Sottoquery ovunque! E buone pratiche!

Manipolazione dei dati in SQL

Mona Khalil

Data Scientist, Greenhouse Software

Quante sottoquery vuoi...

Puoi includere più sottoquery 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');

Formatta le tue query

Allinea SELECT, FROM, WHERE e GROUP BY

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

Annota le tue query

/* Questa query filtra per col1 = 2
e seleziona solo dati da table1 */
SELECT
    col1,
    col2,
    col3
FROM table1
WHERE col1 = 2;

Annota le tue query

SELECT
    col1,
    col2,
    col3
FROM table1 -- questa tabella ha 10.000 righe
WHERE col1 = 2; -- Filtro WHERE valore 2

Indenta le tue query

Indenta le tue sottoquery!

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

Indenta le tue query

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

Quella sottoquery è necessaria?

Le sottoquery richiedono potenza di calcolo
- Quanto è grande il tuo database?
- Quanto è grande la tabella da cui stai interrogando?
La sottoquery è davvero necessaria?

Filtra correttamente ogni sottoquery!

Attento ai tuoi filtri!

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');

Ayo berlatih!

Manipolazione dei dati in SQL