#核心知识点

书写顺序:where——》group by——》having——》order by——》limit

 

许多时候在书写一些复杂的sql语句的时候,尤其是在渐进式推导的时候,经常想要添加一些条件,但是却不知道往哪里加,也许进过许多次尝试之后成功了,但是下次又不知道了,因此总结就显得非常重要了。

首先给出sql语句的查询(或书写)顺序:

(7)    SELECT
(8)    DISTINCT <select_list>   #去重
(1)    FROM  <left_table>
(3)    <join_type> JOIN <right_table>   #左、右、内链接的一种
(2)    ON <join_condition>
(4)    WHERE <where_condition>    #条件查询
(5)    GROUP BY <group_by_list>   #分组
(6)    HAVING <having_condition>  #分组后过滤
(9)    ORDER BY <order_by_condition>  #排序
(10)   LIMIT <limit_number>   #分页

上面从上到下是sql语句的书写顺序,前面括号代表它的执行顺序。

只有弄清楚书写顺序,我们写好的查询语句才不会出现语法错误,

只有弄清楚执行顺序,才能得到想要得到的结果。

关于书写我们要记住关键字:where——》group by——》having——》order by——》limit

下面给出一个例子,帮助理解:

有表table1和table2,需要查询来自杭州,且订单数小于2的客户。

表一:

mysql> select * from table1;
+-------------+----------+
| customer_id | city     |
+-------------+----------+
| 163         | hangzhou |
| 9you        | shanghai |
| baidu       | hangzhou |
| tx          | hangzhou |
+-------------+----------+
4 rows in set (0.00 sec)

表二:

mysql> select * from table2;
+----------+-------------+
| order_id | customer_id |
+----------+-------------+
|        1 | 163         |
|        2 | 163         |
|        3 | 9you        |
|        4 | 9you        |
|        5 | 9you        |
|        6 | tx          |
|        7 | NULL        |
+----------+-------------+
7 rows in set (0.00 sec)

使用两种方法来解答:

第一种,使用子查询

#来自杭州
SELECT * FROM table1 WHERE city = 'hangzhou';
+-------------+----------+
| customer_id | city     |
+-------------+----------+
| 163         | hangzhou |
| baidu       | hangzhou |
| tx          | hangzhou |
+-------------+----------+

#订单小于2
SELECT * FROM table2
    GROUP BY customer_id HAVING COUNT(customer_id)< 2;

+----------+-------------+
| order_id | customer_id |
+----------+-------------+
|        7 | NULL        |
|        6 | tx          |
+----------+-------------+
#再过滤掉其中不符合的就行

#联合在一起
SELECT * FROM table2
    WHERE customer_id in (SELECT customer_id FROM table1 WHERE city = 'hangzhou') 
    GROUP BY customer_id HAVING COUNT(customer_id)< 2;

#结果:
+----------+-------------+
| order_id | customer_id |
+----------+-------------+
|        6 | tx          |
+----------+-------------+

这种方法逻辑性比较鲜明。

 

2.第二种,使用内链接查询

(1)内链接

mysql> SELECT order_id,a.customer_id city FROM table2 a INNER JOIN table1 b
    ->     on a.customer_id = b.customer_id;
+----------+------+
| order_id | city |
+----------+------+
|        1 | 163  |
|        2 | 163  |
|        3 | 9you |
|        4 | 9you |
|        5 | 9you |
|        6 | tx   |
+----------+------+

(2)地点是杭州

mysql> SELECT order_id,a.customer_id city FROM table2 a INNER JOIN table1 b
    ->     on a.customer_id = b.customer_id
    ->     WHERE city = 'hangzhou';
+----------+------+
| order_id | city |
+----------+------+
|        1 | 163  |
|        2 | 163  |
|        6 | tx   |
+----------+------+

(3)小于两件

mysql> SELECT order_id,a.customer_id city FROM table2 a INNER JOIN table1 b
    ->     on a.customer_id = b.customer_id
    ->     WHERE city = 'hangzhou'
    ->     GROUP BY a.customer_id
    ->     HAVING COUNT(1) < 2;
+----------+------+
| order_id | city |
+----------+------+
|        6 | tx   |
+----------+------+

使用内链接查询的效率会比子查询来的高,但是对个人的要求比较高。