380 lines
14 KiB
SQL
380 lines
14 KiB
SQL
---------------------------------------------------------------------------
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-- Practical SQL: A Beginner's Guide to Storytelling with Data, 2nd Edition
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-- by Anthony DeBarros
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-- Try It Yourself Questions and Answers
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----------------------------------------------------------------------------
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----------------------------------------------------------------------------
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-- Chapter 1: Setting Up Your Coding Environment
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----------------------------------------------------------------------------
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-- There are no Try It Yourself exercises in this chapter!
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----------------------------------------------------------------------------
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-- Chapter 2: Creating Your First Database and Table
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----------------------------------------------------------------------------
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-- 1. Imagine you're building a database to catalog all the animals at your
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-- local zoo. You want one table for tracking all the kinds of animals and
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-- another table to track the specifics on each animal. Write CREATE TABLE
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-- statements for each table that include some of the columns you need. Why did
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-- you include the columns you chose?
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-- Answer (yours will vary):
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-- The first table will hold the animal types and their conservation status:
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CREATE TABLE animal_types (
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animal_type_id bigint PRIMARY KEY GENERATED ALWAYS AS IDENTITY,
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common_name text NOT NULL,
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scientific_name text NOT NULL,
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conservation_status text NOT NULL,
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CONSTRAINT common_name_unique UNIQUE (common_name)
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);
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-- It's OK if your answer doesn't have all the keywords in the example above. Those
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-- keywords reference concepts you'll learn in later chapters, including table
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-- constraints, primary keys and and IDENTITY columns. What's important is that you
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-- considered the individual data items you would want to track.
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-- The second table will hold data on individual animals. Note that the
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-- column animal_type_id references the column of the same name in the
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-- table animal types. This is a foreign key, which you will learn about in
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-- Chapter 8.
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CREATE TABLE menagerie (
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menagerie_id bigint PRIMARY KEY GENERATED ALWAYS AS IDENTITY,
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common_name text REFERENCES animal_types (common_name),
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date_acquired date NOT NULL,
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gender text,
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acquired_from text,
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name text,
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notes text
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);
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-- 2. Now create INSERT statements to load sample data into the tables.
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-- How can you view the data via the pgAdmin tool?
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-- Answer (again, yours will vary):
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INSERT INTO animal_types (common_name, scientific_name, conservation_status)
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VALUES ('Bengal Tiger', 'Panthera tigris tigris', 'Endangered'),
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('Arctic Wolf', 'Canis lupus arctos', 'Least Concern');
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-- data source: https://www.worldwildlife.org/species/directory?direction=desc&sort=extinction_status
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INSERT INTO menagerie (common_name, date_acquired, gender, acquired_from, name, notes)
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VALUES
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('Bengal Tiger', '3/12/1996', 'F', 'Dhaka Zoo', 'Ariel', 'Healthy coat at last exam.'),
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('Arctic Wolf', '9/30/2000', 'F', 'National Zoo', 'Freddy', 'Strong appetite.');
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-- To view data via pgAdmin, in the object browser, right-click Tables and
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-- select Refresh. Then right-click the table name and select
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-- View/Edit Data > All Rows
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-- 2b. Create an additional INSERT statement for one of your tables. On purpose,
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-- leave out one of the required commas separating the entries in the VALUES
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-- clause of the query. What is the error message? Does it help you find the
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-- error in the code?
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-- Answer: In this case, the error message points to the missing comma.
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INSERT INTO animal_types (common_name, scientific_name, conservation_status)
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VALUES ('Javan Rhino', 'Rhinoceros sondaicus' 'Critically Endangered');
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----------------------------------------------------------------------------
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-- Chapter 3: Beginning Data Exploration with SELECT
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----------------------------------------------------------------------------
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-- 1. The school district superintendent asks for a list of teachers in each
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-- school. Write a query that lists the schools in alphabetical order along
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-- with teachers ordered by last name A-Z.
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-- Answer:
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SELECT school, first_name, last_name
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FROM teachers
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ORDER BY school, last_name;
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-- 2. Write a query that finds the one teacher whose first name starts
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-- with the letter 'S' and who earns more than $40,000.
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-- Answer:
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SELECT first_name, last_name, school, salary
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FROM teachers
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WHERE first_name LIKE 'S%' -- remember that LIKE is case-sensitive!
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AND salary > 40000;
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-- 3. Rank teachers hired since Jan. 1, 2010, ordered by highest paid to lowest.
