Tom Johnston is an independent consultant specializing in enterprise data architecture, and in relational, object-oriented and data warehouse modeling in various industries, including telecommunications, health care, banking, retailing and transportation. He can be reached at drtomjprof@att.net and through his website at MindfulData.org.

Splitting Episodes, Part 3
June 5, 2009 In this column, we correct a mistake made last time and learn something in the process

Splitting Episodes, Part 2
May 22, 2009 We continue our discussion from the Time and Time Again series of a bitemporal transaction, which results in the split of one...

Splitting Episodes
May 6, 2009 In this column, we discuss a bi-temporal transaction that results in the split of one episode into two episodes

Merging Episodes
April 13, 2009 In this column, we discuss a bi-temporal transaction which results in the merger of two episodes

Proactive Updates, Part 4
April 3, 2009 In previous columns, we completed a proactive update transaction against an open episode. This time, we complete our...

Proactive Updates, Part 3
March 10, 2009 This column continues the discussion of proactively updating closed episodes

Proactive Updates, Part 2
February 18, 2009 Preliminary discussions to proactively updating a closed episode

Proactive Updates, Part 1
February 6, 2009 How to enter an update before it becomes effective

Time and Time Again: Retroactive Updates
January 14, 2009 In this column, we extend an episode backwards in effective time

Time and Time Again: State Transformations
January 2, 2009 This column describes the development of a taxonomy of state transformations on asserted version tables.

Time and Time Again: Scenario 1, Transaction 4
December 17, 2008 In this column, we process a delete transaction against our asserted version table

Time and Time Again: Scenario 1, Transaction 3
December 3, 2008 In this column, we process a second update against our asserted version table

Time and Time Again: Scenario 1, Transaction 2
November 19, 2008 In this column, we begin our discussion of bi-temporal updates

Time and Time Again: Scenario 1
November 5, 2008 With this column, the authors begin an example-driven presentation of asserted versioning.

Time and Time Again: The Logical Data Model for Asserted Versioning Examples
October 15, 2008 In this column, we present the logical data model of the five tables we will use to illustrate asserted versioning.

Time and Time Again: The Origins of Asserted Versioning
October 1, 2008 In this column, we present the schema of asserted versioning tables. Using a one-row policy table, we explain how to read the...

Time and Time Again: The Origins of Asserted Versioning
September 16, 2008 In this column, we discuss the origins of asserted versioning, an approach to temporal data management that has roots in both...

Time and Time Again: Versions and Episodes
September 3, 2008 This column introduces episodes, a core concept in asserted versioning. It explains the correspondences between episodes and...

Time and Time Again: Concluding Our Extended Taxonomy Discussion
August 13, 2008 This wraps up our classification of ways to manage time in databases. Each has its place in enterprise data architecture. Our...

Time and Time Again: The Importance of Bi-Temporal Data Management
July 30, 2008 To emphasize the relevance of our discussions to real-world data management issues, we will consider four hypothetical...

Time and Time Again: Managing Time in Relational Databases - A Revised Roadmap
July 16, 2008 I will present a side-by-side series of inserts, updates and deletes against non-temporal tables and against corresponding...

Time and Time Again
July 2, 2008 We are pleased to welcome Tom as our newest online columnist. This column is a continuation of his previous articles and will...

Time and Time Again: Managing Time in Relational Databases, Part 28 - A Revised Roadmap
June 12, 2008 We will summarize what we've done so far and proceed to develop an amended roadmap.

Time and Time Again: Managing Time in Relational Databases, Part 27 - Original and Temporal Inserts (Concluded)
May 21, 2008 We will proceed now to a discussion of original and temporal inserts.

Time and Time Again - Managing Time in Relational Databases, Part 26: Points in Time and Periods of Time
May 9, 2008 Previous discussions require another digression into how to use pairs of dates to represent periods of time.

Time and Time Again: Managing Time in Relational Databases, Part 25 - Original and Temporal Inserts
April 29, 2008 We turn to temporal integrity constraints as they apply to insert and to upsert transactions.

