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Using SQL with Pandas - Lab

Introduction

In this lab, you will practice using SQL statements and the .query() method provided by Pandas to manipulate datasets.

Objectives

You will be able to:

  • Compare accessing data in a DataFrame using query methods and conditional logic
  • Query DataFrames with SQL using the pandasql library

The Dataset

In this lab, we will continue working with the Titanic Survivors dataset.

Begin by importing pandas as pd, numpy as np, and matplotlib.pyplot as plt, and set the appropriate alias for each. Additionally, set %matplotlib inline.

# Your code here

Next, read in the data from titanic.csv and store it as a DataFrame in df. Display the .head() to ensure that everything loaded correctly.

df = None

Slicing DataFrames Using Conditional Logic

One of the most common ways to query data with pandas is to simply slice the DataFrame so that the object returned contains only the data you're interested in.

In the cell below, slice the DataFrame so that it only contains passengers with 2nd or 3rd class tickets (denoted by the Pclass column).

Be sure to preview values first to ensure proper encoding when slicing

  • Hint: Remember, your conditional logic must be passed into the slicing operator to return a slice of the DataFrame--otherwise, it will just return a table of boolean values based on the conditional statement!
# Preview values first to ensure proper encoding when slicing
no_first_class_df = None

We can also chain conditional statements together by wrapping them in parenthesis and making use of the & and | operators ('and' and 'or' operators, respectively).

In the cell below, slice the DataFrame so that it only contains passengers with a Fare value between 50 and 100, inclusive.

fares_50_to_100_df = None

We could go further and then preview the Fare column of this new subsetted DataFrame:

fares_50_to_100_df['Fare'].hist()
plt.xlabel('Fare', color='red')
plt.ylabel('Frequency', fontsize=12) 
plt.title('Distribution of Fares');

Remember that there are two syntactically correct ways to access a column in a DataFrame. For instance, df['Name'] and df.Name return the same thing.

In the cell below, use the dot notation syntax and slice a DataFrame that contains male passengers that survived that also belong to Pclass 2 or 3. Be sure to preview the column names and content of the Sex column.

# Checking column names for reference
# Checking column values to hardcode query below
poor_male_survivors_df = None

Great! Now that you've reviewed the methods for slicing a DataFrame for querying our data, let's explore a sample use case.

Practical Example: Slicing DataFrames

In this section, you're looking to investigate whether women and children survived more than men, or that rich passengers were more likely to survive than poor passengers. The easiest way to confirm this is to slice the data into DataFrames that contain each subgroup, and then quickly visualize the survival rate of each subgroup with histograms.

In the cell below, create a DataFrame that contains passengers that are female, as well as children (males included) ages 15 and under.

Additionally, create a DataFrame that contains only adult male passengers over the age of 15.

women_and_children_df = None
adult_males_df = None

Great! Now, you can use the matplotlib functionality built into the DataFrame objects to quickly create visualizations of the Survived column for each DataFrame.

In the cell below, create histogram visualizations of the Survived column for both DataFrames. Bonus points if you use plt.title() to label them correctly and make it easy to tell them apart!

# Your code here

Well that seems like a pretty stark difference -- it seems that there was drastically different behavior between the groups! Now, let's repeat the same process, but separating rich and poor passengers.

In the cell below, create one DataFrame containing First Class passengers (Pclass == 1), and another DataFrame containing everyone else.

first_class_df = None
second_third_class_df = None

Now, create histograms of the survival for each subgroup, just as you did above.

# Your code here

To the surprise of absolutely no one, it seems like First Class passengers were more likely to survive than not, while 2nd and 3rd class passengers were more likely to die than not. However, don't read too far into these graphs, as these aren't at the same scale, so they aren't fair comparisons.

Slicing is a useful method for quickly getting DataFrames that contain only the examples we're looking for. It's a quick, easy method that feels intuitive in Python, since we can rely on the same conditional logic that we would if we were just writing if/else statements.

Using the .query() method

Instead of slicing, you can also make use of the DataFrame's built-in .query() method. This method reads a bit more cleanly and allows us to pass in our arguments as a string. For more information or example code on how to use this method, see the pandas documentation.

