sql中的left join及on、where條件關鍵字的區別詳解

2024-08-31 01:05:25

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LEFT JOIN 關鍵字會從左表 (table_name1) 那里返回所有的行，即使在右表 (table_name2) 中沒有匹配的行。

LEFT JOIN 關鍵字語法

SELECT column_name(s)FROM table_name1LEFT JOIN table_name2 ON table_name1.column_name=table_name2.column_name

注釋：在某些數據庫中， LEFT JOIN 稱為 LEFT OUTER JOIN。

創建兩張表并插入一些數據

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，并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，先根據where條件對表進行過濾，然后再執行left join

總結

以上所述是小編給大家介紹的sql中的left join及on、where關鍵字的區別詳解，希望對大家有所幫助，如果大家有任何疑問請給我留言，小編會及時回復大家的。在此也非常感謝大家對VeVb武林網網站的支持！

注：相關教程知識閱讀請移步到MSSQL教程頻道。

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