WHERE are the subqueries?

Data Manipulation in SQL

Mona Khalil

Data Scientist, Greenhouse Software

What is a subquery?

A query nested inside another query

SELECT column
FROM (SELECT column
      FROM table) AS subquery;

Useful for intermediary transformations

What do you do with subqueries?

Can be in any part of a query
- SELECT, FROM, WHERE, GROUP BY
Can return a variety of information
- Scalar quantities (3.14159, -2, 0.001)
- A list (id = (12, 25, 392, 401, 939))
- A table

Why subqueries?

Comparing groups to summarized values
- How did Liverpool compare to the English Premier League's average performance for that year?
Reshaping data
- What is the highest monthly average of goals scored in the Bundesliga?
Combining data that cannot be joined
- How do you get both the home and away team names into a table of match results?

Simple subqueries

Can be evaluated independently from the outer query

SELECT home_goal
FROM match
WHERE home_goal > (
    SELECT AVG(home_goal) 
    FROM match);

SELECT AVG(home_goal) FROM match;

1.56091291478423

Simple subqueries

Is only processed once in the entire statement

SELECT home_goal
FROM match
WHERE home_goal > (
    SELECT AVG(home_goal) 
    FROM match);

Subqueries in the WHERE clause

Which matches in the 2012/2013 season scored home goals higher than overall average?

SELECT AVG(home_goal) FROM match;

1.56091291478423

SELECT date, hometeam_id, awayteam_id, home_goal, away_goal
FROM match
WHERE season = '2012/2013'
      AND home_goal > 1.56091291478423;

Subqueries in the WHERE clause

Which matches in the 2012/2013 season scored home goals higher than overall average?

SELECT date, hometeam_id, awayteam_id, home_goal, away_goal
FROM match
WHERE season = '2012/2013'
      AND home_goal > (SELECT AVG(home_goal)
                     FROM match);

| date       | hometeam_id | awayteam_id | home_goal | away_goal |
|------------|-------------|-------------|-----------|-----------|
| 2012-07-28 | 9998        | 1773        | 5         | 2         |
| 2012-07-29 | 9987        | 9984        | 3         | 3         |
| 2012-10-05 | 9993        | 9991        | 2         | 2         |

Subquery filtering list with IN

Which teams are part of Poland's league?

SELECT
  team_long_name,
  team_short_name AS abbr
FROM team
WHERE 
  team_api_id IN 
  (SELECT hometeam_id
   FROM match
   WHERE country_id = 15722);

| team_long_name | abbr |
|----------------|------|
| Ruch Chorzów   | CHO  |
| Jagiellonia    | BIA  |
| Lech Pozna?    | POZ  |
| P. Warszawa    | PWA  |
| Cracovia       | CKR  |
| Górnik ??czna  | LEC  |
| Polonia Bytom  | GOR  |
| Zag??bie Lubin | ZAG  |
| Pogo? Szczecin | POG  |
| Widzew ?ód?    | WID  |
| ?l?sk Wroc?aw  | SLA  |

Practice time!

Data Manipulation in SQL