http://dev.mysql.com/doc/refman/5.1/zh/functions.html#mathematical-functions
SHOW TABLE STATUS, http://dev.mysql.com/doc/refman/5.1/zh/sql-syntax.html#show-table-status
日期和时间类型概述文档, http://dev.mysql.com/doc/refman/5.1/zh/column-types.html#date-and-time-type-overview
JOIN语法文档, http://dev.mysql.com/doc/refman/5.1/zh/sql-syntax.html#join
DATEDIFF(expr1,expr2)语法文档, http://dev.mysql.com/doc/refman/5.1/en/date-and-time-functions.html#function_datediff
MySQL如何优化LEFT JOIN和RIGHT JOIN文档, http://dev.mysql.com/doc/refman/5.1/zh/optimization.html#left-join-optimization
外键, http://dev.mysql.com/doc/refman/5.1/zh/introduction.html#ansi-diff-foreign-keys
使用AUTO_INCREMENT, http://dev.mysql.com/doc/refman/5.1/zh/tutorial.html#example-auto-increment
列类型, http://dev.mysql.com/doc/refman/5.1/zh/column-types.html
ALTER TABLE语法, http://dev.mysql.com/doc/refman/5.1/zh/sql-syntax.html#alter-table
12.4.2. 数学函数, http://dev.mysql.com/doc/refman/5.1/zh/functions.html#mathematical-functions