标签：customer Shanghai 查询处理 guo Hangzhou 139 id

查询操作是关系数据库中使用最为频繁的操作，也是构成其他构架语句（如DELETE、UPDATE）的基础。当要删除或更新某些记录时，首先查询出这些记录，然后再对其进行相应的SQL操作。因此基于SELECT的查询操作是显得非常重要。对于查询处理，可将其分为逻辑查询处理及物理查询处理。逻辑查询处理表示执行查询应该产生什么样的结果，而物理查询代表MySQL数据库是如何得到该结果的。两种查询的方法可能完全不同，但是得到的结果必定是相同的。

查询处理的各个阶段

1）FROM：对FROM子句中的左表＜left_table>和右表＜right_table>执行笛卡儿积Cartesan product),产生虚拟表 VT1。
2）ON：对虚拟表VT1 应用ON 筛选，只有那些符合＜join_condition>的行才被插入虚拟表 VT2中。
3）JOIN：如果指定了OUTER JOIN (如LEFT OUTER JOIN、RIGHT OUTER JOIN），那么保留表中未匹配的行作为外部行添加到虚拟表VT2中，产生虚拟表VT3。如果FROM子句饮食两个以上表，则对上一个连接生成的结果表VT3和下一个表重复执行步骤1）~步骤3），直到处理完所有的表为止。
4）WHERE：对虚拟表VT3 应用过滤条件，只有符合＜where_condition>的记录才被插入虚拟表VT4中。
5）GROUP BY：根据GROUP BY子句中的列，对VT4中的记录进行分组操作，产生VT5.
6）CUBE | ROLLUP：对表VT5进行CUBE或ROLLUP操作，产生VT6。
7）HAVING：对虚拟表VT6应用HAVING过滤器，只有符合＜having_condition>的记录才被插入虚拟表VT7中。
8）SELECT：对第二次执行SELECT操作，选择指定的列，插入虚拟表VT8中。
9）DISTINCT：去除重复数据，产生虚拟表VT9。
10）ORDER BY：将虚拟表VT9中的记录按照＜order_by_list>进行排序操作，产生虚拟表VT10。
11）LIMIT：取出指定行的记录，产生虚拟表VT11，并返回给查询用户。

建两张表customers、orders并插入数据

create table customers
(
   customer_id varchar(10) not null,
   city varchar(10) not null, 
   primary key(customer_id) 
)engine=innodb;

insert into customers values('139','Hangzhou');
insert into customers values('guo','Shanghai'); 
insert into customers values('tx','Hangzhou');
insert into customers values('baidu','Shanghai'); 

mysql> select * from customers;
+-------------+----------+
| customer_id | city     |
+-------------+----------+
| 139         | Hangzhou |
| baidu       | Shanghai |
| guo         | Shanghai |
| tx          | Hangzhou |
+-------------+----------+
4 rows in set (0.00 sec)

create table orders
(
 order_id int not null auto_increment,
 customer_id varchar(10),
 primary key(order_id)
)engine=innodb;

insert into orders values(null,'139');
insert into orders values(null,'139');
insert into orders values(null,'guo');
insert into orders values(null,'guo');
insert into orders values(null,'guo');
insert into orders values(null,'tx');
insert into orders values(null,null);

mysql> select * from orders;
+----------+-------------+
| order_id | customer_id |
+----------+-------------+
|        1 | 139         |
|        2 | 139         |
|        3 | guo         |
|        4 | guo         |
|        5 | guo         |
|        6 | tx          |
|        7 | NULL        |
+----------+-------------+
7 rows in set (0.00 sec)

执行笛卡儿积

mysql>  select * from customers c join orders o;      
+-------------+----------+----------+-------------+
| customer_id | city     | order_id | customer_id |
+-------------+----------+----------+-------------+
| 139         | Hangzhou |        1 | 139         |
| baidu       | Shanghai |        1 | 139         |
| guo         | Shanghai |        1 | 139         |
| tx          | Hangzhou |        1 | 139         |
| 139         | Hangzhou |        2 | 139         |
| baidu       | Shanghai |        2 | 139         |
| guo         | Shanghai |        2 | 139         |
| tx          | Hangzhou |        2 | 139         |
| 139         | Hangzhou |        3 | guo         |
| baidu       | Shanghai |        3 | guo         |
| guo         | Shanghai |        3 | guo         |
| tx          | Hangzhou |        3 | guo         |
| 139         | Hangzhou |        4 | guo         |
| baidu       | Shanghai |        4 | guo         |
| guo         | Shanghai |        4 | guo         |
| tx          | Hangzhou |        4 | guo         |
| 139         | Hangzhou |        5 | guo         |
| baidu       | Shanghai |        5 | guo         |
| guo         | Shanghai |        5 | guo         |
| tx          | Hangzhou |        5 | guo         |
| 139         | Hangzhou |        6 | tx          |
| baidu       | Shanghai |        6 | tx          |
| guo         | Shanghai |        6 | tx          |
| tx          | Hangzhou |        6 | tx          |
| 139         | Hangzhou |        7 | NULL        |
| baidu       | Shanghai |        7 | NULL        |
| guo         | Shanghai |        7 | NULL        |
| tx          | Hangzhou |        7 | NULL        |
+-------------+----------+----------+-------------+
28 rows in set (0.00 sec)

