW_NUMBER     369
Solution using OFFSET-FETCH and APPLY     370
Conclusion     371
Chapter 6: Data modification     373
Inserting data     373
Bulk import     376
Measuring the amount of logging     377
BULK rowset provider     378
Sequences     381
Characteristics and inflexibilities of the identity property     381
The sequence object     382
Performance considerations     387
Summarizing the comparison of identity with sequence     394
Deleting data     395
Deleting duplicates     399
Updating data     401
Update using table expressions     402
Update using variables     403
Merging data     404
MERGE examples     405
Preventing MERGE conflicts     408
ON isn't a filter     409
USING is similar to FROM     410
The OUTPUT clause     411
Example with INSERT and identity     412
Example for archiving deleted data     413
Example with the MERGE statement     414
Composable DML     417
Conclusion     417
Chapter 7: Working with date and time     419
Date and time data types     419
Date and time functions     422
Challenges working with date and time     434
Literals     434
Identifying weekdays     436
Handling date-only or time-only data with DATETIME and SMALLDATETIME     439
First, last, previous, and next date calculations     440
Search argument      445
Rounding issues     447
Querying date and time data     449
Grouping by the week     449
Intervals     450
Conclusion     471
Chapter 8: T-SQL for BI practitioners     473
Data preparation     473
Sales analysis view     474
Frequencies     476
Frequencies without window functions     476
Frequencies with window functions     477
Descriptive statistics for continuous variables     479
Centers of a distribution     479
Spread of a distribution     482
Higher population moments     487
Linear dependencies     495
Two continuous variables     495
Contingency tables and chi-squared     501
Analysis of variance     505
Definite integration     509
Moving averages and entropy     512
Moving averages     512
Entropy     518
Conclusion     522
Chapter 9: Programmable objects     525
Dynamic SQL     525
Using the EXEC command     525
Using the sp_executesql procedure     529
Dynamic pivot     530
Dynamic search conditions     535
Dynamic sorting     542
User-defined functions     546
Scalar UDFs     546
Multistatement TVFs     550
Stored procedures     553
Compilations, recompilations, and reuse of execution plans     554
Table type and table-valued parameters     571
Triggers     575
Trigger types and uses     575
Efficient trigger programming     581
SQLCLR programming     585
SQLCLR architecture     586
CLR scalar functions and creating your first assembly     588
Streaming table-valued functions     597
SQLCLR stored procedures and triggers     605
SQLCLR user-defined types     617
SQLCLR user-defined aggregates     628
Transaction and concurrency     632
Transactions described     633
Locks and blocking     636
Lock escalation     641
Delayed durability     643
Isolation levels     645
Deadlocks     657
Error handling     662
The TRY-CATCH construct     662
Errors in transactions     666
Retry logic     669
Conclusion     670
Chapter 10: In-Memory OLTP     671
In-Memory OLTP overview     671
Data is always in memory     672
Native compilation     673
Lock and latch-free architecture     673
SQL Server integration     674
Creating memory-optimized tables     675
Creating indexes in memory-optimized tables     676
Clustered vs. nonclustered indexes     677
Nonclustered indexes     677
Hash indexes     680
Execution environments     690
Query interop     690
Natively compiled procedures     699
Surface-area restrictions     703
Table DDL     703
DML     704
Conclusion     705
Chapter 11: Graphs and recursive queries     707
Terminology     707
Graphs     707
Trees     708
Hierarchies     709
Scenarios     709
Employee organizational chart     709
Bill of materials (BOM)     711
Road system     715
Iteration/recursion     718
Subgraph/descendants     719
Ancestors/path     730
Subgraph/descendants with path enumeration     733
Sorting     736
Cycles     740
Materialized path     742
Maintaining data     743
Querying     749
Materialized path with the HIERARCHYID data type     754
Maintaining data     756
Querying     763
Further aspects of working with HIERARCHYID     767
Nested sets     778
Assigning left and right values     778
Querying     784
Transitive closure     787
Directed acyclic graph     787
Conclusion     801
Index     803


Itzik Ben-Gan is a mentor for and co-founder of SolidQ. A SQL Server Microsoft MVP (Most Valuable Professional) since 1999, Itzik has delivered numerous training events around the world focused on T-SQL querying, query tuning, and programming. Itzik has authored several T-SQL books as well as articles for SQL Server Pro, SolidQ Journal, and MSDN. Itzik’s speaking activities include TechEd, SQLPASS, SQL Server Connections, SolidQ events, and various user groups around the world. Itzik is the author of SolidQ’s Advanced T-SQL Querying, Programming and Tuning, and T-SQL Fundamentals courses, along with being a primary resource within the company for its T-SQL-related activities.

Dejan Sarka, MCT and SQL Server MVP, is an independent consultant, trainer, and developer focusing on database and business intelligence applications. His specialties are advanced topics like data modeling, data mining, and data quality. On these tough topics, he works and researches together with SolidQ and the Data Quality Institute. He is the founder of the Slovenian SQL Server and .NET Users Group. Dejan Sarka is the main author or coauthor of 11 books about databases and SQL Server, with more to come. He also has developed and is continuing to develop many courses and seminars for SolidQ and Microsoft. He has been a regular speaker at many conferences worldwide for more than 15 years, including Microsoft TechEd, PASS Summit, and others.

Adam Machanic is a Boston-based SQL Server developer, writer, and speaker. He focuses on large-scale data warehouse performance and development and is author of the award-winning SQL Server monitoring stored procedure, sp_WhoIsActive. Adam has contributed to several books on SQL Server, including T-SQL Querying (Microsoft Press, 2015) and SQL Server 2008 Internals (Microsoft Press, 2009). He regularly speaks at international conferences and training events on a variety of SQL Server topics.

Kevin Farlee has over 25 years in the industry, in both database and storage-management software. In his current role as a Storage Engine Program Manager on the Microsoft SQL Server team, he brings these threads together. His current projects include the SQL Server Project “Hekaton” In-Memory OLTP feature.