MySQL 高级（二）一条SQL语句的执行顺序 SQL中的七种JOIN图解

MySQL 高级（二）一条SQL语句的执行顺序 SQL中的七种JOIN图解

1. SQL的执行顺序

人的书写思路：

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

MySQL的解析思路：

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

总结：SQL的执行顺序

3.SQL中的七种JOIN

七种Join的维恩图帮助理解：

测试用例：

部门表tbl_dept

CREATE TABLE tbl_dept (
	id INT4 NOT NULL auto_increment,
	dept_name VARCHAR ( 255 ) DEFAULT NULL,
	ioc_add VARCHAR ( 255 ) DEFAULT NULL,
PRIMARY KEY ( id ) 
) ENGINE = INNODB AUTO_INCREMENT = 1 DEFAULT CHARSET = utf8;

员工表tbl_emp

CREATE TABLE tbl_emp (
	id INT4 NOT NULL auto_increment,
	`name` VARCHAR ( 255 ) DEFAULT NULL,
	dept_id INT4 DEFAULT NULL,
	PRIMARY KEY ( id ),
	KEY fk_dept_id ( dept_id ) 
#CONSTRAINT fk_dept_id FOREIGN KEY(dept_id) REFERENCES tbl_dept(id)
) ENGINE = INNODB AUTO_INCREMENT = 1 DEFAULT CHARSET = utf8;

测试数据

INSERT INTO tbl_dept ( dept_name, ioc_add ) VALUES ( 'RD', 11 );
INSERT INTO tbl_dept ( dept_name, ioc_add ) VALUES ( 'HR', 12 );
INSERT INTO tbl_dept ( dept_name, ioc_add ) VALUES ( 'MK', 13 );
INSERT INTO tbl_dept ( dept_name, ioc_add ) VALUES ( 'MIS', 14 );
INSERT INTO tbl_dept ( dept_name, ioc_add ) VALUES ( 'FD', 15 );

INSERT INTO tbl_emp ( name, dept_id ) VALUES ( 'e1', 1 );
INSERT INTO tbl_emp ( name, dept_id ) VALUES ( 'e2', 1 );
INSERT INTO tbl_emp ( name, dept_id ) VALUES ( 'e3', 1 );
INSERT INTO tbl_emp ( name, dept_id ) VALUES ( 'e4', 2 );
INSERT INTO tbl_emp ( name, dept_id ) VALUES ( 'e5', 2 );
INSERT INTO tbl_emp ( name, dept_id ) VALUES ( 'e6', 3 );
INSERT INTO tbl_emp ( name, dept_id ) VALUES ( 'e7', 4 );
INSERT INTO tbl_emp ( name, dept_id ) VALUES ( 'e8', 51 );

3.1 内连接 inner join

内连接 inner join 查询获取a和b表中满足on条件的记录

SELECT
	* 
FROM
	tbl_emp a
	INNER JOIN tbl_dept b ON a.dept_id = b.id

3.2 全连接 full jion

mysql 不支持全连接full jion 写法，会报sql语句错误

解决方法：可以通过unoin关键字，将左连接和右连接合并并去重实现

SELECT
	* 
FROM
	tbl_emp a
	LEFT JOIN tbl_dept b ON a.dept_id = b.id 
UNION
SELECT
	* 
FROM
	tbl_emp a
	RIGHT JOIN tbl_dept b ON a.dept_id = b.id

3.3 全外连接

SELECT
	* 
FROM
	tbl_emp a
	LEFT JOIN tbl_dept b ON a.dept_id = b.id 
WHERE
	b.id IS NULL 
UNION
SELECT
	* 
FROM
	tbl_emp a
	RIGHT JOIN tbl_dept b ON a.dept_id = b.id 
WHERE
	a.dept_id IS NULL