If you set link to null, a new database connection is created.
如果将link设为null，就会创建一个新的数据库连接。
You can set link as a reference to an existing database connection link or object.
而link则可被设为对现有数据库连接或对象的一个引用。
You can set link as a reference to an existing database connection link or object.
而link则可被设为对现有数据库连接或对象的一个引用。
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