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-- Answer:
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SELECT last_name, first_name, school, hire_date, salary
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FROM teachers
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WHERE hire_date >= '2010-01-01'
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ORDER BY salary DESC;
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----------------------------------------------------------------------------
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-- Chapter 4: Understanding Data Types
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----------------------------------------------------------------------------
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-- 1. Your company delivers fruit and vegetables to local grocery stores, and
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-- you need to track the mileage driven by each driver each day to a tenth
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-- of a mile. Assuming no driver would ever travel more than 999 miles in
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-- a day, what would be an appropriate data type for the mileage column in your
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-- table. Why?
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-- Answer:
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numeric(4,1)
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-- numeric(4,1) provides four digits total (the precision) and one digit after
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-- the decimal (the scale). That would allow you to store a value as large
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-- as 999.9.
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-- In practice, you may want to consider that the assumption on maximum miles
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-- in a day could conceivably exceed 999.9 and go with the larger numeric(5,1).
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-- 2. In the table listing each driver in your company, what are appropriate
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-- data types for the drivers’ first and last names? Why is it a good idea to
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-- separate first and last names into two columns rather than having one
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-- larger name column?
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-- Answer:
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varchar(50)
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-- or
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text
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-- 50 characters is a reasonable length for names, and using either varchar(50)
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-- or text ensures you will not waste space when names are shorter. Using text will
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-- ensure that if you run into the exceptionally rare circumstance of a name longer
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-- than 50 characters, you'll be covered. Also, separating first and last names
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-- into their own columns will let you later sort on each independently.
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-- 3. Assume you have a text column that includes strings formatted as dates.
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-- One of the strings is written as '4//2017'. What will happen when you try
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-- to convert that string to the timestamp data type?
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-- Answer: Attempting to convert a string of text that does not conform to
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-- accepted date/time formats will result in an error. You can see this with
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-- the below example, which tries to cast the string as a timestamp.
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SELECT CAST('4//2021' AS timestamp with time zone);
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----------------------------------------------------------------------------
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-- Chapter 5: Importing and Exporting Data
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----------------------------------------------------------------------------
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-- 1. Write a WITH statement to include with COPY to handle the import of an
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-- imaginary text file that has a first couple of rows that look like this:
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id:movie:actor
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50:#Mission: Impossible#:Tom Cruise
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-- Answer: The WITH statement will need the options seen here:
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COPY actors
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FROM 'C:\YourDirectory\movies.txt'
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WITH (FORMAT CSV, HEADER, DELIMITER ':', QUOTE '#');
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-- If you'd like to try actually importing this data, save the data in a file
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-- called movies.txt and create the actors table below. You can then run the COPY
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-- statement.
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CREATE TABLE actors (
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id integer,
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movie text,
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actor text
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);
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-- Note: You may never encounter a file that uses a colon as a delimiter and
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-- pound sign for quoting, but anything is possible!
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-- 2. Using the table us_counties_pop_est_2019 you created and filled in this
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-- chapter, export to a CSV file the 20 counties in the United States that had
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-- the most births. Make sure you export only each county’s name, state, and
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-- number of births. (Hint: births are totaled for each county in the column
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-- births_2019.)
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-- Answer:
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COPY (
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SELECT county_name, state_name, births_2019
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FROM us_counties_pop_est_2019 ORDER BY births_2019 DESC LIMIT 20
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)
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TO 'C:\YourDirectory\us_counties_births_export.txt'
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WITH (FORMAT CSV, HEADER);
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-- Note: This COPY statement uses a SELECT statement to limit the output to
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-- only the desired columns and rows.
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-- 3. Imagine you're importing a file that contains a column with these values:
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-- 17519.668
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-- 20084.461
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-- 18976.335
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-- Will a column in your target table with data type numeric(3,8) work for these
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-- values? Why or why not?
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-- Answer:
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-- No, it won't. In fact, you won't even be able to create a column with that
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-- data type because the precision must be larger than the scale. The correct
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-- type for the example data is numeric(8,3).
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----------------------------------------------------------------------------
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-- Chapter 6: Basic Math and Stats with SQL
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----------------------------------------------------------------------------
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-- 1. Write a SQL statement for calculating the area of a circle whose radius is
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-- 5 inches. Do you need parentheses in your calculation? Why or why not?
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-- Answer:
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-- (The formula for the area of a circle is: pi * radius squared.)
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SELECT 3.14 * 5 ^ 2;
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-- The result is an area of 78.5 square inches.
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-- Note: You do not need parentheses because exponents and roots take precedence
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-- over multiplication. However, you could include parentheses for clarity. This
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-- statement produces the same result:
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SELECT 3.14 * (5 ^ 2);
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-- 2. Using the 2019 Census county estimates data, calculate a ratio of births to
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-- deaths for each county in New York state. Which region of the state generally
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-- saw a higher ratio of births to deaths in 2019?
