Progress
SQL-89
Guide and Reference
Null Values
A null value indicates that the value for a particular column is unknown or not available. The Progress/SQL null value is equivalent to the Progress unknown value (?).
Any arithmetic expression that contains a null value evaluates to null. Any comparison expression that contains a null value (in WHERE and HAVING clauses) evaluates to unknown, neither true nor false. When a WHERE or HAVING clause evaluates to unknown for a particular row, Progress does not select or modify that row.
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