The clauses ALWAYS and BY DEFAULT in the column definition determine how explicitly user-specified values are handled in INSERT and UPDATE commands. In an INSERT command, if ALWAYS is selected, a user-specified value is only accepted if the INSERT statement specifies OVERRIDING SYSTEM VALUE. If BY DEFAULT is selected, then the user-specified value takes precedence. Thus, using BY DEFAULT results in a behavior more similar to default values, where the default value can be overridden by an explicit value, whereas ALWAYS provides some more protection against accidentally inserting an explicit value.
列定义中的 ALWAYS 和 BY DEFAULT 子句决定了在 INSERT 和 UPDATE 命令中如何处理明确的用户指定值。 在 INSERT 命令中，如果选择了 ALWAYS，则只有在 INSERT 语句指定了 OVERRIDING SYSTEM VALUE 时，才会接受用户指定的值。 如果选择 BY DEFAULT，则用户指定的值优先。 因此，使用 BY DEFAULT 会产生更类似于默认值的行为，即默认值可以被显式值覆盖，而 ALWAYS 则提供了更多保护，防止意外插入显式值。