Introducing Nested Segments

Published by cce on 2012-03-23

HTSQL now supports nested queries! Generation of multi-layered output is now trivial.


Often you want to organize summary and detailed data in a single hierarchical structure. In HTSQL, the segment (/) operator embeds the result of a correlated query as a nested list. For instance, a list of school records could include associated department records:

code name campus department
code name school_code
art School of Art & Design old stdart Studio Art art
bus School of Business south acc Accounting bus
econ Economics bus
mm Management & Marketing bus
edu College of Education old edpol Educational Policy edu
tched Teacher Education edu

More Examples

This mechanism is not a special case, it is an integral part of our query language. For example, it works with projections.

campus school
north School of Engineering
old School of Art & Design
College of Education
School of Arts and Humanities
School of Natural Sciences
south School of Business
School of Music & Dance

Nested segments play well with aggregates and variables.

name $avg_credits course
credits title
Accounting 3.5 5 Financial Accounting
5 Audit
6 Accounting Internship
Art History 3.5 4 History of Art Criticism I
6 Antique Art: Greece and Rome
4 Antique Art: The Middle East
5 Islamic Art
5 Art of Photography
4 Museum and Gallery Management

Nesting can be arbitrarily deep.

name department
name course
College of Education Educational Policy Introduction to Education
Contemporary Society
Sociology of Childhood
Technology in the Classroom
Technology, Society and Schools
Economics and Education Policy
Politics and Education Policy
Education Policy Analysis
Children’s Literature
Education Policy and Practice
Social Analysis of Education Policy
Classroom Visit
Organizational Analysis of Education Policy
Seminar in Education Policy I
Seminar in Education Policy II
Qualitative Research in Education Policy
Teacher Education Teaching Methodology
Theory and Practice of Early Childhood Education
Methods of Early Science Education
Play as Education Method
Developmental Psychology
Selection of Learning Resources
Teacher Identity
Problems in Education Management
Challenges of Teaching the Gifted and Talented
Techniques of Mathematics Teaching
Techniques of Science Teaching
Techniques of Language Teaching
Problems in Education
Public School Internship
Preschool Internship
Special Topics in Teacher Education
Practice of Mathematics Teaching
Practice of Science Teaching
Practice of Language Teaching

A query may have adjacent nested segments.

name department program
name title
School of Art & Design Studio Art Post Baccalaureate in Art History
Bachelor of Arts in Art History
Bachelor of Arts in Studio Art
School of Business Accounting Master of Arts in Economics
Economics Graduate Certificate in Accounting
Management & Marketing Certificate in Business Administration
B.S. in Accounting
Bachelor of Business Administration
Bachelor of Arts in Economics

For tabular formats, we show parallel tables adjacent to each other. In the query above, programs and departments are shown side-by-side even though they are not correlated with each other.


Nested segments are supported by all HTSQL output formats. The hierarchical structure of the output is very suitable for XML and JSON formats.

<?xml version="1.0" encoding="UTF-8" ?>
<htsql:result xmlns:htsql="">
    <name>School of Art &amp; Design</name>
      <name>Studio Art</name>
    <name>School of Business</name>
      <name>Management &amp; Marketing</name>
  "school": [
      "code": "art",
      "name": "School of Art & Design",
      "campus": "old",
      "department": [
          "code": "stdart",
          "name": "Studio Art",
          "school_code": "art"
      "code": "bus",
      "name": "School of Business",
      "campus": "south",
      "department": [
          "code": "acc",
          "name": "Accounting",
          "school_code": "bus"
          "code": "econ",
          "name": "Economics",
          "school_code": "bus"
          "code": "mm",
          "name": "Management & Marketing",
          "school_code": "bus"
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