sql中的left join以及on、where關鍵字的區別

創建兩張表並插入一些數據

create table class(
  class_id int,
  class_name varchar(20),
  class_grade char(1)
);
 
insert into class values (1,'語文','A');
insert into class values (2,'數學','B');
insert into class values (3,'英語','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);

 

查看表中的數據

mysql> select * from class;
+----------+------------+-------------+
| class_id | class_name | class_grade |
+----------+------------+-------------+
|        1 | 語文       | A           |
|        2 | 數學       | B           |
|        3 | 英語       | 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>

 

比較下面幾組查詢結果
--如果你對left join足夠熟悉的話，先不要看結果，是否可以直接說出下面查詢的結果

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 | 語文       | A           |        1 | A001   |    91 |
|        2 | 數學       | B           |        2 | A001   |    95 |
|        1 | 語文       | A           |        1 | A002   |    82 |
|        2 | 數學       | B           |        2 | A002   |    87 |
|        3 | 英語       | 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 | 語文       | A           |        1 | A001   |    91 |
|        2 | 數學       | B           |        2 | A001   |    95 |
|        1 | 語文       | A           |        1 | A002   |    82 |
|        2 | 數學       | B           |        2 | A002   |    87 |
|        3 | 英語       | 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 | 語文       | A           |     NULL | NULL   |  NULL |
|        2 | 數學       | B           |     NULL | NULL   |  NULL |
|        3 | 英語       | 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 | 語文       | A           |     NULL | NULL   |  NULL |
|        2 | 數學       | B           |     NULL | NULL   |  NULL |
|        3 | 英語       | C           |     NULL | NULL   |  NULL |
+----------+------------+-------------+----------+--------+-------+
3 rows in set (0.00 sec)

mysql> left join的最重要特點是：不管on后面是什么條件，都會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='語文';
+----------+------------+-------------+----------+--------+-------+
| class_id | class_name | class_grade | class_id | stu_id | Score |
+----------+------------+-------------+----------+--------+-------+
|        1 | 語文       | A           |        1 | A001   |    91 |
|        1 | 語文       | A           |        1 | A002   |    82 |
|        2 | 數學       | B           |     NULL | NULL   |  NULL |
|        3 | 英語       | 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='數學';
+----------+------------+-------------+----------+--------+-------+
| class_id | class_name | class_grade | class_id | stu_id | Score |
+----------+------------+-------------+----------+--------+-------+
|        2 | 數學       | B           |        2 | A001   |    95 |
|        2 | 數學       | B           |        2 | A002   |    87 |
|        1 | 語文       | A           |     NULL | NULL   |  NULL |
|        3 | 英語       | 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='英語';
+----------+------------+-------------+----------+--------+-------+
| class_id | class_name | class_grade | class_id | stu_id | Score |
+----------+------------+-------------+----------+--------+-------+
|        3 | 英語       | C           |        3 | B003   |    65 |
|        1 | 語文       | A           |     NULL | NULL   |  NULL |
|        2 | 數學       | 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='體育';
+----------+------------+-------------+----------+--------+-------+
| class_id | class_name | class_grade | class_id | stu_id | Score |
+----------+------------+-------------+----------+--------+-------+
|        1 | 語文       | A           |     NULL | NULL   |  NULL |
|        2 | 數學       | B           |     NULL | NULL   |  NULL |
|        3 | 英語       | C           |     NULL | NULL   |  NULL |
+----------+------------+-------------+----------+--------+-------+
3 rows in set (0.00 sec)

mysql>

如果on后面的條件是左表中的列(and leftTable.colName='***'),左表中滿足條件的行和右表中的行進行匹配(根據on leftTable.id=rightTable.id)；左表中不滿足條件的行，直接輸出，其對應的右表中的列都是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 | 語文       | A           |     NULL | NULL   |  NULL |
|        2 | 數學       | B           |     NULL | NULL   |  NULL |
|        3 | 英語       | 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 | 英語       | C           |        3 | B003   |    65 |
|        1 | 語文       | A           |     NULL | NULL   |  NULL |
|        2 | 數學       | B           |     NULL | NULL   |  NULL |
+----------+------------+-------------+----------+--------+-------+
3 rows in set (0.01 sec)

mysql>

如果on后面的條件是右表中的列(and rightTable.colName='***'),首先會根據(and rightTable.colName='***')過濾掉右表中不滿足條件的行；然后，左表中的行根據(on leftTable.id=rightTable.id)和右表中滿足條件的行進行匹配。

