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