Lab 12: SQL
Due at 11:59pm on 11/19/2015.
Starter Files
Download lab12.zip. Inside the archive, you will find starter files for the questions in this lab, along with a copy of the OK autograder.
Submission
By the end of this lab, you should have submitted the lab with
python3 ok --submit
. You may submit more than once before the
deadline; only the final submission will be graded.
- To receive credit for this lab, you must complete Questions 2, 3, 4, 5, and 6 in lab12.sql and submit through OK.
- The rest are questions that are considered extra practice. They can be found in the lab12_extra.sql file. It is recommended that you complete these problems on your own time.
Setup
The simplest way to start using SQLite is to download a precompiled binary from the SQLite website. The latest version of SQLite at the time of writing is 3.9.2, but you can check for additional updates on the website.
Windows
- Visit the download page linked above and navigate to the section Precompiled Binaries for Windows. Click on the link sqlite-shell-win32-x86-*.zip to download the binary.
- Unzip the file. There should be a
sqlite3.exe
file in the directory after extraction. Navigate to the folder containing the
sqlite3.exe
file and check that the version is at least 3.8.3:$ cd path/to/sqlite $ ./sqlite3 --version 3.9.2 2015-11-02 18:31:15 bda77dda9697c463c3d0704014d51627fceee328
Mac OS X Yosemite (10.10) or El Capitan (10.11)
SQLite comes pre-installed. Check that you have a version that's greater than 3.8.3:
$ sqlite3
SQLite version 3.8.5
Mac OS X Mavericks (10.9) or older
SQLite comes pre-installed, but it is the wrong version.
- Visit the download page linked above and navigate to the section Precompiled Binaries for Mac OS X (x86). Click on the link sqlite-shell-osx-x86-*.zip to download the binary.
- Unzip the file. There should be a
sqlite3
file in the directory after extraction. Navigate to the folder containing the
sqlite3
file and check that the version is at least 3.8.3:$ cd path/to/sqlite $ ./sqlite3 --version 3.9.2 2015-11-02 18:31:15 bda77dda9697c463c3d0704014d51627fceee328
Ubuntu
The easiest way to use SQLite on Ubuntu is to install it straight from the native repositories (the version will be slightly behind the most recent release):
$ sudo apt-get install sqlite3
$ sqlite3 --version
3.8.6 2014-08-15 11:46:33 9491ba7d738528f168657adb43a198238abde19e
Usage
Note: If you downloaded a precompiled binary above, make sure that
sqlite3.exe
file is in the same directory as your.sql
file. (Extract and move it out from the zip file you downloaded.)
After writing your code in the .sql
file, you can test and verify
your output in Terminal or Git Bash with one of the two following
commands.
1.) Runs your code and then exits SQLite immediately afterwards.
Ubuntu / Mac OS X (Yosemite or newer)
sqlite3 < lab12.sql
Windows / Mac OS X (Mavericks or older)
./sqlite3 < lab12.sql
2.) Runs your code and keeps SQLite open for further commands, which
is similar to running Python code with the interactive -i
flag.
You can type .help
to see some of the commands you can run.
Ubuntu / Mac OS X (Yosemite or newer)
sqlite3 --init lab12.sql
Windows / Mac OS X (Mavericks or older)
./sqlite3 --init lab12.sql
To exit out of SQLite after using the second command, you can hit
Ctrl-D
type .exit
or .quit
.
SQL Basics
Creating Tables
Two ways of creating tables in SQL are from scratch or from existing tables.
Below creates the table from scratch, without referencing any other existing tables.
CREATE TABLE [table_name] as
SELECT [val1] as [column1], [val2] as [column2], ... UNION
SELECT [val3] , [val4] , ... UNION
SELECT [val5] , [val6] , ...;
Note: You do not need to repeat the
as
keyword in subsequent SELECT statements when creating the table.
Here is an example where we construct a table with the CREATE TABLE
statement:
CREATE TABLE Football as
SELECT 30 as Berkeley, 7 as Stanford, 2002 as Year UNION
SELECT 28, 16, 2003 UNION
SELECT 17, 38, 2014;
Here we have created a table called Football
, which has three attributes
(columns): Berkeley
, Stanford
, and Year
. We can later access the
values from this table by referencing the table's columns.
