Selecting and sorting data

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

Estimated time: 20 minutes

Overview

Questions

  • What is a query?
  • How do you query databases using SQL?
  • How do you retrieve unique values in SQL?
  • How do you sort results in SQL?

Objectives

  • Understand how SQL can be used to query databases
  • Understand how to build queries, using SQL keywords such as DISTINCT and ORDER BY

What is a query?

A query is a question or request for data. For example, “How many journals does our library subscribe to?”. When we query a database, we can ask the same question using Structured Query Language (SQL) in what is called a statement. Some of the most useful queries - the ones we are introducing in this first section - are used to return results from a table that match specific criteria.

Writing my first query

Let’s start by opening DB Browser for SQLite and the doaj-article-sample database (see Setup). Click “Browse Data” and select the articles table in the “Table” dropdown menu. The articles table contains columns or fields such as Title, Authors, DOI, URL, etc.

Let’s write a SQL query that selects only the Title column from the articles table.

SQL 

SELECT title
FROM articles;

Capitalization and good style

In the first query above, we have capitalized the words SELECT and FROM because they are SQL keywords. Even though capitalization makes no difference to the SQL interpreter, capitalization of these SQL terms helps for readability and is therefore considered good style. As you write and expand your own queries, it might be helpful to pick an option, such as CamelCase, and use that style when naming tables and columns. Some tables and columns require capitalization and some do not. An occasional change of capitalization for these table and column names may be needed.

Example:

SQL 

SELECT Title, Authors, ISSNs, Year
FROM Articles;

instead of

SQL 

SELECT Title, authors, ISSNs, Year
FROM articles;

If we want more information, we can add a new column to the list of fields right after SELECT:

SQL 

SELECT Title, Authors, ISSNs, Year, DOI
FROM articles;

Or we can select all of the columns in a table using the wildcard *:

SQL 

SELECT *
FROM articles;

Unique values

There may be a situation when you need to retrieve unique records and not multiple duplicate records. The SQL DISTINCT keyword is used after SELECT to eliminate duplicate records and fetch only unique records. Let’s return all of the unique ISSNs in a SQL query.

SQL 

SELECT DISTINCT ISSNs
FROM articles;

Note, some database systems require a semicolon ; after each SQL statement. If we select more than one column, then the distinct pairs of values are returned.

SQL 

SELECT DISTINCT ISSNs, Day, Month, Year
FROM articles;

Sorting

We can also sort the results of our queries by using the keyword ORDER BY. Let’s create a query that sorts the articles table in ascending order by ISSNs using the ASC keyword in conjunction with ORDER BY.

SQL 

SELECT *
FROM articles
ORDER BY ISSNs ASC;

The keyword ASC tells us to order it in ascending order. Instead, we can use DESC to get the descending order sorting by First_Author.

SQL 

SELECT *
FROM articles
ORDER BY First_Author DESC;

ASC is the default, so by omitting ASC or DESC, SQLite will sort ascending (ASC).

We can also sort on several fields at once, in different directions. For example, we can order by ISSNs descending and then First_Author ascending in the same query.

SQL 

SELECT *
FROM articles
ORDER BY ISSNs DESC, First_Author ASC;

Challenge

Write a query that returns Title, First_Author, ISSNs and Citation_Count from the articles table, ordered by the top cited article and alphabetically by title.

SQL 

SELECT Title, First_Author, ISSNs, Citation_Count
FROM articles
ORDER BY Citation_Count DESC, Title ASC;

Key Points

  • SQL is ideal for querying databases
  • SQL queries have a basic query structure starting with SELECT field FROM table with additional keywords and criteria that can be used.