Time and Time Again - Managing Time in Relational Databases, Part 24: Original and Temporal Updates (Concluded)
April 9, 2008 As this series continues, context becomes increasingly important so that the thread of the discussion is not lost.

Time and Time Again: Managing Time in Relational Databases, Part 23 - Original and Temporal Updates
March 25, 2008 This article continues our discussion reviewing temporal RI rules.

Time and Time Again: Managing Time in Relational Databases - Part 22: Original and Temporal Deletes
March 6, 2008 This article completes our discussion of delete transactions.

Time and Time Again: Managing Time in Relational Databases, Part 21 - Enforcement Time Options for Temporal RI
February 22, 2008 In this article we continue our discussion of temporal integrity constraints on a database.

Time and Time Again: Managing Time in Relational Databases, Part 20 - RI and Temporal RI (Continued)
February 4, 2008 In this installment, we will begin a discussion of how temporal referential integrity (RI) may be enforced with today's...

Time and Time Again: Managing Time in Relational Databases, Part 19 - RI and Temporal RI
January 25, 2008 In this article, we will consolidate and extend some of the things we have learned about the semantic constraints that apply...

Time and Time Again: Managing Time in Relational Databases, Part 18 - Physical Implementation of Version Pattern 3
January 8, 2008 This article describes the physical implementation of Version Pattern 3.

Time and Time Again: Managing Time in Relational Databases, Part 17 - Physical Implementation of Version Pattern 2
January 2, 2008 In this PDM segment, we will describe the physical implementation of Version Pattern 2. This pattern expands on pattern 1 by...

Time and Time Again: Managing Time in Relational Databases Part 16 - Physical Implementation of Version Pattern 1
December 7, 2007 This installment in our series begins to describe the physical implementation of versioning patterns.

Time and Time Again: Managing Time in Relational Databases, Part 15 - For and Against Surrogate Keys
November 27, 2007 Part 15 addresses the "stong approach" to surrogate keys as well as the standard approach and reasons for dismissing it.

Time and Time Again: Managing Time in Relational Databases, Part 14 - Versioning with Surrogate Keys
November 15, 2007 This article addresses the benefits of using of surrogate keys for versioned tables.

Time and Time Again: Managing Time in Relational Databases, Part 13 - A Roadmap of the Rest of This Series
November 5, 2007 This article maps out the rest of the series.

Time and Time Again: Managing Time in Relational Databases, Part 11 - Foreign Keys in Version Tables Continued
October 23, 2007 This article continues the authors' discussion of foreign keys.

Time and Time Again: Managing Time in Relational Databases, Part 12: Foreign Keys in Version Tables
October 12, 2007 This article completes the discussion of foreign keys in version tables.

Time and Time Again: Managing Time in Relational Databases, Part 10 - Foreign Keys in Version Tables
September 7, 2007 Part 10 discusses the role of foreign keys in versioning.

Time and Time Again: Managing Time in Relational Databases, Part 9: Completing Version Pattern 5
August 17, 2007 The discussion of emantic constraints for Pattern 5 is concluded in this installment of the series.

Time and Time Again: Managing Time in Relational Databases, Part 8: Version Pattern 5
August 3, 2007 Semantic constraints for Pattern 5 are discussed in this installemnt of the series.

Time and Time Again: Managing Time in Relational Databases, Part 7: Completing Version Pattern 4
July 13, 2007 The series completes its examination of version pattern 4.

Time and Time Again: Managing Time in Relational Databases, Part 6: Version Pattern 4
June 29, 2007 The series continues with a look at version pattern 4.

Time and Time Again: Managing Time in Relational Databases, Part 5: Version Pattern 3
June 15, 2007 In this fifth installment, the authors discuss Version Pattern 3.

Time and Time Again: Managing Time in Relational Databases, Part 4 - Preliminaries to Version Pattern 3
June 1, 2007 Version Patterns 1 and 2 were both "update in place" patterns. In this fourth installment, the authors have laid the...

