Duplicarea rapoartelor

Raportare în SQL

Tyler Pernes

Learning & Development Consultant

Ce cauzează duplicarea?

SELECT p.id, SUM(points) AS points, SUM(matches_win) AS matches_won
FROM points AS p
JOIN matches AS m ON p.id = m.id
GROUP BY p.id;

Ce cauzează duplicarea?

+-----+--------+--------------+
| id  | points | matches_won  |
|-----|--------|--------------|
| 1   | 156    | 10           |
+-----+--------+--------------+

¹ va rezulta o valoare a punctelor de trei ori mai mare decât ar trebui, în acest caz, 156.

Ce cauzează duplicarea?

Intermediate Table
+-----+------+--------------+---------+ 
| id  | year | matches_won  | points  |
|-----|------|--------------|---------|    
| 1   | 2016 | 5            | 52      |
| 1   | 2017 | 2            | 52      |  
| 1   | 2018 | 3            | 52      |
+-----+------+--------------+---------+

Ce cauzează duplicarea?

Intermediate Table
+-----+------+--------------+---------+ 
| id  | year | matches_won  | points  |
|-----|------|--------------|---------|    
| 1   | 2016 | 5            | 52      | <--
| 1   | 2017 | 2            | 52      | <-- SUM(points) = 52 x 3 = 156
| 1   | 2018 | 3            | 52      | <-- 
+-----+------+--------------+---------+

Metode de corectare a duplicării

1. Eliminați agregările

SELECT p.id, points, SUM(matches_won) AS matches_won
FROM points AS p
JOIN matches AS m ON p.id = m.id
GROUP BY p.id, points;

+-----+--------+--------------+
| id  | points | matches_won  |
|-----|--------|--------------|
| 1   | 52     | 10           |
+-----+--------+--------------+

Metode de corectare a duplicării

2. Adăugați câmp în clauza JOIN

SELECT p.id, SUM(points) AS points, SUM(matches_win) AS matches_won
FROM points AS p
JOIN matches AS m ON p.id = m.id AND p.year = m.year
GROUP BY p.id;

Metode de corectare a duplicării

2. Adăugați câmp în clauza JOIN

SELECT p.id, SUM(points) AS points, SUM(matches_win) AS matches_won
FROM points AS p
JOIN matches AS m ON p.id = m.id AND p.year = m.year
GROUP BY p.id;

Metode de corectare a duplicării

SELECT id, SUM(matches_won)
FROM matches
GROUP BY id;

+-----+--------------+
| id  | matches_won  |
|-----|--------------|
| 1   | 10           |
| 2   | 7            |
+-----+--------------+

Metode de corectare a duplicării

3. Agregare prin subinterogare

SELECT p.id, points, matches_won
FROM points AS p
JOIN 
    (SELECT id, SUM(matches_won) AS matches_won
    FROM matches
    GROUP BY id) AS m 
ON p.id = m.id;

Metode de corectare a duplicării

Eliminați agregările
Adăugați câmp în clauza JOIN
Agregare prin subinterogare

Identificarea duplicării

Valoare în tabelul original:

SELECT SUM(points) AS total_points
FROM points;

total_points = 52

Valoare în interogare:

SELECT SUM(points) AS total_points
FROM   
    (SELECT p.id, SUM(points) AS points
    FROM points AS p
    JOIN matches AS m ON p.id = m.id
    GROUP BY p.id) AS subquery;

total_points = 156

Obiectivul capitolului

Timp de practică!

Raportare în SQL