SQL逻辑查询语句执行顺序

一、select 语句关键字的定义顺序

View Code

SELECT DISTINCT <select_list>
FROM <left_table>
<join_type> JOIN <right_table>
ON <join_condition>
WHERE <where_condition>
GROUP BY <group_by_list>
HAVING <having_condition>
ORDER BY <order_by_condition>
LIMIT <limit_number>

二、select 语句关键字的执行顺序

执行顺序

(7)     SELECT 
(8)     DISTINCT <select_list>
(1)     FROM <left_table>
(2)     <join_type> JOIN <right_table>
(3)     ON <join_condition>
(4)     WHERE <where_condition>
(5)     GROUP BY <group_by_list>
(6)     HAVING <having_condition>
(9)     ORDER BY <order_by_condition>
(10)    LIMIT <limit_number>

三、准备表和数据

1.新建一个测试数据库TestDB；

create database TestDB;

2.创建测试表table1和table2；

create table table1(
customer_id varchar(10) not null,
city varchar(10) not null,
primary key (customer_id)
)engine=innodb default charset=UTF8;create table table2(
order_id int not null auto_increment,
customer_id varchar(10),
primary key(order_id)
)engine=innodb default charset=utf8;

3.插入测试数据

INSERT INTO table1(customer_id,city) VALUES('163','hangzhou');
INSERT INTO table1(customer_id,city) VALUES('9you','shanghai');
INSERT INTO table1(customer_id,city) VALUES('tx','hangzhou');
INSERT INTO table1(customer_id,city) VALUES('baidu','hangzhou');INSERT INTO table2(customer_id) VALUES('163');
INSERT INTO table2(customer_id) VALUES('163');
INSERT INTO table2(customer_id) VALUES('9you');
INSERT INTO table2(customer_id) VALUES('9you');
INSERT INTO table2(customer_id) VALUES('9you');
INSERT INTO table2(customer_id) VALUES('tx');
INSERT INTO table2(customer_id) VALUES(NULL);

查看table1 , table2

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

四、准备SQL逻辑查询测试语句

查询来自杭州，并且订单数少于2的客户

SELECTa.customer_id,count( b.order_id ) AS total_orders 
FROMtable1 AS aLEFT JOIN table2 AS b ON a.customer_id = b.customer_id 
WHEREa.city = 'hangzhou' 
GROUP BYa.customer_id 
HAVINGcount( b.order_id ) < 2 
ORDER BYtotal_orders DESC;

五、执行顺序分析

在这些SQL语句的执行过程中，都会产生一个虚拟表，用来保存SQL语句的执行结果（这是重点），
我们现在就来跟踪这个虚拟表的变化，得到最终的查询结果的过程，来分析整个SQL逻辑查询的执行顺序和过程。

执行FROM语句

第一步，执行FROM语句。我们首先需要知道最开始从哪个表开始的，这就是FROM告诉我们的。现在有了<left_table>和<right_table>两个表，我们到底从哪个表开始，还是从两个表进行某种联系以后再开始呢？它们之间如何产生联系呢？——笛卡尔积

关于什么是笛卡尔积，请自行Google补脑。经过FROM语句对两个表执行笛卡尔积，会得到一个虚拟表，暂且叫VT1（vitual table 1），内容如下：

 mysql> select * from table1 join table2;

+-------------+----------+----------+-------------+
| customer_id | city     | order_id | customer_id |
+-------------+----------+----------+-------------+
| 163         | hangzhou |        1 | 163         |
| 9you        | shanghai |        1 | 163         |
| baidu       | hangzhou |        1 | 163         |
| tx          | hangzhou |        1 | 163         |
| 163         | hangzhou |        2 | 163         |
| 9you        | shanghai |        2 | 163         |
| baidu       | hangzhou |        2 | 163         |
| tx          | hangzhou |        2 | 163         |
| 163         | hangzhou |        3 | 9you        |
| 9you        | shanghai |        3 | 9you        |
| baidu       | hangzhou |        3 | 9you        |
| tx          | hangzhou |        3 | 9you        |
| 163         | hangzhou |        4 | 9you        |
| 9you        | shanghai |        4 | 9you        |
| baidu       | hangzhou |        4 | 9you        |
| tx          | hangzhou |        4 | 9you        |
| 163         | hangzhou |        5 | 9you        |
| 9you        | shanghai |        5 | 9you        |
| baidu       | hangzhou |        5 | 9you        |
| tx          | hangzhou |        5 | 9you        |
| 163         | hangzhou |        6 | tx          |
| 9you        | shanghai |        6 | tx          |
| baidu       | hangzhou |        6 | tx          |
| tx          | hangzhou |        6 | tx          |
| 163         | hangzhou |        7 | NULL        |
| 9you        | shanghai |        7 | NULL        |
| baidu       | hangzhou |        7 | NULL        |
| tx          | hangzhou |        7 | NULL        |
+-------------+----------+----------+-------------+

总共有28（table1的记录条数 * table2的记录条数）条记录。这就是VT1的结果，接下来的操作就在VT1的基础上进行。