Selecting Data

Introduction

Now that you've gotten a brief introduction to SQL, its time to get some hands-on practice connecting to a database via Python and executing some queries.

Objectives

You will be able to:

Connect to a SQL database using Python
Retrieve all information from a SQL table
Retrieve a subset of records from a table using a WHERE clause
Write SQL queries to filter and order results
Retrieve a subset of columns from a table

Connecting to a Database

First, let's connect to our database by importing sqlite3 and running the following cell in our notebook. You'll need a cursor object (cur) to fetch results. Cursor objects allow you to keep track of which result set is which since it's possible to run multiple queries before you're done fetching the results of the first.

import sqlite3
conn = sqlite3.connect('data.sqlite')
cur = conn.cursor()

# connect database and create cursor here
import sqlite3 
conn = sqlite3.connect('data.sqlite')
cur = conn.cursor()

Schema Overview

The database that you've just connected to is the same database from the previous introduction. Here's an overview of the database:

Querying Via the Connection

Now that you're connected to the database, let's take a look at how you can query the data within.

With your cursor object, you can execute queries

cur.execute("""SELECT * FROM employees LIMIT 5;""")

<sqlite3.Cursor at 0x111deb420>

The execute command itself only returns the cursor object. To see the results, you must use the fetchall method afterwards.

cur.fetchall()

[('1002',
  'Murphy',
  'Diane',
  'x5800',
  'dmurphy@classicmodelcars.com',
  '1',
  '',
  'President'),
 ('1056',
  'Patterson',
  'Mary',
  'x4611',
  'mpatterso@classicmodelcars.com',
  '1',
  '1002',
  'VP Sales'),
 ('1076',
  'Firrelli',
  'Jeff',
  'x9273',
  'jfirrelli@classicmodelcars.com',
  '1',
  '1002',
  'VP Marketing'),
 ('1088',
  'Patterson',
  'William',
  'x4871',
  'wpatterson@classicmodelcars.com',
  '6',
  '1056',
  'Sales Manager (APAC)'),
 ('1102',
  'Bondur',
  'Gerard',
  'x5408',
  'gbondur@classicmodelcars.com',
  '4',
  '1056',
  'Sale Manager (EMEA)')]

It's also possible to combine the previous two cells into one line, like so:

## Uncomment cell to display contents:

# cur.execute("""SELECT * FROM employees LIMIT 5;""").fetchall()

Quick note on formatting syntax:

When working with strings, you may have previously seen a 'string', a "string", a '''string''', or a """string""". While all of these are strings, the triple quotes have the added functionality of being able to use multiple lines within the same string. Sometimes, SQL queries can be much longer than others, in which case it's helpful to use new lines for readability. Here's a short example:

## Uncomment cell to display contents:

# cur.execute("""SELECT * 
#                FROM employees 
#                LIMIT 5;""").fetchall()

Wrapping Results Into Pandas DataFrames

Often, a more convenient output will be to turn these results into pandas DataFrames. To do this, you simply wrap the c.fetchall() output with a pandas DataFrame constructor:

import pandas as pd

cur.execute("""SELECT * FROM employees LIMIT 5;""")
df = pd.DataFrame(cur.fetchall())
df.head()

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	0	1	2	3	4	5	6	7
0	1002	Murphy	Diane	x5800	dmurphy@classicmodelcars.com	1		President
1	1056	Patterson	Mary	x4611	mpatterso@classicmodelcars.com	1	1002	VP Sales
2	1076	Firrelli	Jeff	x9273	jfirrelli@classicmodelcars.com	1	1002	VP Marketing
3	1088	Patterson	William	x4871	wpatterson@classicmodelcars.com	6	1056	Sales Manager (APAC)
4	1102	Bondur	Gerard	x5408	gbondur@classicmodelcars.com	4	1056	Sale Manager (EMEA)

Sadly as you can see this is slightly clunky as we do not have the column names.

We can access the column names by calling cur.description, like so:

cur.execute("""SELECT * FROM employees LIMIT 5;""")
df = pd.DataFrame(cur.fetchall())
df.columns = [x[0] for x in cur.description]
df.head()

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	employeeNumber	lastName	firstName	extension	email	officeCode	reportsTo	jobTitle
0	1002	Murphy	Diane	x5800	dmurphy@classicmodelcars.com	1		President
1	1056	Patterson	Mary	x4611	mpatterso@classicmodelcars.com	1	1002	VP Sales
2	1076	Firrelli	Jeff	x9273	jfirrelli@classicmodelcars.com	1	1002	VP Marketing
3	1088	Patterson	William	x4871	wpatterson@classicmodelcars.com	6	1056	Sales Manager (APAC)
4	1102	Bondur	Gerard	x5408	gbondur@classicmodelcars.com	4	1056	Sale Manager (EMEA)

Check out the documentation for more info on cursor methods and attributes here.