To create tables from existing tables, the SELECT
command references
another table.
Selecting From Tables
More commonly, we will create new tables by selecting specific columns
that we want from existing tables. SELECT
statements can include optional
clauses such as:
FROM
: tells SQL which tables to select values fromWHERE
: filters by some conditionORDER BY
: enforces an ordering by some attribute or attributes (usually a column or columns)LIMIT
: limits the number of rows in the output tableSELECT [columns] FROM [tables] WHERE [condition] ORDER BY [attributes] LIMIT [limit]
Notes about the arguments:
[columns]
: a comma separated list of the columns to select,*
can be used to select all of them[tables]
: a comma separated list of tables to select values from[condition]
: a Boolean expression[attributes]
: a comma separated list of attributes, which are usually columns, but could also be named aggregates (which we will learn later)[limit]
: an integer
We can select all the values of an attribute from a table with the SELECT
statement. In addition, we can apply a filter using the WHERE
clause. Here,
we filter by Year > 2002
, which makes the SELECT
statement keep only the
rows in the table whose Year
value is greater than 2002.
sqlite> SELECT Berkeley FROM Football WHERE Year > 2002;
17
28
Here we selected Berkeley's score for all years after 2002.
Expressions in SQL
Here are some fundamental operations you can perform:
- comparisons:
=
,>
,<
,<=
,>=
,<>
("not equal") - booleans:
and
,or
- arithmatic:
+
,-
,*
,/
We can also perform string concatenation using ||
, which behaves
similarly to +
on strings in Python.
sqlite> select "hello" || " " || "world"
hello world
Getting to Know Your Fellow 61A Students
In the past week, we asked you and your fellow students to complete a brief online survey through Google Forms, which involved relatively random but fun questions. In this lab, we will interact with the results of the survey by using SQL queries to see if we can find interesting things in the data.
First, take a look at data.sql
and examine the tables defined in it. Note
their structures. There are two tables you will be working with:
students
: The main results of the survey. Each column represents a different question from the survey, except for the first column, which is the time of when the result was submitted. This time is a unique identifier for each of the rows in the table.Column Name Question time
The unique timestamp that identifies the submission number
What's your favorite number between 1 and 100? color
What is your favorite color? seven
Choose the number 7 below.
Options:- 7
- You are not the boss of me!
- I do what I want.
- I'm a rebel
- Choose this option instead.
- YOLO!
song
If you could listen to only one of these songs for the rest of your life, which would it be?
Options:- "Hotline Bling" by Drake
- "I Want It That Way" by Backstreet Boys
- "Shake It Off" by Taylor Swift
- "Baby" by Justin Bieber
- "Sandstorm" by Darude
- "Hello" by Adele
- "Thinking Out Loud" by Ed Sheeran
date
Pick a day of the year! pet
If you could have any animal in the world as a pet, what would it be? denero
Choose your favorite photo of John Denero! (Options shown under Question 1) smallest
Try to guess the smallest unique positive INTEGER that anyone will put! checkboxes
: The results from the survey in which students could select more than one option from the numbers listed, which ranged from 1 to 10 and included 2015, 9000, and 9001. Each row has a time (which is again a unique identifier) and has the value'True'
if the student selected the column or'False'
if the student did not. The column names in this table are the following strings, referring to each possible number:'0'
,'1'
,'2'
,'4'
,'5'
,'6'
,'7'
,'8'
,'9'
,'10'
,'2015'
,'9000'
,'9001'
.
A time in students
matches up with a time in checkboxes
. For
example, the row with time "11/11/2015 9:54:03" in students
matches up with
the row with time "11/11/2015 9:54:03" in checkboxes
. These entries come from
the same Google form submission and thus belong to the same student. We used
time to uniquely identify each student, rather than using their name or email.
Note: If you are looking for your personal response within the data, you may have noticed that some of your answers are slightly different from what you inputted. In order to make SQLite accept our data, and to optimize for as many matches as possible during our joins, we did the following things to clean up the data:
number
andsmallest
: If you did not input a number, we put the number-1
in as a placeholder.color
andpet
: We converted all the strings to be completely lowercase.
