用 CASE 寫條件邏輯

Snowflake 中的資料操作

Jake Roach

Field Data Engineer

使用 ELSE

有時會有不符合任何 WHEN ... THEN 條件的值。

用 ELSE 捕捉這些額外情況
很適合用來抓邊界情況

SELECT
    student,
    CASE
        WHEN grade_num = 12 THEN 'Senior'
        WHEN grade_num = 11 THEN 'Junior'
        ...
        ELSE 'Not in HS'  -- Catch others!
    END AS grade
FROM students;

          |   student  |    grade   |
          | ---------- | ---------- |
          |  Viraj     |  Junior    |
          |  Stephanie |  Senior    |
          |  Lewis     |  Not in HS |

評估條件

Operator	Condition	Example
`=`	值等於另一個值	`column_a = 'value_a'`
`IN (...)`	值出現在清單中	`column_a IN ('value_a', 'value_b')`
`>`, `<`	值大於或小於另一個值	`column_a > 0`
`>=`, `<=`	`>`、`<` 的對應，含等於	`column_a >= 0`
`BETWEEN`	值介於兩值之間（含邊界）	`column_a BETWEEN 0 AND 10`

這些比較運算子可與 AND、OR、NOT 組合使用

Snowflake 為欄向式資料庫，CASE 敘述執行效率佳

溫度分類

SELECT
    todays_date,
    temperature,
    CASE

        WHEN temperature BETWEEN 70 AND 90 THEN 'Ideal for Swimming'

        WHEN temperature >= 50 AND tempreature < 70 THEN 'Perfect for Sports'

        WHEN temperature > 32 AND temperature < 50 THEN 'Spring/Fall Temps'
        WHEN temperature > 0 AND temperature <= 32 THEN 'Winter Weather'
        ELSE 'Extreme Temperatures'

    END AS temperature_description
FROM weather;

溫度分類

以 CASE 敘述依溫度分類紀錄的結果表格

合併多個條件

SELECT
    todays_date,
    temperature,
    status,

    CASE
        WHEN temperature > 70 AND status NOT IN ('Rain', 'Wind') THEN 'Beach'

        WHEN temperature BETWEEN 45 AND 70 AND status = 'Sun' THEN 'Sports'

        WHEN temperature <= 32 OR status = 'Snow' THEN 'Skiing'

        ELSE 'Stay In'
    END AS activity   
FROM weather;

你可以在同一個 CASE 敘述中評估多個欄位

合併多個條件

以 CASE 敘述依溫度與戶外狀態分類紀錄的結果表格

一起來練習吧！

Snowflake 中的資料操作