本文主要是介绍【十一】【SQL】外连接（左外连接，右外连接），希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
数据库中的外连接（Outer Join）用于连接两个表，并包括两个表中的匹配行以及左表（LEFT JOIN）或右表（RIGHT JOIN）中未匹配的行。外连接分为两种主要类型：
左外连接（LEFT OUTER JOIN）：返回左表（FROM子句中第一个表）的所有行，即使在右表中没有匹配的行。如果右表中没有匹配的行，则结果集中右表的部分将包含NULL值。
右外连接（RIGHT OUTER JOIN）：返回右表（JOIN子句中指定的表）的所有行，即使在左表中没有匹配的行。如果左表中没有匹配的行，则结果集中左表的部分将包含NULL值。
表的初始化
mysql> create table stu (id int ,name varchar(30));
Query OK, 0 rows affected (0.02 sec)mysql> desc stu;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| name | varchar(30) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)mysql> insert into stu values(1,'Jack'),(2,'Tom'),(3,'Kity'),(4,'Nono');
Query OK, 4 rows affected (0.01 sec)
Records: 4 Duplicates: 0 Warnings: 0mysql> select*from stu;
+------+------+
| id | name |
+------+------+
| 1 | Jack |
| 2 | Tom |
| 3 | Kity |
| 4 | Nono |
+------+------+
4 rows in set (0.00 sec)mysql> create table exam (id int ,grade int );
Query OK, 0 rows affected (0.02 sec)mysql> desc exam;
+-------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| grade | int(11) | YES | | NULL | |
+-------+---------+------+-----+---------+-------+
2 rows in set (0.00 sec)mysql> insert into exam values(1,56),(2,76),(11,82);
Query OK, 3 rows affected (0.01 sec)
Records: 3 Duplicates: 0 Warnings: 0mysql> select*from exam;
+------+-------+
| id | grade |
+------+-------+
| 1 | 56 |
| 2 | 76 |
| 11 | 82 |
+------+-------+
3 rows in set (0.00 sec)mysql>
查询所有学生的成绩，如果这个学生没有成绩，也要将学生的个人信息显示出来
mysql> select *from stu;
+------+------+
| id | name |
+------+------+
| 1 | Jack |
| 2 | Tom |
| 3 | Kity |
| 4 | Nono |
+------+------+
4 rows in set (0.00 sec)mysql> select *from exam;
+------+-------+
| id | grade |
+------+-------+
| 1 | 56 |
| 2 | 76 |
| 11 | 82 |
+------+-------+
3 rows in set (0.00 sec)mysql> select *from stu left join exam on stu.id=exam.id;
+------+------+------+-------+
| id | name | id | grade |
+------+------+------+-------+
| 1 | Jack | 1 | 56 |
| 2 | Tom | 2 | 76 |
| 3 | Kity | NULL | NULL |
| 4 | Nono | NULL | NULL |
+------+------+------+-------+
4 rows in set (0.00 sec)mysql> select *from stu inner join exam on stu.id=exam.id;
+------+------+------+-------+
| id | name | id | grade |
+------+------+------+-------+
| 1 | Jack | 1 | 56 |
| 2 | Tom | 2 | 76 |
+------+------+------+-------+
2 rows in set (0.00 sec)mysql>
左外连接（LEFT JOIN）的查询结果显示了stu表中所有的学生，包括那些在exam表中没有成绩的学生。对于没有成绩的学生（如Kity和Nono），成绩列（grade）显示为NULL。这说明左外连接包括了左表（stu）的所有行，如果右表（exam）中没有匹配的行，则相关列填充NULL。
内连接（INNER JOIN）的查询结果仅显示了那些在stu表和exam表中都有记录的学生。这意味着只有那些既在学生表中也在成绩表中有对应记录的学生才会被显示出来。例如，Jack和Tom的记录被显示了出来，因为他们在两个表中都有对应的记录。而Kity和Nono没有在exam表中的对应记录，所以他们不在查询结果中。
总结来说，外连接（特别是左外连接）用于查询主表（LEFT JOIN左侧的表）中的所有记录，并尝试匹配外表（右侧的表）中的记录。如果外表中没有匹配的记录，则结果集中对应的列会被填充NULL。内连接（INNER JOIN）则仅返回两个表中都有匹配的记录。这些特性使得外连接和内连接在数据查询中各有适用的场景。
对stu表和exam表联合查询，把所有的成绩都显示出来，即使这个成绩没有学生与之对应，也要显示出来
mysql> select*from stu right join exam on stu.id =exam.id;
+------+------+------+-------+
| id | name | id | grade |
+------+------+------+-------+
| 1 | Jack | 1 | 56 |
| 2 | Tom | 2 | 76 |
| NULL | NULL | 11 | 82 |
+------+------+------+-------+
3 rows in set (0.00 sec)mysql> select *from exam left join stu on stu.id=exam.id;
+------+-------+------+------+
| id | grade | id | name |
+------+-------+------+------+
| 1 | 56 | 1 | Jack |
| 2 | 76 | 2 | Tom |
| 11 | 82 | NULL | NULL |
+------+-------+------+------+
3 rows in set (0.01 sec)mysql> 右外连接（RIGHT JOIN）的查询结果展示了exam表中所有的记录，包括那些在stu表中没有对应id的记录。在这个例子中，学生id为11的记录在stu表中没有对应的条目，因此name列显示为NULL。这表明右外连接包括了右表（exam）的所有行，如果左表（stu）中没有匹配的行，则相关列填充NULL。
左外连接（LEFT JOIN）的查询结果与右外连接的查询相反，显示了exam表中所有记录的另一种方式，同时尝试匹配stu表中的记录。与右外连接的结果相同，学生id为11的记录在stu表中没有匹配，导致name列为NULL。
这两个查询展示了外连接的互补性质：
右外连接（RIGHT JOIN）强调了右侧表（在ON关键字右边的表，本例中为exam）的所有行，即使左侧表（stu）中没有匹配的行。
左外连接（LEFT JOIN）强调了左侧表（在ON关键字左边的表，本例中为exam，由于查询的书写方式，实际操作作为左侧表处理）的所有行，即使右侧表（stu）中没有匹配的行。
列出部门名称和这些部门的员工信息，同时列出没有员工的部门
mysql> select *from emp;
+--------+--------+-----------+------+---------------------+---------+---------+--------+
| empno | ename | job | mgr | hiredate | sal | comm | deptno |
+--------+--------+-----------+------+---------------------+---------+---------+--------+
