Databases and SQL

Learning goals

After this lesson, you should be able to:

Develop comfort in composing SQL queries
See the connections between tidyverse verbs and SQL clauses

You can download a template Quarto file to start from here. Put this file in a folder called data_acquisition within a folder for this course.

Introduction to databases

If you find yourself analyzing data within a medium or large organization, you will probably draw on data stored within a centralized data warehouse.

Data warehouses contain vast collections of information–far more than a desktop or laptop computer can easily analyze. These warehouses typically rely on structured data repositories called relational databases (also often called SQL databases because of the use of structured query language (SQL) to access the data).

Connecting to a database with `DBI`

The DBI package (database interface) provides general tools for interacting with databases from R. It is also common for data scientists to interact with databases directly by writing SQL queries. We’ll talk about this in the next section.

For now, we’ll use DBI to connect with an in-process database (duckdb), one that runs locally on your computer. A nice feature of duckdb is that even if your dataset is huge, duckdb can work with it very quickly.

We can set up a database connection with dbConnect() and initialize a temporary database with duckdb():

con <- DBI::dbConnect(duckdb::duckdb())
class(con)

[1] "duckdb_connection"
attr(,"package")
[1] "duckdb"

In a real project we would use duckdb_read_csv() to store data directly into the duckdb database without first having to read it into R. In the toy example below, we have a dataset on Spotify songs (all_spotify_songs.csv) and store in a database table called "songs":

duckdb_read_csv(con, "songs", "../relative/path/to/all_spotify_songs.csv")

Here, we’ll use datasets from the nycflights13 package. The DBI package provides the dbWriteTable() function to write dataset objects to a database:

dbWriteTable(con, "flights", nycflights13::flights)
dbWriteTable(con, "planes", nycflights13::planes)

We can use tbl() to create connections individually to the flights and planes datasets.

flights <- tbl(con, "flights")
planes <- tbl(con, "planes")

Note that the results of tbl() are not quite the same as our normal data frames. Although they have class tbl, note that the number of rows is NA! The full dataset isn’t loaded into memory when we use tbl, so the number of rows is unknown. This behavior is purposeful–it reduces computer resources and allows access to parts of the data only when needed.

class(flights)

[1] "tbl_duckdb_connection" "tbl_dbi"               "tbl_sql"              
[4] "tbl_lazy"              "tbl"

dim(flights)

[1] NA 19

Warming up to SQL with `dplyr`

A really nice feature of dplyr is that we can write R code for wrangling the data and use show_query() to translate that code into SQL.

flights %>%
    show_query()

<SQL>
SELECT *
FROM flights

flights %>%
    mutate(full_date = str_c(year, month, day, sep = "-")) %>%
    show_query()

<SQL>
SELECT *, CONCAT_WS('-', "year", "month", "day") AS full_date
FROM flights

Explore: Using the code examples below, work with your group to create a dplyr<->SQL translation guide (notes document) that allows you to answer the following:

What do SELECT, FROM, WHERE, GROUP BY, and ORDER BY in SQL do? (These uppercase words are called clauses in SQL.) How do these clauses translate to the main tidyverse verbs select, mutate, filter, arrange, summarize, group_by?
What syntax differences are there for logical comparisons? (How do the & and | logical operators in R compare to SQL?)
How does the R syntax for mutate translate to SQL?
How does joining datasets seem to work in SQL?

flights %>% 
    filter(dest == "IAH") %>% 
    arrange(dep_delay) %>% 
    show_query()

<SQL>
SELECT *
FROM flights
WHERE (dest = 'IAH')
ORDER BY dep_delay

flights %>% 
    filter(dest == "IAH") %>% 
    arrange(dep_delay) %>% 
    head(n = 10) %>% 
    show_query()

<SQL>
SELECT *
FROM flights
WHERE (dest = 'IAH')
ORDER BY dep_delay
LIMIT 10

flights %>% 
    filter(dest == "IAH" & origin == "JFK") %>% 
    arrange(dep_delay) %>% 
    show_query()