You will write all of your solutions in the starter file provided. As with other labs, you can test your solutions with OK. In addition, you can use either of the following commands. You may need to refer to the Usage section to find the appropriate command for your OS:
sqlite3 < lab12.sql
sqlite3 --init lab12.sql
Question 1: What Would SQL print?
First, load the tables into sqlite3. If you're on Windows or Mac OS X (Mavericks or older), use the following command:
$ ./sqlite3 --init lab12.sql
If you're on Ubuntu or Mac OS X (Yosemite or newer), use:
$ sqlite3 --init lab12.sql
Before we start, inspect the schema of the tables that we've created for you:
sqlite> .schema
This tells you the name of each of our tables and their attributes.
Let's also take a look at some of the entries in our table. There are a lot of entries though, so let's just output the first 20:
sqlite> SELECT * FROM students LIMIT 20;
If you're curious about some of the answers students put into the Google form,
open up data.sql
in your favorite text editor and take a look!
For each of the SQL queries below, decide to yourself and/or your partner what the query is looking for, then try running the query yourself and see!
sqlite> SELECT * FROM students; -- This is a comment. * is shorthand for all columns!
______selects all records from students;
sqlite> SELECT color FROM students WHERE number = 16;
______selects the color from students who said their favorite number was 16;
sqlite> SELECT song, pet FROM students WHERE color = "blue" AND date = "12/25";
______selects the song and pet from students who said their favorite color was blue and picked December 25th;
Question 2: Obedience
To warm-up, let's ask a simple question related to our data: Is there a correlation between whether students do as they're told and their favorite image of John DeNero?
Write a SQL query to create a table that contains the columns seven
(this
column representing "obedience") and denero
(the image students selected)
from the students
table.
You should get the following output:
sqlite> SELECT * FROM obedience LIMIT 10;
7|Image 1
7|Image 2
Choose this option instead.|Image 2
7|Image 5
Choose this option instead.|Image 5
7|Image 5
7|Image 2
7|Image 4
I'm a rebel|Image 4
YOLO!|Image 5
CREATE TABLE obedience as
-- REPLACE THIS LINE
SELECT 'YOUR CODE HERE';
SELECT seven, denero FROM students;
Use OK to test your code:
python3 ok -q obedience
Question 3: Go Bears! (And Dogs?)
Now that we have learned how to select columns from a SQL table, let's filter the results to see some more interesting results!
It turns out that 61A students have a lot of school spirit: the most popular favorite
color was 'blue'
. You would think that this school spirit would carry over to the
pet answer, and everyone would want a pet bear! Unfortunately, this was not the case,
and the majority of students opted to have a pet 'dog'
instead. That is the more
sensible choice, I suppose...
Write a SQL query to create a table that contains both the column color
and the
column pet
, using the keyword WHERE
to restrict the answers to the most popular
results of color being 'blue'
and pet being 'dog'
.
You should get the following output:
sqlite> SELECT * FROM blue_dog;
blue|dog
blue|dog
blue|dog
blue|dog
blue|dog
blue|dog
blue|dog
blue|dog
blue|dog
blue|dog
blue|dog
blue|dog
blue|dog
blue|dog
blue|dog
blue|dog
blue|dog
blue|dog
blue|dog
CREATE TABLE blue_dog as
-- REPLACE THIS LINE
SELECT 'YOUR CODE HERE';
SELECT color, pet FROM students WHERE color = 'blue' AND pet = 'dog';
Use OK to test your code:
python3 ok -q bluedog
Question 4: The Smallest Unique Integer
Who successfully managed to guess the smallest unique integer value? Let's find out!
Unfortunately we have not learned how to do aggregations, which can help us count the number of times a specific value was selected, in SQL just yet. As such, we can only hand inspect our data to determine it. However, an anonymous elf has informed us that the smallest unique value is greater than 6!
Write a SQL query with the columns time
and smallest
to try to determine what
the smallest integer value is. In order to make it easier for us to inspect these
values, use ORDER BY
to sort the numerical values, and LIMIT
your result to
the first 20 values that are greater than the number 6.