| 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 |
| 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 |
| 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 |
| 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 |
| 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 |
| 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 |
| 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 |
| 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 |
| 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 |
| 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 |
| 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 |
| 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 |
| 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 |
| 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 |
+--------+--------+-----------+------+---------------------+---------+---------+--------+
14 rows in set (0.00 sec)mysql> select distinct deptno from emp;
+--------+
| deptno |
+--------+
| 20 |
| 30 |
| 10 |
+--------+
3 rows in set (0.00 sec)mysql> select*from dept;
+--------+------------+----------+
| deptno | dname | loc |
+--------+------------+----------+
| 10 | ACCOUNTING | NEW YORK |
| 20 | RESEARCH | DALLAS |
| 30 | SALES | CHICAGO |
| 40 | OPERATIONS | BOSTON |
+--------+------------+----------+
4 rows in set (0.00 sec)mysql> select * from dept left join emp on dept.deptno=emp.deptno;
+--------+------------+----------+--------+--------+-----------+------+---------------------+---------+---------+--------+
| deptno | dname | loc | empno | ename | job | mgr | hiredate | sal | comm | deptno |
+--------+------------+----------+--------+--------+-----------+------+---------------------+---------+---------+--------+
| 20 | RESEARCH | DALLAS | 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 |
| 30 | SALES | CHICAGO | 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 |
| 30 | SALES | CHICAGO | 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 |
| 20 | RESEARCH | DALLAS | 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 |
| 30 | SALES | CHICAGO | 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 |
| 30 | SALES | CHICAGO | 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 |
| 10 | ACCOUNTING | NEW YORK | 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 |
| 20 | RESEARCH | DALLAS | 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 |
| 10 | ACCOUNTING | NEW YORK | 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 |
| 30 | SALES | CHICAGO | 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 |
| 20 | RESEARCH | DALLAS | 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 |
| 30 | SALES | CHICAGO | 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 |
| 20 | RESEARCH | DALLAS | 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 |
| 10 | ACCOUNTING | NEW YORK | 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 |
| 40 | OPERATIONS | BOSTON | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL |
+--------+------------+----------+--------+--------+-----------+------+---------------------+---------+---------+--------+
15 rows in set (0.00 sec)mysql> select * from dept left join emp on dept.deptno=emp.deptno order by dept.deptno desc;
+--------+------------+----------+--------+--------+-----------+------+---------------------+---------+---------+--------+
| deptno | dname | loc | empno | ename | job | mgr | hiredate | sal | comm | deptno |
+--------+------------+----------+--------+--------+-----------+------+---------------------+---------+---------+--------+
| 40 | OPERATIONS | BOSTON | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL |
| 30 | SALES | CHICAGO | 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 |
| 30 | SALES | CHICAGO | 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 |
| 30 | SALES | CHICAGO | 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 |
| 30 | SALES | CHICAGO | 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 |
| 30 | SALES | CHICAGO | 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 |
| 30 | SALES | CHICAGO | 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 |
| 20 | RESEARCH | DALLAS | 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 |