Time and Time Again: Managing Time in Relational Databases, Part 3 - Version Patterns 1 and 2
May 18, 2007 This third article in the series discusses Version Patterns 1 and 2.

Time and Time Again: Managing Time in Relational Databases, Part 2 - Preliminaries
May 4, 2007 Part 2 of this series sets the stage for a continuining discussion of version patterns that satisfy distinct sets of business...

Time and Time Again: Managing Time in Relational Databases, Part 1
May 1, 2007 This is the first in a series of articles that will show how to provide real-time historical access using the relational...

Logical Tables, Physical Files and Flaws in Relational DBMSs: Fabian Pascal vs. the Original Source Material
December 6, 2002 Tom Johnston, a monthly columnist on dataWarehouse.com, takes issue with statements made by Fabian Pascal. This is a...

Unobvious Redundancies in Relational Data Models, Part 5
October 12, 2001 The final segment of this five-part series discusses the ways to eliminate redundancies in data models.

Unobvious Redundancies in Relational Data Models, Part 4
October 5, 2001 Part 4 of this five-part series emphasizes redundancies which are the most difficult to spot – those that involve a mix...

Unobvious Redundancies in Relational Data Models, Part 3
September 28, 2001 Part 3 of this five-part series focuses on redundancies involving relationships.

Unobvious Redundancies in Relational Data Models, Part 2
September 21, 2001 Part 2 of this five-part series considers examples of redundancies among attributes that are not all in the same entity....

Unobvious Redundancies in Relational Data Models, Part 1
September 14, 2001 In the first article of a five-part series, Johnston explains how to recognize – and correct – unobvious redundancies in...

Business Rules, Meta Data and Late-Binding: How to Build Stable and Flexible Information Systems, Part 8
April 27, 2001 In this final installment of the business rules, meta data and late binding series, Johnston explains how a late-binding...

Business Rules, Meta Data and Late Binding: How to Build Stable and Flexible Information Systems, Part 7
April 6, 2001 In this installment of his series, Johnston discusses H3 and its importance to organizational structure and business rules.

Business Rules, Meta Data and Late Binding: How to Build Stable and Flexible Information Systems, Part 6
March 23, 2001 In this installment of his series, Johnston discusses the importance of meta data as another way to express business rules.

Business Rules, Meta Data and Late Binding, Part 5
March 16, 2001 In this installment of his series, Johnston discusses how the issue between expressing business rules in data schemas and...

Business Rules, Meta Data and Late Binding, Part 4
March 9, 2001 In this installment of his series, Johnston reaches a controversial conclusion regarding the best implementation (flexible vs....

Business Rules, Meta Data and Late Binding, Part 3
March 2, 2001 In an era of rapid change, many business rules are highly volatile. Because acquisitions, mergers and simple growth can...

Business Rules, Meta Data and Late Binding, Part 2
February 23, 2001 In this article – the second in an eight-part series – author Tom Johnston analyzes the data model of his fictitious company’s...

Business Rules, Meta Data and Late Binding, Part 1
February 16, 2001 In his introduction to this eight-part series, author Tom Johnston discusses that late-binding perspective on business rules,...

De-Embedding Foreign Keys, Part 4
June 23, 2000 Against a nonembedded foreign key database, standard SQL cannot be used to express complex queries or to express insert or...

De-Embedding Foreign Keys, Part 3
June 16, 2000 This is the third in a four-part series focusing on de-embedding foreign keys. The author describes what SQL might look like...

De-Embedding Foreign Keys: Part 2
June 9, 2000 This article is the second in a four-part series focusing on de-embedding foreign keys. In this installment, the author...

De-Embedding Foreign Keys: Part 1
June 2, 2000 This article is the first in a four-part series focusing on de-embedding foreign keys. In this installment, the author...

Primary Key Reengineering Projects
March 1, 2000

Primary Key Reengineering Projects
February 1, 2000 Tom Johnston proposes that organizations would be better served by reengineering from unintelligent keys to enterprise keys...