The Where Clause

In general, the WHERE clause filters query results by some condition. As you are starting to see, you can also combine multiple conditions.

Selecting Customers From a Specific City

cur.execute("""SELECT * FROM customers WHERE city = 'Boston';""")
df = pd.DataFrame(cur.fetchall())
df.columns = [x[0] for x in cur.description]
df

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	customerNumber	customerName	contactLastName	contactFirstName	phone	addressLine1	addressLine2	city	state	postalCode	country	salesRepEmployeeNumber	creditLimit
0	362	Gifts4AllAges.com	Yoshido	Juri	6175559555	8616 Spinnaker Dr.		Boston	MA	51003	USA	1216	41900.00
1	495	Diecast Collectables	Franco	Valarie	6175552555	6251 Ingle Ln.		Boston	MA	51003	USA	1188	85100.00

Selecting Multiple Cities

cur.execute("""SELECT * FROM customers WHERE city = 'Boston' OR city = 'Madrid';""")
df = pd.DataFrame(cur.fetchall())
df.columns = [x[0] for x in cur.description]
df

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	customerNumber	customerName	contactLastName	contactFirstName	phone	addressLine1	addressLine2	city	state	postalCode	country	salesRepEmployeeNumber	creditLimit
0	141	Euro+ Shopping Channel	Freyre	Diego	(91) 555 94 44	C/ Moralzarzal, 86		Madrid		28034	Spain	1370	227600.00
1	237	ANG Resellers	Camino	Alejandra	(91) 745 6555	Gran Vía, 1		Madrid		28001	Spain		0.00
2	344	CAF Imports	Fernandez	Jesus	+34 913 728 555	Merchants House	27-30 Merchant's Quay	Madrid		28023	Spain	1702	59600.00
3	362	Gifts4AllAges.com	Yoshido	Juri	6175559555	8616 Spinnaker Dr.		Boston	MA	51003	USA	1216	41900.00
4	458	Corrida Auto Replicas, Ltd	Sommer	Martín	(91) 555 22 82	C/ Araquil, 67		Madrid		28023	Spain	1702	104600.00
5	465	Anton Designs, Ltd.	Anton	Carmen	+34 913 728555	c/ Gobelas, 19-1 Urb. La Florida		Madrid		28023	Spain		0.00
6	495	Diecast Collectables	Franco	Valarie	6175552555	6251 Ingle Ln.		Boston	MA	51003	USA	1188	85100.00

The Order By and Limit Clauses

Two additional keywords that you can use to refine your searches are the ORDER BY and LIMIT clauses. The order by clause allows you to sort the results by a particular feature. For example, you could sort by the customerName column if you wished to get results in alphabetical order. By default, ORDER BY is ascending. So, as with the above example, if you want the opposite, use the additional parameter DESC. Finally, the limit clause is typically the last argument in a SQL query and simply limits the output to a set number of results.

Selecting Specific Columns With Complex Criteria

cur.execute("""SELECT customerNumber, customerName, city, creditLimit
               FROM customers
               WHERE (city = 'Boston' OR city = 'Madrid') AND (creditLimit >= 50000.00)
               ORDER BY creditLimit DESC
               LIMIT 15
               ;""")
df = pd.DataFrame(cur.fetchall())
df.columns = [x[0] for x in cur.description]
df

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	customerNumber	customerName	city	creditLimit
0	495	Diecast Collectables	Boston	85100.00
1	344	CAF Imports	Madrid	59600.00
2	362	Gifts4AllAges.com	Boston	41900.00
3	141	Euro+ Shopping Channel	Madrid	227600.00
4	458	Corrida Auto Replicas, Ltd	Madrid	104600.00
5	237	ANG Resellers	Madrid	0.00
6	465	Anton Designs, Ltd.	Madrid	0.00

You might notice that the output of this query doesn't seem to respect our credit limit criterion. A little investigation shows that this is because the number is actually stored as a string!

type(df.creditLimit.iloc[0])

str

This is an annoying problem to encounter and also underlines the importance of setting up a database in an appropriate manner at the get-go. For now, it's time to practice some of your SQL querying skills!

Summary

In this lesson, you saw how to connect to a SQL database via Python and how to subsequently execute queries against that database. Going forward, you'll continue to learn additional keywords for specifying your query parameters!

HeeebsInc/dsc-selecting-data-wdc01-dtsc-ft-060120

Selecting Data

Introduction

Objectives

Connecting to a Database

Schema Overview

Querying Via the Connection

Quick note on formatting syntax:

Wrapping Results Into Pandas DataFrames

The Where Clause

Selecting Customers From a Specific City

Selecting Multiple Cities

The Order By and Limit Clauses

Selecting Specific Columns With Complex Criteria

Summary