Selecting Data

Overview

Teaching: 10 min
Exercises: 5 min

Questions

• How can I get data from a database?

Objectives

• Explain the difference between a table, a record, and a field.

• Explain the difference between a database and a database manager.

• Write a query to select all values for specific fields from a single table.

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.

Changing Database Managers

Many database managers — Oracle, IBM DB2, PostgreSQL, MySQL, Microsoft Access, and SQLite — understand SQL but each stores data in a different way, so a database created with one cannot be used directly by another. However, every database manager can import and export data in a variety of formats like .csv, SQL, so it is possible to move information from one to another.

Getting Into and Out Of SQLite

In order to use the SQLite commands interactively, we need to enter into the SQLite console. So, open up a terminal, and run

$ cd /path/to/survey/data/
$ sqlite3 survey.db

The SQLite command is sqlite3 and you are telling SQLite to open up the survey.db. You need to specify the .db file, otherwise SQLite will open up a temporary, empty database.

To get out of SQLite, type out .exit or .quit. For some terminals, Ctrl-D can also work. If you forget any SQLite . (dot) command, type .help.

Before we get into using SQLite to select the data, let’s take a look at the tables of the database we will use in our examples:

Notice that three entries — one in the Visit table, and two in the Measurement table — don’t contain any actual data, but instead have a special -null- entry: we’ll return to these missing values later.

Checking If Data is Available

On the shell command line, change the working directory to the one where you saved survey.db. If you saved it at your Desktop you should use

$ cd Desktop
$ ls | grep survey.db

survey.db

If you get the same output, you can run

$ sqlite3 survey.db

SQLite version 3.8.8 2015-01-16 12:08:06
Enter ".help" for usage hints.
sqlite>

that instructs SQLite to load the database in the survey.db file.

For a list of useful system commands, enter .help.

All SQLite-specific commands are prefixed with a . to distinguish them from SQL commands.

Type .tables to list the tables in the database.

.tables

Measurement  Person       Site         Visit

If you had the above tables, you might be curious what information was stored in each table. To get more information on the tables, type .schema to see the SQL statements used to create the tables in the database. The statements will have a list of the columns and the data types each column stores.

.schema

CREATE TABLE Person ( person_id text, personal_name text, family_name text );
CREATE TABLE Site ( site_name text, lat real, long real );
CREATE TABLE Visit ( visit_id integer, site_name text, visit_date text );
CREATE TABLE Measurement ( visit_id integer, person_id text, type text, value real );

The output is formatted as <columnName dataType>. Thus we can see from the first line that the table Person has three columns:

• person_id with type text
• personal_name with type text
• family_name with type text

Note: The available data types vary based on the database manager - you can search online for what data types are supported.

You can change some SQLite settings to make the output easier to read. First, set the output mode to display left-aligned columns. Then turn on the display of column headers.

.mode column
.header on

Alternatively, you can get the settings automatically by creating a .sqliterc file. Add the commands above and reopen SQLite. For Windows, use C:\Users\<yourusername>.sqliterc. For Linux/MacOS, use /Users/<yourusername>/.sqliterc.

To exit SQLite and return to the shell command line, you can use either .quit or .exit.

For now, let’s write an SQL query that displays scientists’ 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_name, personal_name FROM Person;

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 in upper case and the names for the table and columns in lower case, but we don’t have to: as the example below shows, SQL is case insensitive.

SeLeCt FaMiLy_NaMe, PeRsOnAl_NaMe FrOm PeRsOn;

You can use SQL’s case insensitivity to distinguish between different parts of an SQL statement. In this lesson, we use the convention of using UPPER CASE for SQL keywords (such as SELECT and FROM), Title Case for table names, and lower case for field names. Whatever casing convention you choose, please be consistent: complex queries are hard enough to read without the extra cognitive load of random capitalization.

While we are on the topic of SQL’s syntax, one aspect of SQL’s syntax that can frustrate novices and experts alike is forgetting to finish a command with ; (semicolon). When you press enter for a command without adding the ; to the end, it can look something like this:

SELECT person_id FROM Person
...>
...>

This is SQL’s prompt, where it is waiting for additional commands or for a ; to let SQL know to finish. This is easy to fix! Just type ; and press enter!

Now, 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_name, family_name FROM Person;

or even repeat columns:

SELECT person_id, person_id, person_id FROM Person;

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

SELECT * FROM Person;

Understanding CREATE statements

Use the .schema to identify column that contains integers.

Solution

.schema

CREATE TABLE Person ( person_id text, personal_name text, family_name text );
CREATE TABLE Site ( site_name text, lat real, long real );
CREATE TABLE Visit ( visit_id integer, site_name text, visit_date text );
CREATE TABLE Measurement ( visit_id integer, person_id text, type text, value real );

From the output, we see that the visit_id column in the Measurement table (3rd line) is composed of integers.

Selecting Site Names

Write a query that selects only the site_name column from the Site table.

Solution

SELECT site_name FROM Site;

site_name
DR-1
DR-3
MSK-4

Query Style

Many people format queries as:

SELECT personal_name, family_name FROM person;

or as:

select Personal_Name, Family_Name from PERSON;

What style do you find easiest to read, and why?

Key Points

• A relational database stores information in tables, each of which has a fixed set of columns and a variable number of records.

• A database manager is a program that manipulates information stored in a database.

• We write queries in a specialized language called SQL to extract information from databases.

• Use SELECT… FROM… to get values from a database table.

• SQL is case-insensitive (but data is case-sensitive).