問題描述
我在表中有一個 Status 列，它有 3 個值 - 'N/A'、'Single'、'Multiple'.某些行的狀態(tài)列具有 NULL 值.
I've a Status column in a table which has 3 values - 'N/A' , 'Single' ,'Multiple'. Some rows have a NULL value for the Status column.
我需要提取 Status 不為空且不為N/A"的所有行.基本上，我需要狀態(tài)為單個"或多個"的所有行.
I need to pull up all the rows in which Status is not null and is not 'N/A'. Basically, I need all the rows whose status is "Single" or "Multiple".
我剛剛了解到 NULL 實(shí)際上等同于UNKNOWN".
I've been just reading up about NULL actually being equivalent to 'UNKNOWN'.
如果我說
SELECT *
FROM t_userstatus
WHERE status <> 'N/A'
我得到了結(jié)果(僅包含單個"或多個"的所有行).
I get the results (All rows containing "Single" or "Multiple" only).
我想知道的是，上面的 WHERE 子句是否總是排除具有 NULL 值的行?這是預(yù)期的行為嗎?
What I would like to know is that , does the above WHERE clause always exclude the rows having NULL values?Is that the expected behaviour?
是什么導(dǎo)致即使我沒有明確指定它也會排除空行?
在我的查詢中，我是否必須明確說狀態(tài) IS NOT NULL ?
In my query,do I have to explicitly say status IS NOT NULL ?
我對編程比較陌生，不勝感激.
I am relatively new to programming, any help is appreciated.
推薦答案
SQL 使用 三值邏輯:真、假、未知.與 null 的任何比較都會導(dǎo)致 unknown.
SQL uses three-valued logic: true, false, and unknown. Any comparison to null results in unknown.
所以 null <>'N/A' 計算結(jié)果為 unknown.由于 unknown 不正確，這意味著該行被排除在外.
So null <> 'N/A' evaluates to unknown. Since unknown is not true, that means the row gets excluded.
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