昨天看到的,这不是一个什么多深的技术问题,多么牛叉的编程能力。这跟一个人的开发能力也没有非常必然的直接关系,但是知道这些会对你的SQL编写,排忧及优化上会有很大的帮助。它不是一个复杂的知识点,但是一个非常基础的SQL根基。
看下面的几段SQL语句:
SELECT ID,COUNT(ID) AS TOTAL FROM STUDENT GROUP BY ID HAVING TOTAL>2
SELECT ID,COUNT(ID) AS TOTAL FROM STUDENT GROUP BY ID ORDER BY TOTAL
SELECT FIRSTNAME+' '+LASTNAME AS NAME, COUNT(*) AS COUNT FROM STUDENT GROUP BY NAME
你觉得哪一个不能够成功执行?
下面是SELECT语句的逻辑执行顺序:
- FROM
- ON
- JOIN
- WHERE
- GROUP BY
- WITH CUBE or WITH ROLLUP
- HAVING
- SELECT
- DISTINCT
- ORDER BY
- TOP
MICROSOFT指出,SELECT语句的实际物理执行顺序可能会由于查询处理器的不同而与这个顺序有所出入。
示例一
SELECT ID,COUNT(ID) AS TOTAL FROM STUDENT GROUP BY ID HAVING TOTAL>2
觉得这个SQL语句眼熟吗?对,非常基础的分组查询。但它不能执行成功,因为HAVING的执行顺序在SELECT之上。
实际执行顺序如下:
- FROM STUDENT
- GROUP BY ID
- HAVING TOTAL>2
- SELECT ID,COUNT(ID) AS TOTAL
很明显,TOTAL是在最后一句SELECT ID,COUNT(ID) AS TOTAL执行过后生成的新别名。因此,在HAVING TOTAL>2执行时是不能识别TOTAL的。
示例二
SELECT ID,COUNT(ID) AS TOTAL FROM STUDENT GROUP BY ID ORDER BY TOTAL
这个的实际执行顺序是:
- FROM STUDENT
- GROUP BY ID
- SELECT ID,COUNT(ID) AS TOTAL
- ORDER BY TOTAL
这一次没有任何问题,能够成功执行。如果把ORDER BY TOTAL换成ORDER BY COUNT(ID)呢?
SELECT ID,COUNT(ID) AS TOTAL FROM STUDENT GROUP BY ID ORDER BY COUNT(ID)
实际执行顺序:
- FROM STUDENT
- GROUP BY ID
- SELECT ID,COUNT(ID) AS TOTAL
- ORDER BY COUNT(ID)
没错,它是能够成功执行的,看SQL执行计划,它与上面ORDER BY TOTAL是一样的。ORDER BY 是在SELECT后执行,因此可以用别名TOTAL。
示例三
SELECT FIRSTNAME+' '+LASTNAME AS NAME, COUNT(*) AS COUNT FROM STUDENT GROUP BY NAME
实际执行顺序:
FROM STUDENT GROUP BY NAME SELECT FIRSTNAME+' '+LASTNAME AS NAME,COUNT(*) AS COUNT
很明显,执行GROUP BY NAME时别名NAME还没有创建,因此它是不能执行成功的。