说明：  　　这是一个sql的reference,收集得很全的，是初学者不可少的工具的，　真的很好的，我以前总是不懂的去查书，现在不用了，　有这个就足够了，
(This is a sql of reference, the entire collection made it is indispensable tools beginners, really good, I never used to understand to go, not now, this is sufficient,)