说明：  第一章是实践篇，大量SQL代码，一口气2天读完，已经来到330页。 第二章是理论篇，主要涉及关系数据库历史，集合论， 谓词逻辑、三值逻辑的由来， 想深入理解可能得花2个月、2年、20年不定。 第三章是附录，详细的习题解答。
(The first chapter is practice, a large number of SQL code, read 2 days at a time, has come to 330 pages. The second chapter is about theory, which mainly deals with the history of relational database, the origin of set theory, predicate logic and ternary logic. It may take two months, two years or 20 years to understand deeply. The third chapter is the appendix, detailed exercise solutions.)