<SQL>
SELECT *
FROM flights
WHERE (dest = 'IAH' AND origin = 'JFK')
ORDER BY dep_delay

flights %>% 
    filter(dest == "IAH" | origin == "JFK") %>% 
    arrange(year, month, day, desc(dep_delay)) %>% 
    show_query()

<SQL>
SELECT *
FROM flights
WHERE (dest = 'IAH' OR origin = 'JFK')
ORDER BY "year", "month", "day", dep_delay DESC

flights %>% 
    filter(dest %in% c("IAH", "HOU")) %>% 
    show_query()

<SQL>
SELECT *
FROM flights
WHERE (dest IN ('IAH', 'HOU'))

flights %>% 
    filter(!is.na(dep_delay)) %>% 
    show_query()

<SQL>
SELECT *
FROM flights
WHERE (NOT((dep_delay IS NULL)))

planes %>% 
    select(tailnum, type, manufacturer, model, year) %>% 
    show_query()

<SQL>
SELECT tailnum, "type", manufacturer, model, "year"
FROM planes

planes %>% 
    select(tailnum, type, manufacturer, model, year) %>% 
    rename(year_built = year) %>% 
    show_query()

<SQL>
SELECT tailnum, "type", manufacturer, model, "year" AS year_built
FROM planes

flights %>% 
    mutate(
        speed = distance / (air_time / 60)
    ) %>% 
    show_query()

<SQL>
SELECT *, distance / (air_time / 60.0) AS speed
FROM flights

flights %>% 
    left_join(planes, by = "tailnum") %>% 
    show_query()

<SQL>
SELECT
  flights."year" AS "year.x",
  "month",
  "day",
  dep_time,
  sched_dep_time,
  dep_delay,
  arr_time,
  sched_arr_time,
  arr_delay,
  carrier,
  flight,
  flights.tailnum AS tailnum,
  origin,
  dest,
  air_time,
  distance,
  "hour",
  "minute",
  time_hour,
  planes."year" AS "year.y",
  "type",
  manufacturer,
  model,
  engines,
  seats,
  speed,
  engine
FROM flights
LEFT JOIN planes
  ON (flights.tailnum = planes.tailnum)

SQL Practice: Stack Exchange Data Explorer

We will experiment with the Stack Exchange Data Explorer, a website that provides a SQL interface for all the data in StackExchange. StackExchange powers the StackOverflow programming question and answer site, but it also powers question and answer sites related to 126 topics including English, Travel, Bicycles, and Parenting.

StackExchange provides an in-depth Data Explorer Tutorial. We will use this interface to construct SQL queries on the Travel Data Explorer.

Head to the Stack Exchange Data Explorer for Travel. You will see a list of queries other users have created in the past. These queries are for all Stack Exchange sites, so some may not be relevant. Queries about your activity (for example, “How many upvotes do I have for each tag?”) will not be useful either if you do not have activity for the particular site.

Click on one of them and you see the SQL code for the query. Then click the “Run Query” button to get results. For example, you might look at the number of up vs down votes for questions and answers by weekday and notice that for questions, Tuesday has the highest up vs. down vote ratio and Saturday has the lowest. You can contemplate hypotheses for this difference!

Select Queries

Let’s experiment with our own queries. Click on “Compose Query” in the upper right, and notice the tables are shown in the right. As a reminder, a table is similar to a data frame. Each table lists the columns stored within the table and the data types for the columns. Look through the tables for Posts, Users, and Comments. Do the columns generally make sense, and correspond to the StackOverflow website? There’s a description of the tables and columns (called a schema) available on StackExchange’s Meta Q&A Site.

Now enter your first query in the text box and click the “Run Query” button:

SELECT TOP(100) Id, Title, Score, Body, Tags
FROM Posts

In this query we already see several important features of SQL:

SELECT tells SQL that a query is coming.
TOP(100) only returns the first 100 rows.
- Note: The StackExchange data explorer uses a variant of SQL called Transact SQL that is supported by Microsoft databases. TOP(100) is a non-standard SQL feature supported by T-SQL. For most databases you would accomplish the same goal by adding LIMIT 100 to the end of the query.
Id, Title, Score, Body, Tags determines what columns are included in the result
FROM Posts determines the source dataset.

