Effects of NULL

The numeric variable X, perhaps of type INTEGER, might be assigned NULL. In that case the result of evaluating X + 1 is NULL, and so SET Y = X + 1 assigns NULL to Y. A curious consequence of this is that immediately following this assignment the comparison Y = X + 1 does not evaluate to TRUE. For that matter, neither does the comparison Y = Y. In fact they both evaluate to UNKNOWN.

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