Yap Han Chiang
A techie's ramblings

A techie's ramblings

Data mining: SQL for data practitioners

Data mining: SQL for data practitioners

Yap Han Chiang's photo
Yap Han Chiang
·Apr 19, 2021·

5 min read

Subscribe to my newsletter and never miss my upcoming articles

Play this article

Table of contents

  • Practical SQL
  • Outstanding chapters
  • Closing

Data practitioners use SQL for data mining: From simply displaying data to telling stories with data.

As a software engineer, most of my interactions with SQL are for simple transactional queries, rather than analytical queries.

The main difference between them is that transactional queries are usually simple and involves a few JOINs because they are meant to serve live user requests, while analytics queries are more complex and computationally intensive.

Practical SQL

I wanted to learn more about the analytic side of things with SQL, beyond the SELECT, WHERE, GROUP BY, JOIN, and came across the book Practical SQL, which provides a great foundation in SQL.

It is written by Anthony DeBarros, a data analyst and journalist, who uses SQL(PostgreSQL) to supercharge his storytelling abilities.

Anthony goes through the problem-solving processes of acquiring and analysing data using real-world datasets and scenarios such as U.S. Census demographics, crime statistics, and data about taxi rides in New York City.

This book is a great read for people who work with data or looking to get into such roles. It is useful even for non-technical people such as business analysts, marketing managers, and C-level executives.

Outstanding chapters

While the the entire book is a great read, there are a few chapters that stood out to me, as these cover the topics that I wanted to learn.

Chaper 10: Statistical functions in SQL

This chapter explains the concept and usefulness of window functions to perform calculations across groups of data.

This is similar to GROUP BY, with one important difference that the rows are not grouped into a single output row. Instead, each row of the group is available for processing.

Let's take an example of an employee table, from PostgreSQL's website, which contains the department, employee number, and salary. The rank() is used to produce a numerical rank of the salary of each employee by the department.

employee-rank-query.png

With GROUP BY department, we can use aggregate functions to compute a single value from the grouped rows, but unable to access each individual row.

Chapter 13: Mining text to find meaningful data - Full text search in PostgreSQL

The chapter introduces full-text search capabilities in PostgreSQL. The general idea is to store the text that we want to search for as a list of lexemes, and query for the text by transforming it to lexeme before matching against the inverted index.

I was particularly interested in full-text search as it is one of the first few domain areas that I worked on as a software engineer and enjoyed very much, which is described in my first software engineering role and pagination: the deceptively simple task.

Basic full text search

tsvector is the data type that represents a sorted list of lexemes.

SELECT to_tsvector('I am walking across the sitting room to sit with you.');
→ 'across':4 'room':7 'sit':6,9 'walk':3

ts_query is the data type that represents a full text search query as a list of lexemes.

SELECT to_tsquery('walking & sitting');
→ 'walk' & 'sit'

To perform a full text query, we can use the '@@' operator

SELECT to_tsvector('I am walking across the sitting room') @@ to_tsquery('walking & sitting');
→ true

Showing results fragment relative to query

Apart from text matches, we can also show fragments of the search results that relate to the search term with ts_headline.

select president, speech_date,
ts_headline(speech_text, to_tsquery('Vietnam'),
 'StartSel = <,
 StopSel = >,
 MinWords=5,
 MaxWords=7,
 MaxFragments=1')
from "practical-sql".public.president_speeches ps
WHERE search_speech_text @@ to_tsquery('Vietnam');

Note that speech_text is text type, not tsvector type, while search_speech_text is tsvector type.

ts_headline-query.png

Distance operator

We can query documents with search terms that are close to one another using the <-> operator. A number can be used in place of the hyphen to indicate the distance between the search terms.

SELECT president,
speech_date,
ts_headline(speech_text, to_tsquery('military <-> defense'),
 'StartSel = <,
 StopSel = >,
 MinWords=5,
 MaxWords=7,
 MaxFragments=1')
FROM president_speeches
WHERE search_speech_text @@ to_tsquery('military <-> defense');

distance-operator.png

Ranking search results

Finally, we can also rank the search results by relevance with ts_rank and ts_rank_cd.

The difference between them is that ts_rank ranks the results based on how often the search terms appear in the text, while ts_rank_cd ranks the results based on the proximity of the search terms to each other.

Chapter 15: View, functions, triggers

One of the principles of software development is to keep the things DRY while automating processes.

Views

A view is a virtual table that is created using a saved query, which is run every time the view is accessed. Note that it does not save the result of a query, which is what materialized view does.

Main benefits of using a view:

  • Avoid code duplication
  • Reduce complexity in nested queries
  • Provide security by limiting access to only certain columns in a table

We can also insert, update, delete a row from a view, and the underlying table will be updated too because a view is created from a saved query that is run every time it is accessed.

Functions

We can use functions to simplify query, update table, or use it as a response to events(trigger)

Trigger

Triggers allow us to automate workflows whenever events happen, such as saving a log of changes made to a table.

It can be configured to fire on INSERT/UPDATE/DELETE/TRUNCATE events, before or after it happens, once per row or one per entire operation.

Using the example in the book, we can create a trigger function that logs grades change for students.

CREATE OR REPLACE FUNCTION record_if_grade_changed()
RETURNS trigger AS
$$
BEGIN
IF NEW.grade <> OLD.grade THEN
    INSERT INTO grades_history (
    student_id,
    course_id,
    change_time,
    course,
    old_grade,
    new_grade
    )
    VALUES
    (OLD.student_id,
    OLD.course_id,
    now(),
    OLD.course,
    OLD.grade,
    NEW.grade);
END IF;
RETURN NEW;
END;
$$ LANGUAGE plpgsql;

Next, create the trigger


CREATE TRIGGER grades_update
AFTER UPDATE
ON grades
FOR EACH ROW
EXECUTE PROCEDURE record_if_grade_changed();

Now, the function record_if_grade_changed will run after the grades table is updated each time.

Closing

SQL is an essential tool in every data professional's arsenal of technologies. I highly recommend giving Practical SQL a try, whether it is an introduction or a refresher.

Did you find this article valuable?

Support Yap Han Chiang by becoming a sponsor. Any amount is appreciated!

Learn more about Hashnode Sponsors
 
Share this