 

mysql> select * from class A left join score B on A.class_id=B.class_id and A.class_name='語文' and B.Score=90;
+----------+------------+-------------+----------+--------+-------+
| class_id | class_name | class_grade | class_id | stu_id | Score |
+----------+------------+-------------+----------+--------+-------+
|        1 | 語文       | A           |     NULL | NULL   |  NULL |
|        2 | 數學       | B           |     NULL | NULL   |  NULL |
|        3 | 英語       | 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='語文' and B.Score=91;
+----------+------------+-------------+----------+--------+-------+
| class_id | class_name | class_grade | class_id | stu_id | Score |
+----------+------------+-------------+----------+--------+-------+
|        1 | 語文       | A           |        1 | A001   |    91 |
|        2 | 數學       | B           |     NULL | NULL   |  NULL |
|        3 | 英語       | 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='體育' and B.Score=90;
+----------+------------+-------------+----------+--------+-------+
| class_id | class_name | class_grade | class_id | stu_id | Score |
+----------+------------+-------------+----------+--------+-------+
|        1 | 語文       | A           |     NULL | NULL   |  NULL |
|        2 | 數學       | B           |     NULL | NULL   |  NULL |
|        3 | 英語       | 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='體育' and B.Score=82;
+----------+------------+-------------+----------+--------+-------+
| class_id | class_name | class_grade | class_id | stu_id | Score |
+----------+------------+-------------+----------+--------+-------+
|        1 | 語文       | A           |     NULL | NULL   |  NULL |
|        2 | 數學       | B           |     NULL | NULL   |  NULL |
|        3 | 英語       | C           |     NULL | NULL   |  NULL |
+----------+------------+-------------+----------+--------+-------+
3 rows in set (0.00 sec)

mysql>

/**********************過濾條件在on中時**********************/
總結一下，如果 left join on leftTable.id=rightTable.id 后還有其他條件：
(1)and leftTable.colName='***'，過濾左表，但是左表不滿足條件的行直接輸出，並將右表對應部分置為null
(2)and rightTable.colName='***'，過濾右表，對左表沒有影響
(3)and leftTable.colName='***' and rightTable.colName='***'，就是上面(1)和(2)一起發揮作用

不管on后面有哪些條件，left join都要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 where A.class_name='語文';
+----------+------------+-------------+----------+--------+-------+
| class_id | class_name | class_grade | class_id | stu_id | Score |
+----------+------------+-------------+----------+--------+-------+
|        1 | 語文       | A           |        1 | A001   |    91 |
|        1 | 語文       | 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='數學';
+----------+------------+-------------+----------+--------+-------+
| class_id | class_name | class_grade | class_id | stu_id | Score |
+----------+------------+-------------+----------+--------+-------+
|        2 | 數學       | B           |        2 | A001   |    95 |
|        2 | 數學       | 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='英語';
+----------+------------+-------------+----------+--------+-------+
| class_id | class_name | class_grade | class_id | stu_id | Score |
+----------+------------+-------------+----------+--------+-------+
|        3 | 英語       | 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='體育';
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 | 語文       | 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='語文' 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='語文' and B.Score=91;
+----------+------------+-------------+----------+--------+-------+
| class_id | class_name | class_grade | class_id | stu_id | Score |
+----------+------------+-------------+----------+--------+-------+
|        1 | 語文       | 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='體育' 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='體育' and B.Score=91;
Empty set (0.00 sec)

mysql> 

/**********************過濾條件在where中時**********************/

過濾條件寫在where中時，先執行left join，然后再根據where條件進行過濾