From glancing at the results, it appears that this table contains both questions and answers. Let’s try to focus on answers. Looking again at the Schema Description, notice that there is a PostTypeId column in Posts, and a value of 1 corresponds to questions. Let’s update our query to only include questions:

SELECT TOP(100)
Id, Title, Score, Body, Tags
FROM Posts
WHERE PostTypeId = 1

The SQL command WHERE is like the filter command we have been using in dplyr.

Note that whereas we used the double equals == for comparison in R, the SQL WHERE command takes just a single =.

Exercise: Find the title and score of Posts that have a score of at least 110. Hint: TOP is not necessary here because you want all result rows.

Solution

SELECT Title, Score
FROM Posts
WHERE PostTypeId = 1 AND Score >= 110

Exercise: Find posts whose title contains some place you are interested in (you pick!). Hint: use SQL’s LIKE operator.

Solution

SELECT Title, Score
FROM Posts
WHERE PostTypeId = 1 AND Title LIKE '%Paris%'

Note that you can look up the actual webpage for any question using its Id. For example, if the Id is 19591, the webpage URL would be https://travel.stackexchange.com/questions/19591/. Look up a few of the questions by their Id.

It’s unclear how the 100 questions we saw were selected from among the over 43,000 total questions.

To count the number of posts, we can use COUNT in SQL: SELECT COUNT(Id) FROM Posts Where PostTypeId = 1.

Let’s try to arrange the Posts by score. The following query surfaces the top scoring question: OK we’re all adults here, so really, how on earth should I use a squat toilet?

SELECT TOP(100)
Id, Title, Score, Body, Tags
FROM Posts
WHERE PostTypeId = 1
ORDER BY Score DESC

The ORDER BY ??? DESC syntax is similar to R’s arrange(). You can leave off the DESC if you want the results ordered smallest to largest.

We could also find the highest rated questions tagged “italy” (the top question is Does Venice Smell?):

SELECT TOP(100)
Id, Title, Score, Body, Tags
FROM Posts
WHERE PostTypeId = 1 AND Tags LIKE '%italy%'
ORDER BY Score DESC

Exercise: Pick two tags that interest you and you think will occur together and find the top voted posts that contain both.

Solution

SELECT Title, Score, Tags
FROM Posts
WHERE PostTypeId = 1 AND Tags LIKE '%paris%' AND Tags LIKE '%france%'

SQL Summarization

So far, we have covered the equivalent of R’s selecting, filtering, and arranging. Let’s take a look at grouping and summarizing now, which has similar structures in both R and SQL. Imagine we want to see how many posts of each type there are. This query shows us that there are 44K questions and 71K answers.

SELECT 
PostTypeId, COUNT(Id) numPosts
FROM posts
GROUP BY PostTypeId 
ORDER BY PostTypeId

Note two characteristics of SQL summarization here:

The GROUP BYclause indicates the table column for grouping, much like R’s group_by.
There is no explicit summarize. Instead, all columns that appear in the SELECT except for those listed in GROUP BY must make use of an aggregate function. COUNT(*) is one of these, and is the equivalent of R’s n(). Many other aggregate functions exist, including MAX, SUM, AVG, and many others. Every aggregate function requires a column as an argument (even COUNT() which doesn’t logically need one).
The aggregate column (in this case COUNT(Id)) must immediately be followed by a name that will be used for it in the results (in this case numPosts). This can be particularly useful if you want to order by the aggregated value.

Exercise: Change the previous query so it orders the result rows by the number of posts of that type. Hint: Reuse the name you assigned to the aggregate function.

Solution

SELECT 
PostTypeId, COUNT(Id) numPosts
FROM posts
GROUP BY PostTypeId 
ORDER BY numPosts

Exercise: Find the most commonly used tagsets (sets/combinations of tags) applied to posts. Note that this is not asking you to count the most common individual tags — this would be more complex because multiple tags are squashed into the Tags field.

Solution

SELECT
Tags, COUNT(Tags) numTagsets
FROM posts
GROUP BY Tags
ORDER BY numTagsets

SQL Joins

Finally, as with R, we often want to join data from two or more tables. The types of joins in SQL are the same as we saw with R (inner, outer, left, right). Most commonly we want to perform an INNER join, which is the default if you just say JOIN. (We can look up the inner_join() documentation to remind ourselves what an inner join does.)

