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LEFT JOIN 關(guān)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(guān)鍵字語法

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

注釋：在某些數(shù)據(jù)庫中， LEFT JOIN 稱為 LEFT OUTER JOIN。

創(chuàng)建兩張表并插入一些數(shù)據(jù)

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(shù)學(xué)','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(shù)據(jù)

mysql> select * from class;
+----------+------------+-------------+
| class_id | class_name | class_grade |
+----------+------------+-------------+
|    1 | 語文    | A      |
|    2 | 數(shù)學(xué)    | 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(jié)果

--如果你對left join足夠熟悉的話，先不要看結(jié)果，是否可以直接說出下面查詢的結(jié)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(shù)學(xué)    | B      |    2 | A001  |  95 |
|    1 | 語文    | A      |    1 | A002  |  82 |
|    2 | 數(shù)學(xué)    | 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(shù)學(xué)    | B      |    2 | A001  |  95 |
|    1 | 語文    | A      |    1 | A002  |  82 |
|    2 | 數(shù)學(xué)    | 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(shù)學(xué)    | 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(shù)學(xué)    | 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(diǎn)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(shù)學(xué)    | 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(shù)學(xué)';
+----------+------------+-------------+----------+--------+-------+
| class_id | class_name | class_grade | class_id | stu_id | Score |
+----------+------------+-------------+----------+--------+-------+
|    2 | 數(shù)學(xué)    | B      |    2 | A001  |  95 |
|    2 | 數(shù)學(xué)    | 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(shù)學(xué)    | 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(shù)學(xué)    | 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(jìn)行匹配(根據(jù)on leftTable.id=rightTable.id)；左表中不滿足條件的行，直接輸出，其對應(yīng)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(shù)學(xué)    | 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(shù)學(xué)    | 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(jù)(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(jù)(on leftTable.id=rightTable.id)和右表中滿足條件的行進(jìn)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(shù)學(xué)    | 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(shù)學(xué)    | 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(shù)學(xué)    | 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(shù)學(xué)    | 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(shí)**********************/

總結(jié)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(yīng)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(fā)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(shù)學(xué)';
+----------+------------+-------------+----------+--------+-------+
| class_id | class_name | class_grade | class_id | stu_id | Score |
+----------+------------+-------------+----------+--------+-------+
|    2 | 數(shù)學(xué)    | B      |    2 | A001  |  95 |
|    2 | 數(shù)學(xué)    | 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(shí)**********************/

過濾條件寫在where中時(shí)，先根據(jù)where條件對表進(jìn)行過濾，然后再執(zhí)行l(wèi)eft join

總結(jié)

以上所述是小編給大家介紹的sql中的left join及on、where關(guān)鍵字的區(qū)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(shí)回復(fù)大家的。在此也非常感謝大家對腳本之家網(wǎng)站的支持！

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