The first 5 lines of your output should look like this:
sqlite> SELECT * FROM smallest_int LIMIT 5;
11/11/2015 10:01:03|7
11/11/2015 13:53:36|7
11/11/2015 14:52:07|7
11/11/2015 15:36:00|7
11/11/2015 15:46:03|7
CREATE TABLE smallest_int as
-- REPLACE THIS LINE
SELECT 'YOUR CODE HERE';
SELECT time, smallest FROM students WHERE smallest > 6 ORDER BY smallest LIMIT 20;
Use OK to test your code:
python3 ok -q smallest-int
After you've successfully passed the OK test, actually take a look at the table
smallest_int
that you just created and find the smallest unique integer value!
To do this, try the following:
$ sqlite3 --init lab12.sql
sqlite> SELECT * FROM smallest_int; -- No LIMIT this time!
Joins
We can use joins to include rows from another table that satisfy the where
predicate. Joins can either be on different tables, or the same table if we
include an alias. Here we are referencing the football table twice, once as
the alias a
and once as the alias b
.
sqlite> SELECT a.Berkeley - b.Berkeley, a.Stanford - b.Stanford, a.Year, b.Year
...> FROM Football as a, Football as b WHERE a.Year > b.Year;
-11|22|2014|2003
-13|21|2014|2002
-2|9|2003|2002
What is this query asking for?
You may notice that it does not seem like we actually performed any operations
to do the join. However, the join is implicit in the fact that we listed more
than one table after the FROM. In this example, we joined the table Football
with itself and gave each instance of the table an alias, a
and b
so that
we could distinctly refer to each table's attributes and perform selections and
comparisons on them, such as a.Year > b.Year
.
One way to think of a join is that it produces a cross-product between the two tables by matching each row from the first table with every other row in the second table, which creates a new, larger joined table.
Here's an illustration of what happened in the joining process during the above query.
From here, the select statement examines the joined table and selects the values
it desires: a.Berkeley - b.Berkeley
and a.Stanford - b.Stanford
but only
from the rows WHERE a.Year > b.Year
. This prevents duplicate results from
appearing in our output!
Question 5: Sevens
Let's take a look at data from both of our tables, students
and checkboxes
,
to find out if students that really like the number 7 also chose '7'
for the
obedience question. Specifically, we want to look at the students that fulfill
the below conditions and see if they also chose '7'
in the question that asked
students to choose the number 7 (column seven
in students
).
- reported that their favorite number (column
number
instudents
) was 7 - have
'True'
in column'7'
incheckboxes
, meaning they checked the number7
during the survey
In order to examine rows from both the students
and the checkboxes
table, we will
need to perform a join.
How would you specify the WHERE
clause to make the SELECT
statement only consider
rows in the joined table whose values all correspond to the same student? If
you find that your output is massive and overwhelming, then you are probably missing
the necessary condition in your WHERE
clause to ensure this.
Note: The columns in the
checkboxes
table are strings with the associated number, so you must put quotes around the column name to refer to it. For example if you alias the table asa
, to get the column to see if a student checked 9001, you must writea.'9001'
.
Write a SQL query to create a table with just the column seven
from students
, filtering
first for students who said their favorite number (column number
) was 7 in the students
table and who checked the box for seven (column '7'
) in the checkboxes
table.
You should get the following output:
sqlite> SELECT * FROM sevens;
7
7
7
Choose this option instead.
I do what I want.
Choose this option instead.
7
YOLO!
You are not the boss of me!
Choose this option instead.
7
Choose this option instead.
You are not the boss of me!
7
CREATE TABLE sevens as
-- REPLACE THIS LINE
SELECT 'YOUR CODE HERE';
SELECT s.seven FROM students as s, checkboxes as c WHERE s.number = 7 AND c.'7' = 'True' AND s.time = c.time;
Use OK to test your code:
python3 ok -q sevens
Question 6: Matchmaker, Matchmaker
Did you take 61A with the hope of finding your soul mate? Well you're in luck! With all this data in hand, it's easy for us to find your perfect match. If two students want the same pet and have the same taste in music, they are clearly meant to be together! In order to provide some more information for the potential lovebirds to converse about, let's include the favorite colors of the two individuals as well!
In order to match up students, you will have to do a join on the students
table
with itself. When you do a join, SQLite will match every single row with every single
other row, so make sure you do not match anyone with themselves, or match any given pair
twice!