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-- Answer:
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SELECT county_name,
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state_name,
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births_2019 AS births,
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deaths_2019 AS DEATHS,
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births_2019::numeric / deaths_2019 AS birth_death_ratio
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FROM us_counties_pop_est_2019
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WHERE state_name = 'New York'
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ORDER BY birth_death_ratio DESC;
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-- Generally, counties in and around New York City had the highest ratio of births
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-- to deaths in the 2019 estimates. One exception to the trend is Jefferson County,
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-- which is upstate on the U.S./Canadian border.
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-- 3. Was the 2019 median county population estimate higher in California or New York?
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-- Answer:
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-- California had a median county population estimate of 187,029 in 2019, almost double
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-- that of New York, at 86,687. Here are two solutions:
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-- First, you can find the median for each state one at a time:
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SELECT percentile_cont(.5)
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WITHIN GROUP (ORDER BY pop_est_2019)
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FROM us_counties_pop_est_2019
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WHERE state_name = 'New York';
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SELECT percentile_cont(.5)
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WITHIN GROUP (ORDER BY pop_est_2019)
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FROM us_counties_pop_est_2019
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WHERE state_name = 'California';
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-- Or both in one query (credit: https://github.com/Kennith-eng)
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SELECT state_name,
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percentile_cont(0.5)
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WITHIN GROUP (ORDER BY pop_est_2019) AS median
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FROM us_counties_pop_est_2019
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WHERE state_name IN ('New York', 'California')
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GROUP BY state_name;
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-- Finally, this query shows the median for each state:
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SELECT state_name,
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percentile_cont(0.5)
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WITHIN GROUP (ORDER BY pop_est_2019) AS median
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FROM us_counties_pop_est_2019
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GROUP BY state_name;
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----------------------------------------------------------------------------
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-- Chapter 7: Joining Tables in a Relational Database
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----------------------------------------------------------------------------
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-- 1. According to the Census population estimates, which county had the
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-- greatest percentage loss of population between 2010 and 2019? Try
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-- an Internet search to find out what happened. (Hint: The loss is related
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-- to a particular type of facility.)
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-- Answer:
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-- Concho County, Texas, lost 33 percent of its population from 2010 to
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-- 2019, the result of the closure of Eden Detention Center.
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-- https://www.texasstandard.org/stories/after-edens-prison-closes-what-comes-next-for-this-small-texas-town/
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-- Simply use ASC in the ORDER BY clause to re-order the results, like this:
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SELECT c2019.county_name,
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c2019.state_name,
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c2019.pop_est_2019 AS pop_2019,
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c2010.estimates_base_2010 AS pop_2010,
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c2019.pop_est_2019 - c2010.estimates_base_2010 AS raw_change,
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round( (c2019.pop_est_2019::numeric - c2010.estimates_base_2010)
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/ c2010.estimates_base_2010 * 100, 1 ) AS pct_change
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FROM us_counties_pop_est_2019 AS c2019
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JOIN us_counties_pop_est_2010 AS c2010
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ON c2019.state_fips = c2010.state_fips
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AND c2019.county_fips = c2010.county_fips
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ORDER BY pct_change ASC;
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-- 2. Apply the concepts you learned about UNION to create query
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-- results that merge queries of the Census county population estimates
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-- for 2010 and 2019. Your results should include a column called year
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-- that specifies the year of the estimate for each row in the results.
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-- Answer:
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-- Note that you pass a string for the year in both queries.
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SELECT '2010' AS year,
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state_fips,
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county_fips,
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county_name,
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state_name,
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estimates_base_2010 AS estimate
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FROM us_counties_pop_est_2010
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UNION
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SELECT '2019' AS year,
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state_fips,
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county_fips,
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county_name,
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state_name,
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pop_est_2019 AS estimate
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FROM us_counties_pop_est_2019
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ORDER BY state_fips, county_fips, year;
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-- 3. Using the percentile_cont() function from Chapter 6,
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-- determine the median of the percent change in estimated county
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-- population between 2010 and 2019.
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-- Answer: -0.5%
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SELECT percentile_cont(.5)
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WITHIN GROUP (ORDER BY round( (c2019.pop_est_2019::numeric - c2010.estimates_base_2010)
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/ c2010.estimates_base_2010 * 100, 1 )) AS percentile_50th
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FROM us_counties_pop_est_2019 AS c2019
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JOIN us_counties_pop_est_2010 AS c2010
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ON c2019.state_fips = c2010.state_fips
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AND c2019.county_fips = c2010.county_fips;
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