创建两张表并插入一些数据

创造两张表并插入一些数量

create table class(
  class_id int,
  class_name varchar(20),
  class_grade char(1)
);

insert into class values (1,'语文','A');
insert into class values (2,'数学','B');
insert into class values (3,'英语','C');

create table score(
  class_id int,
  stu_id varchar(20),
  Score int
);

insert into score values (1,'A001',91);
insert into score values (2,'A001',95);
insert into score values (1,'A002',82);
insert into score values (2,'A002',87);
insert into score values (3,'B003',65);

 

查阅表中的多少

mysql> select * from class;
+----------+------------+-------------+
| class_id | class_name | class_grade |
+----------+------------+-------------+
|        1 | 语文       | A           |
|        2 | 数学       | B           |
|        3 | 英语       | C           |
+----------+------------+-------------+
3 rows in set (0.00 sec)

mysql> select * from score;
+----------+--------+-------+
| class_id | stu_id | Score |
+----------+--------+-------+
|        1 | A001   |    91 |
|        2 | A001   |    95 |
|        1 | A002   |    82 |
|        2 | A002   |    87 |
|        3 | B003   |    65 |
+----------+--------+-------+
5 rows in set (0.00 sec)

mysql>

 

正如下边几组查询结果
–假如你对left
join丰硕熟识的话,先不要看结果,是不是足以直接揭露下边查询的结果

mysql> select * from class A left join score B on A.class_id=B.class_id;
+----------+------------+-------------+----------+--------+-------+
| class_id | class_name | class_grade | class_id | stu_id | Score |
+----------+------------+-------------+----------+--------+-------+
|        1 | 语文       | A           |        1 | A001   |    91 |
|        2 | 数学       | B           |        2 | A001   |    95 |
|        1 | 语文       | A           |        1 | A002   |    82 |
|        2 | 数学       | B           |        2 | A002   |    87 |
|        3 | 英语       | C           |        3 | B003   |    65 |
+----------+------------+-------------+----------+--------+-------+
5 rows in set (0.00 sec)

mysql> select * from class A left join score B on A.class_id=B.class_id and 1=1;
+----------+------------+-------------+----------+--------+-------+
| class_id | class_name | class_grade | class_id | stu_id | Score |
+----------+------------+-------------+----------+--------+-------+
|        1 | 语文       | A           |        1 | A001   |    91 |
|        2 | 数学       | B           |        2 | A001   |    95 |
|        1 | 语文       | A           |        1 | A002   |    82 |
|        2 | 数学       | B           |        2 | A002   |    87 |
|        3 | 英语       | C           |        3 | B003   |    65 |
+----------+------------+-------------+----------+--------+-------+
5 rows in set (0.01 sec)

mysql> select * from class A left join score B on A.class_id=B.class_id and 1=0;
+----------+------------+-------------+----------+--------+-------+
| class_id | class_name | class_grade | class_id | stu_id | Score |
+----------+------------+-------------+----------+--------+-------+
|        1 | 语文       | A           |     NULL | NULL   |  NULL |
|        2 | 数学       | B           |     NULL | NULL   |  NULL |
|        3 | 英语       | C           |     NULL | NULL   |  NULL |
+----------+------------+-------------+----------+--------+-------+
3 rows in set (0.00 sec)

mysql> select * from class A left join score B on 1=0;
+----------+------------+-------------+----------+--------+-------+
| class_id | class_name | class_grade | class_id | stu_id | Score |
+----------+------------+-------------+----------+--------+-------+
|        1 | 语文       | A           |     NULL | NULL   |  NULL |
|        2 | 数学       | B           |     NULL | NULL   |  NULL |
|        3 | 英语       | C           |     NULL | NULL   |  NULL |
+----------+------------+-------------+----------+--------+-------+
3 rows in set (0.00 sec)

mysql> left join的最重要特点是:不管on后面是什么条件,都会返回左表中的所有行!

 

mysql> select * from class A left join score B on A.class_id=B.class_id and A.class_name='语文';
+----------+------------+-------------+----------+--------+-------+
| class_id | class_name | class_grade | class_id | stu_id | Score |
+----------+------------+-------------+----------+--------+-------+
|        1 | 语文       | A           |        1 | A001   |    91 |
|        1 | 语文       | A           |        1 | A002   |    82 |
|        2 | 数学       | B           |     NULL | NULL   |  NULL |
|        3 | 英语       | C           |     NULL | NULL   |  NULL |
+----------+------------+-------------+----------+--------+-------+
4 rows in set (0.00 sec)

