限制群組結果

Oracle SQL 入門

Hadrien Lacroix

Content Developer

回到範例

SELECT Composer, AVG(Milliseconds)
FROM Track
WHERE UnitPrice = 0.99
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         |
| ...             | ...               |

那群組之後再過濾呢？

WHERE 的限制

WHERE 不能用來過濾群組
- 群組函式不能用在 WHERE 子句中

WHERE 限制範例

SELECT Composer, AVG(Milliseconds)
FROM Track
GROUP BY Composer
WHERE AVG(Milliseconds) > 200000

syntax error at or near "WHERE"
LINE 4: WHERE AVG(Milliseconds) > 200000
        ^

HAVING

SELECT Composer, AVG(Milliseconds)
FROM Track
GROUP BY Composer
HAVING AVG(Milliseconds) > 200000

| composer    | avg    |
|-------------|--------|
| George Duke | 274155 |
| Miles Davis | 391146 |
| R. Carless  | 251585 |
| ...         | ...    |

用 HAVING 限制群組結果

HAVING 子句流程

HAVING

SELECT Composer, MAX(Milliseconds)
FROM Track
GROUP BY Composer
HAVING MAX(Milliseconds) > 200000

| composer    | max    |
|-------------|--------|
| George Duke | 274155 |
| Miles Davis | 907520 |
| R. Carless  | 251585 |
| ...         | ...    |

另一個例子

SELECT Composer, SUM(UnitPrice)
FROM Track
WHERE GenreId = 1
GROUP BY Composer
HAVING COUNT(*) > 4

| composer      | sum  |
|---------------|------|
| AC/DC         | 7.92 |
| Chris Cornell | 9.90 |
| Eddie Vedder  | 8.91 |
| ...           | ...  |

指引

SELECT 與 HAVING 可使用不同的群組函式
HAVING 一定要搭配群組函式

執行順序：

先由 WHERE 與 GROUP BY 過濾資料列
對各群組套用群組函式
用 HAVING 過濾群組，然後輸出

一起來練習吧！

Oracle SQL 入門