PLSQL Language Referenc-SQL資料型別-子型別與基型別具有相同的資料型別家族

DECLARE

    SUBTYPE Word IS CHAR(6);

    SUBTYPE Text IS VARCHAR2(15);

    verb       Word := 'run';

    sentence1  Text;

    sentence2  Text := 'Hurry!';

    sentence3  Text := 'See Tom run.';

BEGIN

    sentence1 := verb; 

    verb := sentence2; 

    verb := sentence3;  -- 12個字元值，6字元約束

END;