APEX Tutorial 6 : Learn Constraint in Object Browser | Oracle APEX
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Use a constraint to define an integrity constraint--a rule that restricts the values in a database. Oracle Database lets you create six types of constraints and lets you declare them in two ways.
1) A primary key constraint combines a NOT NULL constraint and a unique constraint in a single declaration. That is, it prohibits multiple rows from having the same value in the same column or combination of columns and prohibits values from being null.
2) A unique constraint prohibits multiple rows from having the same value in the same column or combination of columns but allows some values to be null.
3) A foreign key constraint requires values in one table to match values in another table.
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