在宏内开头定义DATABASE变量告诉它要连接到哪一个数据库。
Define a database variable initially within the macro to tell it which database to connect to.
包含文件(或者头文件)定义了在操作系统中或特定的库中使用的常量、宏以及其他结构。
Include files (or header files) define constants, macros, and other structures used in the operating system or in a particular library.
如果在首次包含本库头文件之前用户未显式定义该宏，则缺省为20。
Defaults to 20 if not explicitly defined by the user before inclusion of the first library header.
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