| 20 | RESEARCH | DALLAS | 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 |
| 20 | RESEARCH | DALLAS | 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 |
| 20 | RESEARCH | DALLAS | 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 |
| 20 | RESEARCH | DALLAS | 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 |
| 10 | ACCOUNTING | NEW YORK | 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 |
| 10 | ACCOUNTING | NEW YORK | 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 |
| 10 | ACCOUNTING | NEW YORK | 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 |
+--------+------------+----------+--------+--------+-----------+------+---------------------+---------+---------+--------+
15 rows in set (0.00 sec)mysql> select * from dept left join emp on dept.deptno=emp.deptno order by dept.deptno asc;
+--------+------------+----------+--------+--------+-----------+------+---------------------+---------+---------+--------+
| deptno | dname | loc | empno | ename | job | mgr | hiredate | sal | comm | deptno |
+--------+------------+----------+--------+--------+-----------+------+---------------------+---------+---------+--------+
| 10 | ACCOUNTING | NEW YORK | 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 |
| 10 | ACCOUNTING | NEW YORK | 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 |
| 10 | ACCOUNTING | NEW YORK | 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 |
| 20 | RESEARCH | DALLAS | 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 |
| 20 | RESEARCH | DALLAS | 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 |
| 20 | RESEARCH | DALLAS | 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 |
| 20 | RESEARCH | DALLAS | 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 |
| 20 | RESEARCH | DALLAS | 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 |
| 30 | SALES | CHICAGO | 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 |
| 30 | SALES | CHICAGO | 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 |
| 30 | SALES | CHICAGO | 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 |
| 30 | SALES | CHICAGO | 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 |
| 30 | SALES | CHICAGO | 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 |
| 30 | SALES | CHICAGO | 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 |
| 40 | OPERATIONS | BOSTON | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL |
+--------+------------+----------+--------+--------+-----------+------+---------------------+---------+---------+--------+
15 rows in set (0.00 sec)mysql> select dname,ename,dept.deptno from dept left join emp on dept.deptno=emp.deptno order by dept.deptno asc;
+------------+--------+--------+
| dname | ename | deptno |
+------------+--------+--------+
| ACCOUNTING | CLARK | 10 |
| ACCOUNTING | MILLER | 10 |
| ACCOUNTING | KING | 10 |
| RESEARCH | FORD | 20 |
| RESEARCH | JONES | 20 |
| RESEARCH | SMITH | 20 |
| RESEARCH | ADAMS | 20 |
| RESEARCH | SCOTT | 20 |
| SALES | BLAKE | 30 |
| SALES | WARD | 30 |
| SALES | TURNER | 30 |
| SALES | MARTIN | 30 |
| SALES | ALLEN | 30 |
| SALES | JAMES | 30 |
| OPERATIONS | NULL | 40 |
+------------+--------+--------+
15 rows in set (0.00 sec)mysql>
查询员工表 (emp) 和部门表 (dept):
select * from emp; 查询显示了emp表中所有的员工记录。
select * from dept; 查询显示了dept表中所有的部门记录。
使用DISTINCT关键字查询唯一的部门编号 (deptno):
select distinct deptno from emp; 查询从emp表中检索不重复的部门编号。这可以帮助了解有多少不同的部门拥有员工。
左外连接查询:
select * from dept left join emp on dept.deptno=emp.deptno; 这个查询使用左外连接来检索dept表中的所有部门记录以及与之匹配的emp表中的员工记录。如果某个部门没有员工，对应的员工字段将为NULL。这确保了即使某些部门没有员工，部门信息也会被检索出来。
根据部门编号 (deptno) 降序排序:
select * from dept left join emp on dept.deptno=emp.deptno order by dept.deptno desc; 这个查询不仅执行了左外连接，还按照部门编号降序排列了结果。
根据部门编号 (deptno) 升序排序:
select * from dept left join emp on dept.deptno=emp.deptno order by dept.deptno asc; 类似地，这个查询执行了左外连接，但是按照部门编号升序排列了结果。
选择特定字段进行展示:
select dname, ename, dept.deptno from dept left join emp on dept.deptno=emp.deptno order by dept.deptno asc; 这个查询选择了部门名称(dname)、员工名称(ename)和部门编号(dept.deptno)作为输出，并按部门编号升序排列。这样的查询有助于快速查看每个部门及其员工的概览。
结尾
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