Selecting and sorting data
OverviewTeaching: 15 min
Exercises: 5 minQuestions
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
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 a common language called Structured Query Language or 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). Choose
Browse Data and the
articles table. The articles table contains columns or fields such as
Let’s write a SQL query that selects only the
Title column from the
SELECT title FROM articles;
We have capitalised the words
FROM because they are SQL keywords. This makes no difference to the SQL interpreter as it is case-insensitive, but it helps for readability and is therefore considered good style.
If we want more information, we can add a new column to the list of fields right after
SELECT Title, Authors, ISSNs, Year FROM articles;
Or we can select all of the columns in a table using the wildcard
SELECT * FROM articles;
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.
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.
SELECT DISTINCT ISSNs, Day, Month, Year FROM articles;
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 alphabetically by ISSNs using the
ASC keyword in conjunction with
SELECT * FROM articles ORDER BY ISSNs ASC;
ASC tells us to order it in ascending order. Instead, we can use
DESC to get the descending order sorting by
SELECT * FROM articles ORDER BY First_Author DESC;
ASC is the default, so by omitting
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.
SELECT * FROM articles ORDER BY ISSNs DESC, First_Author ASC;
Write a query that returns
Citation_Countfrom the articles table, ordered by the top cited article and alphabetically by title.
SELECT Title, First_Author, ISSNs, Citation_Count FROM articles ORDER BY Citation_Count DESC, Title ASC;
SQL is ideal for querying databases
SQL queries have a basic query structure starting with
SELECTfield FROM table with additional keywords and criteria that can be used.