mysql> select * from class A left join score B on A.class_id=B.class_id and A.class_name='数学';
+----------+------------+-------------+----------+--------+-------+
| class_id | class_name | class_grade | class_id | stu_id | Score |
+----------+------------+-------------+----------+--------+-------+
|        2 | 数学       | B           |        2 | A001   |    95 |
|        2 | 数学       | B           |        2 | A002   |    87 |
|        1 | 语文       | A           |     NULL | NULL   |  NULL |
|        3 | 英语       | C           |     NULL | NULL   |  NULL |
+----------+------------+-------------+----------+--------+-------+
4 rows in set (0.00 sec)

mysql> select * from class A left join score B on A.class_id=B.class_id and A.class_name='英语';
+----------+------------+-------------+----------+--------+-------+
| class_id | class_name | class_grade | class_id | stu_id | Score |
+----------+------------+-------------+----------+--------+-------+
|        3 | 英语       | C           |        3 | B003   |    65 |
|        1 | 语文       | A           |     NULL | NULL   |  NULL |
|        2 | 数学       | B           |     NULL | NULL   |  NULL |
+----------+------------+-------------+----------+--------+-------+
3 rows in set (0.01 sec)

mysql> select * from class A left join score B on A.class_id=B.class_id and A.class_name='体育';
+----------+------------+-------------+----------+--------+-------+
| class_id | class_name | class_grade | class_id | stu_id | Score |
+----------+------------+-------------+----------+--------+-------+
|        1 | 语文       | A           |     NULL | NULL   |  NULL |
|        2 | 数学       | B           |     NULL | NULL   |  NULL |
|        3 | 英语       | C           |     NULL | NULL   |  NULL |
+----------+------------+-------------+----------+--------+-------+
3 rows in set (0.00 sec)

mysql>

假使on后边的标准是左表中的列(and
leftTable.colName=’***’),左表中满意条件的行和右表中的行开展相配(依照on
leftTable.id=rightTable.id);左表中不满意条件的行,间接出口,其对应的右表中的列都以null。

 

mysql> select * from class A left join score B on A.class_id=B.class_id and B.Score=90;
+----------+------------+-------------+----------+--------+-------+
| class_id | class_name | class_grade | class_id | stu_id | Score |
+----------+------------+-------------+----------+--------+-------+
|        1 | 语文       | A           |     NULL | NULL   |  NULL |
|        2 | 数学       | B           |     NULL | NULL   |  NULL |
|        3 | 英语       | C           |     NULL | NULL   |  NULL |
+----------+------------+-------------+----------+--------+-------+
3 rows in set (0.01 sec)

mysql> select * from class A left join score B on A.class_id=B.class_id and B.Score=65;
+----------+------------+-------------+----------+--------+-------+
| class_id | class_name | class_grade | class_id | stu_id | Score |
+----------+------------+-------------+----------+--------+-------+
|        3 | 英语       | C           |        3 | B003   |    65 |
|        1 | 语文       | A           |     NULL | NULL   |  NULL |
|        2 | 数学       | B           |     NULL | NULL   |  NULL |
+----------+------------+-------------+----------+--------+-------+
3 rows in set (0.01 sec)

mysql>

即使on后边的规范是右表中的列(and
rightTable.colName=’***’),首先会凭仗(and
rightTable.colName=’***’)过滤掉右表中不满意条件的行;然后,左表中的行依据(on
leftTable.id=rightTable.id)和右表中满足条件的行开展相称。

 

mysql> select * from class A left join score B on A.class_id=B.class_id and A.class_name='语文' and B.Score=90;
+----------+------------+-------------+----------+--------+-------+
| class_id | class_name | class_grade | class_id | stu_id | Score |
+----------+------------+-------------+----------+--------+-------+
|        1 | 语文       | A           |     NULL | NULL   |  NULL |
|        2 | 数学       | B           |     NULL | NULL   |  NULL |
|        3 | 英语       | C           |     NULL | NULL   |  NULL |
+----------+------------+-------------+----------+--------+-------+
3 rows in set (0.00 sec)

mysql> select * from class A left join score B on A.class_id=B.class_id and A.class_name='语文' and B.Score=91;
+----------+------------+-------------+----------+--------+-------+
| class_id | class_name | class_grade | class_id | stu_id | Score |
+----------+------------+-------------+----------+--------+-------+
|        1 | 语文       | A           |        1 | A001   |    91 |
|        2 | 数学       | B           |     NULL | NULL   |  NULL |
|        3 | 英语       | C           |     NULL | NULL   |  NULL |
+----------+------------+-------------+----------+--------+-------+
3 rows in set (0.01 sec)

