Databases and SQL - Library edition

Selecting Data

A relational database is a way to store and manipulate information. Databases are arranged as tables. Each table has columns (also known as fields) that describe the data, and rows (also known as records) which contain the data.

When we are using a spreadsheet, we put formulas into cells to calculate new values based on old ones. When we are using a database, we send commands (usually called queries) to a database manager: a program that manipulates the database for us. The database manager does whatever lookups and calculations the query specifies, returning the results in a tabular form that we can then use as a starting point for further queries.

Queries are written in a language called SQL, which stands for “Structured Query Language”. SQL provides hundreds of different ways to analyze and recombine data. We will only look at a handful of queries, but that handful accounts for most of what scientists do.

The tables below show the database we will use in our examples:

Works: table containing bibliographic details for the library’s books

Work_ID	Title	ISBN	Date	Place	Publisher	Edition	Pages
1	SQL in a nutshell	9780596518844	2009	Sebastopol	O’Reilly	3rd ed.	578
2	SQL for dummies	9781118607961	2013	Hoboken	Wiley	8th ed.	-null-
3	PHP & MySQL	9781449325572	2013	Sebastopol	O’Reilly	2nd ed.	532
4	Using SQLite	9780596521189	2010	Sebastopol	O’Reilly	1st ed.	503
5	Geek sublime	9780571310302	2014	London	Faber & Fa	-null-	258
6	Capital in the 21	9780674430006	2014	Cambridge	Belknap Pr	-null-	685
7	SQL	9780071548649	2009	New York	McGraw-Hil	3rd ed.	534
…

Items: table containing details on copies of Works owned by the library

Item_ID	Work_ID	Barcode	Acquired	Status
1	1	081722942611	2009	Loaned
2	1	492437609065	2011	On shelf
3	2	172480710952	2013	On shelf
4	3	708014968732	2013	Missing
…

Authors: table containing details on authors and contributors

Author_ID	Family	Personal	Occupation	Birth	Death
1	Kline	Kevin E.	-null-	1966	-null-
2	Kline	Daniel	-null-	-null-	-null-
3	Hunt	Brand	-null-	-null-	-null-
4	Taylor	Allen G.	-null-	1945	-null-
…

Works_Authors: table linking works with authors

Work_ID	Author_ID	Role
1	1	Author
1	2	Contributor
1	3	Contributor
2	4	Author
…

Notice that several entries don’t contain any actual data, but instead have a special -null- entry: we’ll return to these missing values later. For now, let’s write an SQL query that displays authors’ names. We do this using the SQL command SELECT, giving it the names of the columns we want and the table we want them from. Our query and its output look like this:

SELECT family, personal FROM Authors;

The semicolon at the end of the query tells the database manager that the query is complete and ready to run. We have written our commands and column names in lower case, and the table name in Title Case, but we don’t have to: as the example below shows, SQL is case insensitive.

SeLeCt FaMiLy, PeRsOnAl FrOm wORks;

You can use SQL’s case insensitivity to your advantage. For instance, some people choose to write SQL keywords (such as SELECT and FROM) in capital letters and field and table names in lower case. This can make it easier to locate parts of an SQL statement. For instance, you can scan the statement, quickly locate the prominent FROM keyword and know the table name follows. Whatever casing convention you choose, please be consistent: complex queries are hard enough to read without the extra cognitive load of random capitalization. One convention is to use UPPER CASE for SQL statements, to distinguish them from tables and column names. This is the convention that we will use for this lesson.

Going back to our query, it’s important to understand that the rows and columns in a database table aren’t actually stored in any particular order. They will always be displayed in some order, but we can control that in various ways. For example, we could swap the columns in the output by writing our query as:

SELECT personal, family FROM Authors;

or even repeat columns:

SELECT personal, personal FROM Authors;

As a shortcut, we can select all of the columns in a table using *:

SELECT * FROM Authors;

SELECT personal, family FROM authors;

select Personal, Family from AUTHORS;