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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

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