如果没有指定默认值,则初始变量为NULL。
If a default value is not specified, the variable is initialized to NULL.
如果没有指定默认的动作,则打开一个选项窗口。
If no action has been defined as default, a window of options opens.
如果没有指定模式,则遵守特定规则来确定模式名称。
If no schema is specified, special rules are followed to determine the schema name.
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