Introduction to SQL

Last updated on 2024-03-26 | Edit this page

Overview

Questions

What is SQL?
Why is it significant?
What is the relationship between a relational database and SQL?

Objectives

Define a relational database.
Explain what SQL is and why to use it.
Identify library and information skills that relate to using SQL

What is SQL?

Structured Query Language, or SQL (sometimes pronounced “sequel”), is a powerful language used to interrogate and manipulate relational databases. It is not a general programming language that you can use to write an entire program. However, SQL queries can be called from programming languages to let any program interact with databases. There are several variants of SQL, but all support the same basic statements that we will be covering today.

Relational databases

Relational databases consist of one or more tables of data. These tables have fields (columns) and records (rows). Every field has a data type. Every value in the same field of each record has the same type. These tables can be linked to each other when a field in one table can be matched to a field in another table. SQL queries are the commands that let you look up data in a database or make calculations based on columns.

Why use SQL?

SQL is well established and has been around since the 1970s. It is still widely used in a variety of settings.

SQL lets you keep the data separate from the analysis. There is no risk of accidentally changing data when you are analysing it. If the data is updated, a saved query can be re-run to analyse the new data.

SQL is optimised for handling large amounts of data. Data types help quality control of entries - you will receive an error if you try to enter a word into a field that should contain a number. Understanding the nature of relational databases, and using SQL, will help you in using databases in programming languages such as R or Python.

Many web applications (including WordPress and e-commerce sites like Amazon) run on a SQL (relational) database. Understanding SQL is the first step in eventually building custom web applications that can serve data to users.

Library work is about information management. As library workers, we help sort and organise information, and we help people find information. Most of us go through mediated queries to help people find the information they need, e.g., we assist with or conduct a search via a library catalogue. With SQL, you can directly construct your database queries without the constraints (e.g., field name or search limitations) imposed by a mediated search interface. Library workers are good at searching information – constructing queries using SQL is a different and more direct way of finding information.

What are some of the uses for SQL in libraries?

You can use SQL to make macro or large scale changes to metadata records in library databases, for instance, updating journal names so that they are consistent or normalised throughout the database.
For situations where you have to interact with database administrators in your library or wider organisation, you can learn the fundamentals of SQL to become more fluent in the terminology.
Because SQL querying is similar to natural language queries, you can use it to organise a variety of projects (e.g. programme evaluation) and ask questions of the data before going on to use other data analysis tools.
You can use SQL to query your library database and explore new views that are not necessarily provided via library systems patron facing interfaces.
SQL can be used to keep an inventory of items, for instance, for a library’s makerspace, or it can be used to track licences for journals.
For projects involving migrating and cleaning data from one system to another, SQL can be a handy tool.
With spreadsheets of data scattered throughout folders, SQL can be a useful tool for connecting this data and bringing it together in a database or central data warehouse where it can be accessible to different roles in the library and queried in one place.
It can also help with initial exposure with interacting with a system, in preparation for interacting with a application programming interface or API later on.

In addition, you can read about these two perspectives:

Database Management Systems

There are a number of different database management systems for working with relational data. We’re going to use SQLite today, but basically everything we teach you will apply to the other database systems as well (e.g., MySQL, PostgreSQL, MS Access, Filemaker Pro). The only things that will differ are the details of exactly how to import and export data and possibly some differences in datatype.

Introduction to DB Browser for SQLite

Let’s all open the database we downloaded via the setup in DB Browser for SQLite.

You can see the tables in the database by looking at the left hand side of the screen under Tables.

To see the contents of a table, click on “Browse Data” then select the table in the “Table” dropdown in the upper left corner.

If we want to write a query, we click on the “Execute SQL” tab.

There are two ways to add new data to a table without writing SQL:

Enter data into a CSV file and append
Click the “Browse Data” tab, then click the “New Record” button.

To add data from a CSV file:

Choose “File” > “Import” > “Table from CSV file…”
Select a CSV file to import
Review the import settings and confirm that the column names and fields are correct
Click “OK” to import the data. If the table name matches an existing table and the number of columns match, DB Browser will ask if you want to add the data to the existing table.

Dataset Description

The data we will use was created from 5 csv files that contain tables of article titles, journals, languages, licences, and publishers. The information in these tables are from a sample of 51 different journals published during 2015.

articles

Contains individual article titles and the associated citations and metadata.
(16 fields, 1001 records)
Field names: id, Title, Authors, DOI, URL, Subjects, ISSNs, Citation, LanguageID, LicenceID, Author_Count, First_Author, Citation_Count, Day, Month, Year

journals

Contains various journal titles and associated metadata. The table also associates Journal Titles with ISSN numbers that are then referenced in the ‘articles’ table by the ISSNs field.
(5 fields, 51 records)
Field names: id, ISSN-L,ISSNs, PublisherID, Journal_Title

languages

ID table which associates language codes with id numbers. These id numbers are then referenced in the ‘articles’ table by the LanguageID field.
(2 fields, 4 records)
Field names: id, Language

licences

ID table which associates Licence codes with id numbers. These id numbers are then referenced in the ‘articles’ table by the LicenceID field.
(2 fields, 4 records)
Field names: id, Licence

publishers

ID table which associates Publisher names with id numbers. These id numbers are then referenced in the ‘journals’ table by the PublisherID field.
(2 fields, 6 records)
Field names: id, Publisher

A Note About Data Types

The main data types that are used in doaj-article-sample database are INTEGER and TEXT which define what value the table column can hold.

SQL Data Type Quick Reference

Different database software/platforms have different names and sometimes different definitions of data types, so you’ll need to understand the data types for any platform you are using. The following table explains some of the common data types and how they are represented in SQLite; more details available on the SQLite website.

Data type	Details	Name in SQLite
boolean or binary	this variable type is often used to represent variables that can only have two values: yes or no, true or false.	doesn’t exist - need to use integer data type and values of 0 or 1.
integer	sometimes called whole numbers or counting numbers. Can be 1, 2, 3, etc., as well as 0 and negative whole numbers: -1, -2, -3, etc.	INTEGER
float, real, or double	a decimal number or a floating point value. The largest possible size of the number may be specified.	REAL
text or string	any combination of numbers, letters, symbols. Platforms may have different data types: one for variables with a set number of characters - e.g., a zip code or postal code, and one for variables with an open number of characters, e.g., an address or description variable.	TEXT
date or datetime	depending on the platform, may represent the date and time or the number of days since a specified date. This field often has a specified format, e.g., YYYY-MM-DD	doesn’t exist - need to use built-in date and time functions and store dates in real, integer, or text formats. See Section 2.2 of SQLite documentation for more details.
blob	a Binary Large OBject can store a large amount of data, documents, audio or video files.	BLOB

Key Points

SQL is a powerful language used to interrogate and manipulate relational databases.
People working in library- and information-related roles have skills that allow them to use SQL to organize and access data.