mysql> select * from class A left join score B on A.class_id=B.class_id and A.class_name='体育' and B.Score=90;
+----------+------------+-------------+----------+--------+-------+
| class_id | class_name | class_grade | class_id | stu_id | Score |
+----------+------------+-------------+----------+--------+-------+
|        1 | 语文       | A           |     NULL | NULL   |  NULL |
|        2 | 数学       | B           |     NULL | NULL   |  NULL |
|        3 | 英语       | C           |     NULL | NULL   |  NULL |
+----------+------------+-------------+----------+--------+-------+
3 rows in set (0.00 sec)

mysql> select * from class A left join score B on A.class_id=B.class_id and A.class_name='体育' and B.Score=82;
+----------+------------+-------------+----------+--------+-------+
| class_id | class_name | class_grade | class_id | stu_id | Score |
+----------+------------+-------------+----------+--------+-------+
|        1 | 语文       | A           |     NULL | NULL   |  NULL |
|        2 | 数学       | B           |     NULL | NULL   |  NULL |
|        3 | 英语       | C           |     NULL | NULL   |  NULL |
+----------+------------+-------------+----------+--------+-------+
3 rows in set (0.00 sec)

mysql>

/**********************过滤条件在on中时**********************/
小结一下,要是 left join on leftTable.id=rightTable.id
后还也会有其余规范:
(1)and
leftTable.colName=’***’,过滤左表,不过左表不满意条件的行间接出口,并将右表对应部分置为null
(2)and rightTable.colName=’***’,过滤右表,对左表未有影响
(3)and leftTable.colName=’***’ and
rightTable.colName=’***’,就是地点(1)和(2)一同发挥作用

不管on后边有哪些条件,left join都要赶回左表中的全体行!

 

mysql> select * from class A left join score B on A.class_id=B.class_id where A.class_name='语文';
+----------+------------+-------------+----------+--------+-------+
| class_id | class_name | class_grade | class_id | stu_id | Score |
+----------+------------+-------------+----------+--------+-------+
|        1 | 语文       | A           |        1 | A001   |    91 |
|        1 | 语文       | A           |        1 | A002   |    82 |
+----------+------------+-------------+----------+--------+-------+
2 rows in set (0.01 sec)

mysql> select * from class A left join score B on A.class_id=B.class_id where A.class_name='数学';
+----------+------------+-------------+----------+--------+-------+
| class_id | class_name | class_grade | class_id | stu_id | Score |
+----------+------------+-------------+----------+--------+-------+
|        2 | 数学       | B           |        2 | A001   |    95 |
|        2 | 数学       | B           |        2 | A002   |    87 |
+----------+------------+-------------+----------+--------+-------+
2 rows in set (0.00 sec)

mysql> select * from class A left join score B on A.class_id=B.class_id where A.class_name='英语';
+----------+------------+-------------+----------+--------+-------+
| class_id | class_name | class_grade | class_id | stu_id | Score |
+----------+------------+-------------+----------+--------+-------+
|        3 | 英语       | C           |        3 | B003   |    65 |
+----------+------------+-------------+----------+--------+-------+
1 row in set (0.00 sec)

mysql> select * from class A left join score B on A.class_id=B.class_id where A.class_name='体育';
Empty set (0.00 sec)

mysql> select * from class A left join score B on A.class_id=B.class_id where B.Score=90;
Empty set (0.01 sec)

mysql> select * from class A left join score B on A.class_id=B.class_id where B.Score=91;
+----------+------------+-------------+----------+--------+-------+
| class_id | class_name | class_grade | class_id | stu_id | Score |
+----------+------------+-------------+----------+--------+-------+
|        1 | 语文       | A           |        1 | A001   |    91 |
+----------+------------+-------------+----------+--------+-------+
1 row in set (0.00 sec)

mysql> select * from class A left join score B on A.class_id=B.class_id where A.class_name='语文' and B.Score=90;
Empty set (0.00 sec)

mysql> select * from class A left join score B on A.class_id=B.class_id where A.class_name='语文' and B.Score=91;
+----------+------------+-------------+----------+--------+-------+
| class_id | class_name | class_grade | class_id | stu_id | Score |
+----------+------------+-------------+----------+--------+-------+
|        1 | 语文       | A           |        1 | A001   |    91 |
+----------+------------+-------------+----------+--------+-------+
1 row in set (0.00 sec)

mysql> select * from class A left join score B on A.class_id=B.class_id where A.class_name='体育' and B.Score=90;
Empty set (0.00 sec)

mysql> select * from class A left join score B on A.class_id=B.class_id where A.class_name='体育' and B.Score=91;
Empty set (0.00 sec)

mysql> 

/**********************过滤条件在where中时**********************/

过滤条件写在where中时,先依照where条件对表进行过滤,然后再实践left join

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