In the cell below, use the .query() method to slice a DataFrame that contains only passengers who have a PassengerId greater than or equal to 500.

query_string = None
high_passenger_number_df = None
high_passenger_number_df.head()

Just as with slicing, you can pass in queries with multiple conditions. One unique difference between using the .query() method and conditional slicing is that you can use and or & as well as or or | (for fun, try reading this last sentence out loud), while you are limited to the & and | symbols to denote and/or operations with conditional slicing.

In the cell below, use the query() method to return a DataFrame that contains only female passengers of ages 15 and under.

Hint: Although the entire query is a string, you'll still need to denote that female is also a string, within the string. (String-Ception?)

female_children_df = None
female_children_df.head()

A cousin of the query() method, eval() allows you to use the same string-filled syntax as querying for creating new columns. For instance:

some_df.eval('C = A + B')

would return a copy of the some_df dataframe, but will now include a column C where all values are equal to the sum of the A and B values for any given row. This method also allows the user to specify if the operation should be done in place or not, providing a quick, easy syntax for simple feature engineering.

In the cell below, use the DataFrame's eval() method in place to add a column called Age_x_Fare, and set it equal to Age multiplied by Fare.

df = None
df.head()

Great! Now, let's move on the coolest part of this lab--querying DataFrames with SQL!

Querying DataFrames With SQL

For the final section of the lab, you'll make use of the pandasql library. Pandasql is a library designed to make it easy to query DataFrames directly with SQL syntax, which was open-sourced by the company, Yhat, in late 2016. It's very straightforward to use, but you are still encouraged to take a look at the documentation as needed.

If you're using the pre-built virtual environment, you should already have the package ready to import. If not, uncomment and run the cell below to pip install pandasql so that it is available to import.

# !pip install pandasql

That should have installed everything correctly. This library has a few dependencies, which you should already have installed. If you don't, just pip install them in your terminal and you'll be good to go!

In the cell below, import sqldf from pandasql.

# Your code here

Great! Now, it's time to get some practice with this handy library.

pandasql allows you to pass in SQL queries in the form of a string to directly query your database. Each time you make a query, you need to pass an additional parameter that gives it access to the other variables in the session/environment. You can use a lambda function to pass locals() or globals() so that you don't have to type this every time.

In the cell below, create a variable called pysqldf and set it equal to a lambda function q that returns sqldf(q, globals()). If you're unsure of how to do this, see the example in the documentation.

pysqldf = None

Great! That will save you from having to pass globals() as an argument every time you query, which can get a bit tedious.

Now write a basic query to get a list of passenger names from df, limit 10. If you would prefer to format your query on multiple lines and style it as canonical SQL, that's fine -- remember that multi-line strings in Python are denoted by """ -- for example:

"""
This is a 
Multi-Line String
"""

In the cell below, write a SQL query that returns the names of the first 10 passengers.

q = None

passenger_names = None
passenger_names

Great! Now, for a harder one:

In the cell below, query the DataFrame for names and fares of any male passengers that survived, limit 30.

q2 = None

sql_surviving_males = None
sql_surviving_males

This library is really powerful! This makes it easy for us to leverage all of your SQL knowledge to quickly query any DataFrame, especially when you only want to select certain columns. This saves from having to slice/query the DataFrame and then slice the columns you want (or drop the ones you don't want).

Although it's outside the scope of this lab, it's also worth noting that both pandas and pandasql provide built-in functionality for join operations, too!

Practical Example: SQL in Pandas

In the cell below, create 2 separate DataFrames using pandasql. One should contain the Pclass of all female passengers that survived, and the other should contain the Pclass of all female passengers that died.

Then, create a horizontal bar graph visualizations of the Pclass column for each DataFrame to compare the two. Bonus points for taking the time to make the graphs extra readable by adding titles, labeling each axis, and cleaning up the number of ticks on the X-axis!

# Write your queries in these variables to keep your code well-formatted and readable
q3 = None
q4 = None

survived_females_by_pclass_df = None
died_females_by_pclass_df = None

# Create and label the histograms for each below!

Summary

In this lab, you practiced how to query Pandas DataFrames using SQL.