用ON过滤

mysql>  select * from  大专栏  查询处理customers c join orders o on c.customer_id=o.customer_id;
+-------------+----------+----------+-------------+
| customer_id | city     | order_id | customer_id |
+-------------+----------+----------+-------------+
| 139         | Hangzhou |        1 | 139         |
| 139         | Hangzhou |        2 | 139         |
| guo         | Shanghai |        3 | guo         |
| guo         | Shanghai |        4 | guo         |
| guo         | Shanghai |        5 | guo         |
| tx          | Hangzhou |        6 | tx          |
+-------------+----------+----------+-------------+
6 rows in set (0.00 sec)

连接（join）
连接分为内连接、外连接、交叉连接，其中外连接又分为左外连接、右外连接、全外连接。左外连接、右外连接中outer关键字都可省略。
左外连接，左表保留，右表符全条件的保留

mysql>  select * from customers c left join orders o on c.customer_id=o.customer_id;
+-------------+----------+----------+-------------+
| customer_id | city     | order_id | customer_id |
+-------------+----------+----------+-------------+
| 139         | Hangzhou |        1 | 139         |
| 139         | Hangzhou |        2 | 139         |
| guo         | Shanghai |        3 | guo         |
| guo         | Shanghai |        4 | guo         |
| guo         | Shanghai |        5 | guo         |
| tx          | Hangzhou |        6 | tx          |
| baidu       | Shanghai |     NULL | NULL        |
+-------------+----------+----------+-------------+
7 rows in set (0.00 sec)

右外连接，右表保留，左表符全条件的保留

mysql>  select * from customers c right join orders o on c.customer_id=o.customer_id;    
+-------------+----------+----------+-------------+
| customer_id | city     | order_id | customer_id |
+-------------+----------+----------+-------------+
| 139         | Hangzhou |        1 | 139         |
| 139         | Hangzhou |        2 | 139         |
| guo         | Shanghai |        3 | guo         |
| guo         | Shanghai |        4 | guo         |
| guo         | Shanghai |        5 | guo         |
| tx          | Hangzhou |        6 | tx          |
| NULL        | NULL     |        7 | NULL        |
+-------------+----------+----------+-------------+
7 rows in set (0.00 sec)

到目前为止MySQL 5.7仍不支持全外连接，

mysql> select @@version;
+-----------+
| @@version |
+-----------+
| 5.7.10    |
+-----------+
1 row in set (0.00 sec)

mysql> select * from a full outer join b on a.id=b.id;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'outer join b on a.id=b.id' at line 1
mysql> select * from a left outer join b on a.id=b.id;    
+------+------+------+------+
| ID   | QTY  | ID   | QTY  |
+------+------+------+------+
|    2 |   20 |    2 |   30 |
|    1 |   10 | NULL | NULL |
+------+------+------+------+
2 rows in set (0.00 sec)

mysql>

where过滤

mysql>  select * from customers c left outer join orders o on c.customer_id=o.customer_id where city='Hangzhou';
+-------------+----------+----------+-------------+
| customer_id | city     | order_id | customer_id |
+-------------+----------+----------+-------------+
| 139         | Hangzhou |        1 | 139         |
| 139         | Hangzhou |        2 | 139         |
| tx          | Hangzhou |        6 | tx          |
+-------------+----------+----------+-------------+
3 rows in set (0.00 sec)

分组
分组会自动去重

mysql>  select * from customers c left outer join orders o on c.customer_id=o.customer_id where city='Hangzhou' group by c.customer_id;
+-------------+----------+----------+-------------+
| customer_id | city     | order_id | customer_id |
+-------------+----------+----------+-------------+
| 139         | Hangzhou |        1 | 139         |
| tx          | Hangzhou |        6 | tx          |
+-------------+----------+----------+-------------+
2 rows in set (0.00 sec)

having过滤
having过滤是对分组条件进行过滤的筛选器

mysql>  select * from customers c left outer join orders o on c.customer_id=o.customer_id where city='Hangzhou' group by c.customer_id having count(o.order_id)<2;
+-------------+----------+----------+-------------+
| customer_id | city     | order_id | customer_id |
+-------------+----------+----------+-------------+
| tx          | Hangzhou |        6 | tx          |
+-------------+----------+----------+-------------+
1 row in set (0.00 sec)

虽然SELECT是查询中最先指定的部分，但是直到步骤8）时才真正进行处理。

mysql>  select c.customer_id ,count(o.order_id) from customers c left outer join orders o on c.customer_id=o.customer_id  group by c.customer_id having count(o.order_id)<2;
+-------------+-------------------+
| customer_id | count(o.order_id) |
+-------------+-------------------+
| baidu       |                 0 |
| tx          |                 1 |
+-------------+-------------------+
2 rows in set (0.00 sec)

排序

mysql>  select c.customer_id ,count(o.order_id) total_order from customers c left outer join orders o on c.customer_id=o.customer_id  group by c.customer_id having count(o.order_id)<2 order by total_order desc;
+-------------+-------------+
| customer_id | total_order |
+-------------+-------------+
| tx          |           1 |
| baidu       |           0 |
+-------------+-------------+
2 rows in set (0.00 sec)

limit

mysql>  select c.customer_id ,count(o.order_id) total_order from customers c left outer join orders o on c.customer_id=o.customer_id  group by c.customer_id having count(o.order_id)<2 order by total_order desc limit 1;
+-------------+-------------+
| customer_id | total_order |
+-------------+-------------+
| tx          |           1 |
+-------------+-------------+
1 row in set (0.00 sec)

标签：customer,Shanghai,查询处理,guo,Hangzhou,139,id
来源： https://www.cnblogs.com/lijianming180/p/12239755.html

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