LEFT JOIN 關(guān)鍵字會(huì)從左表 (table_name1) 那里返回所有的行，即使在右表 (table_name2) 中沒(méi)有匹配的行。
LEFT JOIN 關(guān)鍵字語(yǔ)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(shù)據(jù)庫(kù)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(yǔ)文','A');insert into class values (2,'數(shù)學(xué)','B');insert into class values (3,'英語(yǔ)','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(yǔ)文 | A || 2 | 數(shù)學(xué) | B || 3 | 英語(yǔ) | 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(duì)left join足夠熟悉的話，先不要看結(jié)果，是否可以直接說(shuō)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(yǔ)文 | A | 1 | A001 | 91 || 2 | 數(shù)學(xué) | B | 2 | A001 | 95 || 1 | 語(yǔ)文 | A | 1 | A002 | 82 || 2 | 數(shù)學(xué) | B | 2 | A002 | 87 || 3 | 英語(yǔ) | 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(yǔ)文 | A | 1 | A001 | 91 || 2 | 數(shù)學(xué) | B | 2 | A001 | 95 || 1 | 語(yǔ)文 | A | 1 | A002 | 82 || 2 | 數(shù)學(xué) | B | 2 | A002 | 87 || 3 | 英語(yǔ) | 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(yǔ)文 | A | NULL | NULL | NULL || 2 | 數(shù)學(xué) | B | NULL | NULL | NULL || 3 | 英語(yǔ) | 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(yǔ)文 | A | NULL | NULL | NULL || 2 | 數(shù)學(xué) | B | NULL | NULL | NULL || 3 | 英語(yǔ) | 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(huì)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(yǔ)文';+----------+------------+-------------+----------+--------+-------+| class_id | class_name | class_grade | class_id | stu_id | Score |+----------+------------+-------------+----------+--------+-------+| 1 | 語(yǔ)文 | A | 1 | A001 | 91 || 1 | 語(yǔ)文 | A | 1 | A002 | 82 || 2 | 數(shù)學(xué) | B | NULL | NULL | NULL || 3 | 英語(yǔ) | 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(yǔ)文 | A | NULL | NULL | NULL || 3 | 英語(yǔ) | 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(yǔ)';+----------+------------+-------------+----------+--------+-------+| class_id | class_name | class_grade | class_id | stu_id | Score |+----------+------------+-------------+----------+--------+-------+| 3 | 英語(yǔ) | C | 3 | B003 | 65 || 1 | 語(yǔ)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(yǔ)文 | A | NULL | NULL | NULL || 2 | 數(shù)學(xué) | B | NULL | NULL | NULL || 3 | 英語(yǔ) | 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(duì)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(yǔ)文 | A | NULL | NULL | NULL || 2 | 數(shù)學(xué) | B | NULL | NULL | NULL || 3 | 英語(yǔ) | 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(yǔ) | C | 3 | B003 | 65 || 1 | 語(yǔ)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(huì)根據(jù)(and rightTable.colName='***')過(guò)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(yǔ)文' and B.Score=90;+----------+------------+-------------+----------+--------+-------+| class_id | class_name | class_grade | class_id | stu_id | Score |+----------+------------+-------------+----------+--------+-------+| 1 | 語(yǔ)文 | A | NULL | NULL | NULL || 2 | 數(shù)學(xué) | B | NULL | NULL | NULL || 3 | 英語(yǔ) | 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(yǔ)文' and B.Score=91;+----------+------------+-------------+----------+--------+-------+| class_id | class_name | class_grade | class_id | stu_id | Score |+----------+------------+-------------+----------+--------+-------+| 1 | 語(yǔ)文 | A | 1 | A001 | 91 || 2 | 數(shù)學(xué) | B | NULL | NULL | NULL || 3 | 英語(yǔ) | 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(yǔ)文 | A | NULL | NULL | NULL || 2 | 數(shù)學(xué) | B | NULL | NULL | NULL || 3 | 英語(yǔ) | 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(yǔ)文 | A | NULL | NULL | NULL || 2 | 數(shù)學(xué) | B | NULL | NULL | NULL || 3 | 英語(yǔ) | C | NULL | NULL | NULL |+----------+------------+-------------+----------+--------+-------+3 rows in set (0.00 sec)mysql>
/**********************過(guò)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(guò)濾左表，但是左表不滿足條件的行直接輸出，并將右表對(duì)應(yīng)部分置為null
(2)and rightTable.colName='***'，過(guò)濾右表，對(duì)左表沒(méi)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(yǔ)文';+----------+------------+-------------+----------+--------+-------+| class_id | class_name | class_grade | class_id | stu_id | Score |+----------+------------+-------------+----------+--------+-------+| 1 | 語(yǔ)文 | A | 1 | A001 | 91 || 1 | 語(yǔ)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(yǔ)';+----------+------------+-------------+----------+--------+-------+| class_id | class_name | class_grade | class_id | stu_id | Score |+----------+------------+-------------+----------+--------+-------+| 3 | 英語(yǔ) | 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(yǔ)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(yǔ)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(yǔ)文' and B.Score=91;+----------+------------+-------------+----------+--------+-------+| class_id | class_name | class_grade | class_id | stu_id | Score |+----------+------------+-------------+----------+--------+-------+| 1 | 語(yǔ)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(guò)濾條件在where中時(shí)**********************/
過(guò)濾條件寫(xiě)在where中時(shí)，先根據(jù)where條件對(duì)表進(jìn)行過(guò)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(duì)大家有所幫助，如果大家有任何疑問(wèn)請(qǐng)給我留言，小編會(huì)及時(shí)回復(fù)大家的。在此也非常感謝大家對(duì)VeVb武林網(wǎng)網(wǎng)站的支持！
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