Subqueries everywhere! And best practices!

Data Manipulation in SQL

Mona Khalil

Data Scientist, Greenhouse Software

As many subqueries as you want...

Can include multiple subqueries in SELECT, FROM, WHERE

SELECT 
    s.stage,
    ROUND(s.avg_goals,2) AS avg_goal,
    (SELECT AVG(home_goal + away_goal)
     FROM match WHERE season = '2013/2014') AS overall_avg
FROM 
    (SELECT
         stage,
         AVG(home_goal + away_goal) AS avg_goals
     FROM match
     WHERE season = '2013/2014'
     GROUP BY stage) AS s
WHERE 
    s.avg_goals > (SELECT AVG(home_goal + away_goal) 
                   FROM match WHERE season = '2013/2014');

Format your queries

Line up SELECT, FROM, WHERE, and GROUP BY

SELECT
    col1,
    col2,
    col3
FROM table1
WHERE col1 = 2;

Annotate your queries

/* This query filters for col1 = 2
and only selects data from table1 */
SELECT
    col1,
    col2,
    col3
FROM table1
WHERE col1 = 2;

Annotate your queries

SELECT
    col1,
    col2,
    col3
FROM table1 -- this table has 10,000 rows
WHERE col1 = 2; -- Filter WHERE value 2

Indent your queries

Indent your subqueries!

SELECT
    col1,
    col2,
    col3
FROM table1
WHERE col1 IN
        (SELECT id
         FROM table2
         WHERE year = 1991);

Indent your queries

SELECT 
  date, 
  hometeam_id, 
  awayteam_id,
  CASE WHEN hometeam_id = 8455 AND home_goal > away_goal 
            THEN 'Chelsea home win'
       WHEN awayteam_id = 8455 AND home_goal < away_goal
            THEN 'Chelsea away win'
       WHEN hometeam_id = 8455 AND home_goal < away_goal
            THEN 'Chelsea home loss'
       WHEN awayteam_id = 8455 AND home_goal > away_goal
            THEN 'Chelsea away loss'
       WHEN (hometeam_id = 8455 OR awayteam_id = 8455) 
            AND home_goal = away_goal THEN 'Chelsea Tie'
       END AS outcome
FROM match
WHERE hometeam_id = 8455 OR awayteam_id = 8455;

Holywell's SQL Style Guide

Is that subquery necessary?

Subqueries require computing power
- How big is your database?
- How big is the table you're querying from?
Is the subquery actually necessary?

Properly filter each subquery!

Watch your filters!

SELECT 
    s.stage,
    ROUND(s.avg_goals,2) AS avg_goal,
    (SELECT AVG(home_goal + away_goal)
     FROM match WHERE season = '2013/2014') AS overall_avg
FROM 
    (SELECT
         stage,
         AVG(home_goal + away_goal) AS avg_goals
     FROM match
     WHERE season = '2013/2014'
     GROUP BY stage) AS s
WHERE 
    s.avg_goals > (SELECT AVG(home_goal + away_goal) 
                   FROM match WHERE season = '2013/2014');

Let's practice!

Data Manipulation in SQL