Let’s say we wanted to enhance the earlier query to find the highest scoring answers with some information about each user.

SELECT TOP(100)
Title, Score, DisplayName, Reputation
FROM Posts p
JOIN Users u
ON p.OwnerUserId = u.Id
WHERE PostTypeId = 1
ORDER BY Score Desc

We see a few notable items here:

The JOIN keyword must go in between the two tables we want to join.
Each table must be named. In this case we named posts p and users u.
We need to specify the relationship that joins the two tables. In this case, a posts OwnerUserId column refers to the Id column in the users table.

Exercise: Create a query similar to the one above that identifies the authors of the top rated comments instead of posts.

Solution

SELECT TOP(100)
Text, Score, DisplayName, Reputation, AboutMe, Views, UpVotes, DownVotes
FROM Comments c
JOIN Users u
ON c.UserId = u.Id
ORDER BY Score Desc

Additional Exercises (Homework 9)

The first few exercises will ask you to analyze Stack Exchange badges. Start at https://data.stackexchange.com/stackoverflow/query/new. For each exercise, record the query you used.

Exercise: Count the number of total badges that have been given out. Hint: count the number of rows in the relevant table.

ANSWER:

Exercise: Find how many times each badge has been awarded, sorted from most awarded to least awarded.

ANSWER:

Exercise: Find a badge that looks interesting to you. Find all the user DisplayNames that have received the badge, along with the date at which they received it.

ANSWER:

Exercise: Show the users who have received the most badges, along with how many they have received.

ANSWER:

The next few activities analyze user activity. These activities mimic the common workflow of creating datasets in SQL that you analyze in R.

Exercise: Export a CSV file containing information about each user: DisplayName, Id, Reputation, and CreationDate. Because any queries that return more than 50,000 results return a random subset of 50,000 results, include a sort on the user ID variable to ensure that the first 50,000 users are retrieved. Name your file users.csv.

ANSWER:

Exercise: Make a table that has each user’s total number of posts and total number of upvotes, and export this file as a CSV named posts.csv. Continue to include a sort on the user ID variable. Hint: Start with the posts table, join information from users, and perform some grouped summaries.

ANSWER:

Exercise: Calculate the number of comments per user, and the total number of upvotes across all comments per user (this is the sum of the Score variable under the Comments table) and export this as a CSV file named comments.csv. Continue to include a sort on the user ID variable.

ANSWER:

Exercise: Import these three datasets into R. Visualize the relationship between the three datasets. Include at least one visualization comparing each of:

information from the user CSV and the post CSV, and
information from the user CSV and comment CSV

Your goal should be to eave together your visualizations and exposition to tell a compelling story.

ANSWER:

Exploring cloud DBMS’s

Redshift is Amazon’s cloud database management system (DBMS).

To try out Redshift, you can sign up for a free AWS Educate account. Once your account is confirmed, you will have access to many tutorials about cloud computing.
In the Getting Started section of your AWS Educate main page, navigate to the Getting Started with Databases (Lab) tutorial on the second page of tutorials.
Various Redshift resources can be found here.

BigQuery is Google’s DBMS.

BigQuery can be tried for free through Big Query sandbox.
On the main BigQuery page you’ll see a big blue button that says “Try BigQuery free”.
On the cloud welcome page under the Products section, you’ll see a button for “Analyze and manage data - BigQuery”.
Accessing public data within BigQuery
- In your “Welcome to BigQuery Studio!” window, you’ll see a “Try the Google Trends Demo Query” section.
- Click the “Open this query” blue button to get an example SQL statement for the Google Trends dataset. You’ll also see on the left panel a list of all public datasets available through BigQuery.

Learning goals

Introduction to databases

Connecting to a database with DBI

Warming up to SQL with dplyr

SQL Practice: Stack Exchange Data Explorer

Select Queries

SQL Summarization

SQL Joins

Additional Exercises (Homework 9)

Exploring cloud DBMS’s

Connecting to a database with `DBI`

Warming up to SQL with `dplyr`