Tworzenie grup danych
Wprowadzenie do Oracle SQL
Hadrien Lacroix
Content Developer
Grupowanie danych
Informacje o grupach
| Composer | Milliseconds |
|-----------------|--------------|
| Antonio Vivaldi | 199,086 |
| Pearl Jam | 122,801 |
| Pearl Jam | 65,593 |
| Jimmy Page | 401,920 |
| Jimmy Page | 386,063 |
| Jimmy Page | 132,702 |
| Jimmy Page | 189,675 |
| Jimmy Page | 126,641 |
| Carlos Santana | 318,432 |
| Carlos Santana | 296,437 |
| Carlos Santana | 882,834 |
| ... | ... |
Jaka jest średnia długość utworów napisanych przez każdego kompozytora?
GROUP BY- dzieli wiersze tabeli na grupy
- używa funkcji grupowych do uzyskania informacji zbiorczych dla każdej grupy
GROUP BY
SELECT Composer, AVG(Milliseconds)
FROM Track
GROUP BY Composer
| Composer | AVG(Milliseconds) |
|-----------------|-------------------|
| Antonio Vivaldi | 199,086.0 |
| Pearl Jam | 94,197.0 |
| Jimmy Page | 474,888.3 |
| Carlos Santana | 499,234.3 |
| ... | ... |
GROUP BY i WHERE
SELECT Composer, AVG(Milliseconds)
FROM Track
WHERE Genre = 1
GROUP BY Composer
| Composer | AVG(Milliseconds) |
|-----------------|-------------------|
| Antonio Vivaldi | 199,086.0 |
| Pearl Jam | 94,197.0 |
| Jimmy Page | 474,888.3 |
| Carlos Santana | 499,234.3 |
| ... | ... |
GROUP BY i ORDER BY
SELECT Composer, AVG(Milliseconds) AS Average
FROM Track
GROUP BY Composer
ORDER BY AVG(Milliseconds)
| Composer | AVG(Milliseconds) |
|-----------------|-------------------|
| Pearl Jam | 94,197.0 |
| Antonio Vivaldi | 199,086.0 |
| Jimmy Page | 474,888.3 |
| Carlos Santana | 499,234.3 |
| ... | ... |
GROUP BY i ORDER BY
SELECT Composer, AVG(Milliseconds) AS Average
FROM Track
GROUP BY Composer
ORDER BY 2
| Composer | AVG(Milliseconds) |
|-----------------|-------------------|
| Pearl Jam | 94,197.0 |
| Antonio Vivaldi | 199,086.0 |
| Jimmy Page | 474,888.3 |
| Carlos Santana | 499,234.3 |
| ... | ... |
GROUP BY i ORDER BY
SELECT Composer, AVG(Milliseconds) AS Average
FROM Track
GROUP BY Composer
ORDER BY Average
| Composer | Average |
|-----------------|-------------------|
| Pearl Jam | 94,197.0 |
| Antonio Vivaldi | 199,086.0 |
| Jimmy Page | 474,888.3 |
| Carlos Santana | 499,234.3 |
| ... | ... |
Wytyczne
Każda kolumna lub wyrażenie w instrukcji SELECT, która nie jest funkcją agregującą, musi znaleźć się w klauzuli GROUP BY
SELECT Composer, AVG(Milliseconds), UnitPrice
FROM Track
GROUP BY Composer
column "track.unitprice" must appear in the GROUP BY clause or be used in an aggregate function
LINE 2: SELECT Composer, AVG(Milliseconds), UnitPrice
Wytyczne
Każda kolumna lub wyrażenie na liście SELECT, która nie jest funkcją agregującą, musi znaleźć się w klauzuli GROUP BY
SELECT Composer, AVG(Milliseconds), MAX(UnitPrice)
FROM Track
GROUP BY Composer
| Composer | AVG(Milliseconds) | MAX(UnitPrice) |
|-----------------|-------------------|----------------|
| Antonio Vivaldi | 199,086.0 | 0.99 |
| Pearl Jam | 94,197.0 | 0.99 |
| Jimmy Page | 474,888.3 | 0.99 |
| Carlos Santana | 499,234.3 | 0.99 |
| ... | ... | ... |
Wytyczne
Wyrażenia podane w klauzuli GROUP BY nie muszą być uwzględnione w instrukcji SELECT
SELECT AVG(Milliseconds)
FROM Track
GROUP BY Composer
| AVG(Milliseconds) |
|-------------------|
| 199,086.0 |
| 94,197.0 |
| 474,888.3 |
| 499,234.3 |
| ... |
Wiele kolumn
SELECT Country, City, COUNT(CustomerId)
FROM Customer
GROUP BY Country, City
| Country | City | COUNT(CustomerId) |
|-----------|---------------------|-------------------|
| Argentina | Buenos Aires | 1 |
| Australia | Sidney | 1 |
| Austria | Vienne | 1 |
| Belgium | Brussels | 1 |
| Brazil | Brasilia | 1 |
| Brazil | São José dos Campos | 1 |
| Brazil | São Paulo | 2 |
| ... | ... | ... |
Czas na ćwiczenia!
Wprowadzenie do Oracle SQL