Hint: You may want to enforce a sort of "ordering" on the column time
(which
is a unique identifier) from your joined tables, in order to do the above correctly.
Write a SQL query to create a table that has 4 columns:
- The shared preferred pet of the couple
- The shared favorite song of the couple
- The favorite color of the first person
- The favorite color of the second person
You should get the following output:
sqlite> SELECT * FROM matchmaker LIMIT 20;
dog|Shake It Off|blue|pink
dog|Shake It Off|blue|blue
dog|Shake It Off|blue|blue
dog|Shake It Off|blue|red
dog|Shake It Off|blue|orange
dog|Shake It Off|blue|blue
dog|Shake It Off|blue|blue
dog|Shake It Off|blue|blue
rabbit|Hello|turquoise|blue
john denero|Hotline Bling|teal|orange
john denero|Hotline Bling|teal|seafoam
dragon|Sandstorm|dark blue|blue
dragon|Sandstorm|dark blue|blu
dog|Hotline Bling|grey|red
dog|Hotline Bling|grey|green
dog|Hello|green|green
dog|Hello|green|white
dog|Hello|green|blue
dog|Hello|green|black
dog|Hello|green|blue
CREATE TABLE matchmaker as
-- REPLACE THIS LINE
SELECT 'YOUR CODE HERE';
SELECT a.pet, a.song, a.color, b.color FROM students AS a, students AS b WHERE a.time < b.time AND a.pet = b.pet AND a.song = b.song;
Use OK to test your code:
python3 ok -q matchmaker
Extra Questions
The following questions are for extra practice — they can be found in the lab12_extra.sql file. It is recommended that you complete these problems as well, but you do not need to turn them in for credit.
The COUNT aggregate
Recall how finding the smallest integer anyone chose was rather painful, because we could not simply count up how many times each integer was chosen by anyone.
Bring in SQL aggregation, which is commonly used to aggregate values in order to answer these types of questions!
In order to perform SQL aggregation, we need to group rows in our table by one or more attributes. Once
we have groups, we can aggregate over the groups in our table and find things like the maximum value (MAX
),
the minimum value (MIN
), the number of rows in the group (COUNT
), the average over all of the values
(AVG
), and more! SELECT
statements that use aggregation are marked by two things: an aggregate function
(MAX
, MIN
, COUNT
, AVG
, etc.) and a GROUP BY
clause. For example:
sqlite> SELECT song, MAX(number) FROM students GROUP BY song;
|69
Baby|76
Hello|100
Hotline Bling|100
Sandstorm|100
Shake It Off|98
That Way|100
Thinking Out Loud|99
This SELECT
statement groups all of the rows in our table students
by song
. Then, within each
group, we perform aggregation by MAXing over the attribute number
. By selecting song
and MAX(number)
,
we then can see the highest number
any student chose for any given song
.
Question 7: The Smallest Unique Integer (Part 2)
Now, let's revisit the previous problem of finding the smallest integer that anyone chose, and take
a closer look at the COUNT
aggregate.
Just like MAX
above, we can select a COUNT
of some attribute in our query after grouping the rows
in our table by some attribute.
Write a SQL query that uses the COUNT
aggregate to create a table that pairs the attribute smallest
with
the number of times it was chosen by a student (this is the aggregation part). In order to cut out the
people who chose not to respond, and the sneaky cheaters that tried to put small non-integer values,
limit your results to the number 1 and greater!
Hint: You may find that there isn't a particular attribute you should have to perform the COUNT
aggregation over. If you are only interested in counting the number of rows in a group, you can just
say COUNT(*)
.
Hint: Think about what attribute you need to GROUP BY
.
After you've defined your table, you should get something like:
sqlite> SELECT * FROM smallest_int_count LIMIT 10;
1|190
2|18
3|21
4|7
5|5
6|7
7|7
8|2
9|1
10|3
CREATE TABLE smallest_int_count as
-- REPLACE THIS LINE
SELECT 'YOUR CODE HERE';
SELECT smallest, COUNT(*) FROM students WHERE smallest >= 1 GROUP BY smallest;
Use OK to test your code:
python3 ok -q smallest-int-count
It looks like the number 9
only had one person choose it! Were you the lucky student that put
it down?