Principle of Simplest Query Operations

We present a simple style rule for SQL select queries with the intention to make it easier to develop queries. The rule also applies to Access Query Builder in which context the rule was developed. I am tempted to call it the Law of Demeter for Queries. The Law of Demeter promotes simple methods in object-oriented systems while the principle of least information promotes simple queries in relational data bases.


A query joins the appropriate number of tables to provide the required fields and their correct values. A query also projects the minimum number of fields needed. In addition, a query does exactly one of four things Leads to queries that are easier to develop and debug. We want to have minimum information per query to not overload and confuse. Simple queries make it explicit in which order the steps are applied.

What is minimized?

The minimum number of fields are selected (projection) to achieve the correct result. Each query implements one task involving a join, and a minimum projection and one of widening, aggregation, elimination of duplicates and row selection. We minimize the operations performed by a query to one of the above four.

Note that the style rule does not constrain what can be expressed. However, it requires more nested queries. But the significant benefits are (1) each nested query can be easily tested and (2) less knowledge is needed about how query evaluation works and (3) the confusing feature combinations are eliminated.

If you need a pure join of tables, do a row selection without a condition.


Richard Rasala: Access Queries. Focus on: Our Query Philosophy and Our Query Style.

Karl Lieberherr and the Demeter Team: Law of Demeter

Private Communication with Ken Baclawski fall 2014.

Teaching Materials

PowerPoint Slides

Files: PrincipleOfLeastInformation2.pptx and DebuggingWithPrincipleOfLeastInformation.pptx

Karl Lieberherr, Spring 2015