书写顺序：
SELECT -> FROM -> JOIN -> ON -> WHERE -> GROUP BY -> HAVING -> UNION -> ORDER BY ->LIMIT
主要记着下面这些
WHERE -> GROUP BY -> HAVING -> ORDER BY ->LIMIT
这几个语句的相对顺序，其实和下面的执行相对顺序是一致的
先用where过滤一遍所有行，再进行分组，再用having过滤一遍所有组，接着排序，最后取limit个
执行顺序：
FROM -> ON -> JOIN -> WHERE -> GROUP BY -> HAVING -> SELECT -> UNION -> ORDER BY ->LIMIT