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周休二日與平日旅遊(舉例)在MySQL上一段SQL 語句Count的運算 ~ 迪貝之家
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| 不錯的思路 |
| 觸發我在想怎麼運用在其他的資料庫 |
| 應該是case when吧~~ |
| The IF() expressions determine, for each column value, whether it should be counted. |
| If so, the expression evaluates to 1 and COUNT() counts it. If not, the expression evaluates |
| to NULL and COUNT() ignores it. The effect is to count the number of values that satisfy |
| the condition given as the first argument to IF(). |
| 類似的邏輯碰到是否有品項資料未key in而造成NULL值 |
| 不過坦白說, 只要做一次group by,就可以看到null的count數了 |
| 坦白說我沒寫過底下的邏輯應用 |
| 寫出來只是為了我將來參考用 |
| 就只為了把null加到count(distinct)裏頭 |
| 不就直接加1就好了..... |
| COUNT(DISTINCT) ignores NULL values. To count NULL as one of the values in the set if |
| it’s present, use one of the following expressions: |
| COUNT(DISTINCT val) + IF(COUNT(IF(val IS NULL,1,NULL))=0,0,1) |
| COUNT(DISTINCT val) + IF(SUM(ISNULL(val))=0,0,1) |
COUNT(DISTINCT val) + (SUM(ISNULL(val))<>0)
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