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rules to be applied to NULLs while doing comparisons

I want to know the rules which we need to follow when doing comparisions
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1) NULL is never TRUE or FALSE

2) NULL cannot be equal or unequal to other values

3) If a value in an expression is NULL, then the expression itself evaluates to NULL except for concatenation operator (||)

4) when ever joining tables and one table is having nulls in column which is mentioned in join then we should mention column is not null

Answer is