SQL 中的双重否定 - NOT (Food != "Apple") 的含义
Double negative in SQL - meaning of NOT (Food != "Apple")
双重否定含义有问题。
这些语句是否等价？
- 不是（食物 <> "Apple" 不是（食物 = "Banana"））
===
- （食物 = "Apple" 和食物 = "Banana"）
您正在寻找De Morgan's laws
NOT( A OR B ) = NOT A AND NOT B
NOT( A AND B ) = NOT A OR NOT B
所以第一句应该是：
NOT (Food <> "Apple" AND NOT (Food = "Banana"))
==> NOT (Food <> "Apple" AND Food <> "Banana") -- SOLVE inner NOT
==> NOT (Food <> "Apple") OR NOT (Food <> "Banana") -- MORGAN LAW
==> Food = "Apple" OR Food = "Banana" -- APPLY NOT
双重否定含义有问题。
这些语句是否等价？
- 不是（食物 <> "Apple" 不是（食物 = "Banana"））
===
- （食物 = "Apple" 和食物 = "Banana"）
您正在寻找De Morgan's laws
NOT( A OR B ) = NOT A AND NOT B
NOT( A AND B ) = NOT A OR NOT B
所以第一句应该是：
NOT (Food <> "Apple" AND NOT (Food = "Banana"))
==> NOT (Food <> "Apple" AND Food <> "Banana") -- SOLVE inner NOT
==> NOT (Food <> "Apple") OR NOT (Food <> "Banana") -- MORGAN LAW
==> Food = "Apple" OR Food = "Banana" -- APPLY NOT