DML语句

Posted 2021-11-26 qiuxirufeng

DML 操作是指对数据库中表记录的操作，主要包括表记录的插入（insert）、更新（update）、删除（delete）和查询（select），是开发人员日常使用最频繁的操作。

插入记录

表创建好后，就可以往里插入记录了，插入记录的基本语法如下

INSERT INTO tablename (field1, field2, .…, fieldn）VALUES (value1, value2, ..……, valuen);

向表 emp 中插入：ename 为 zzx1，hiredate 为 2000-01-01，sal 为 2000，deptno 为 1

mysql> desc emp;
+----------+---------------+------+-----+---------+-------+
| Field    | Type          | Null | Key | Default | Extra |
+----------+---------------+------+-----+---------+-------+
| ename    | varchar(10)   | YES  |     | NULL    |       |
| hiredate | date          | YES  |     | NULL    |       |
| sal      | decimal(10,2) | YES  |     | NULL    |       |
| deptno   | int(2)        | YES  |     | NULL    |       |
+----------+---------------+------+-----+---------+-------+
4 rows in set (0.01 sec)

mysql> insert into emp(ename,hiredate,sal,deptno) values('zzx1','2000-01-01','2000',1);
Query OK, 1 row affected (0.01 sec)

也可以不用指定字段名称，但是 values 后面的顺序应该和字段的排列顺序一致

mysql> insert into emp values('lisa','2003-02-01','3000',2);
Query OK, 1 row affected (0.01 sec)

mysql> insert into emp values('bjguan','2004-04-02','4000',1);
Query OK, 1 row affected (0.01 sec)

只对表中的 ename 和 sal 字段显式插入值

mysql> insert into emp(ename,sal) values('dony',1000);
Query OK, 1 row affected (0.01 sec)

查看实际插入的值

mysql> select * from emp;
+--------+------------+---------+--------+
| ename  | hiredate   | sal     | deptno |
+--------+------------+---------+--------+
| zzx1   | 2000-01-01 | 2000.00 |      1 |
| lisa   | 2003-02-01 | 3000.00 |      2 |
| bjguan | 2004-04-02 | 4000.00 |      1 |
| dony   | NULL       | 1000.00 |   NULL |
+--------+------------+---------+--------+
4 rows in set (0.00 sec)

可以一次性插入多条记录，每条记录之间用逗号分隔

INSERT INTO tablename (field1, field2, ..…, fieldn)
VALUES
(record1_value1, record1_value2, .…, record1_valuen),
(record2_value1, record2_value2, .…, record2_valuen),
...
(recordn_value1, recordn_value2, .…, recordn_valuen);

首先创建表 dept

mysql> create table dept(deptno int(2), deptname varchar(10));
Query OK, 0 rows affected (0.03 sec)

mysql> desc dept;
+----------+-------------+------+-----+---------+-------+
| Field    | Type        | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| deptno   | int(2)      | YES  |     | NULL    |       |
| deptname | varchar(10) | YES  |     | NULL    |       |
+----------+-------------+------+-----+---------+-------+
2 rows in set (0.01 sec)

对 dept 表一次插入多条记录

mysql> insert into dept values(1,'tech'),(2,'sale'),(5,'fin'),(5,'dept5'),(6,'dept6');
Query OK, 5 rows affected (0.01 sec)
Records: 5  Duplicates: 0  Warnings: 0

mysql> select * from dept;
+--------+----------+
| deptno | deptname |
+--------+----------+
|      1 | tech     |
|      2 | sale     |
|      5 | fin      |
|      5 | dept5    |
|      6 | dept6    |
+--------+----------+
5 rows in set (0.00 sec)

更新记录

UPDATE tablename SET field1=value1, field2=value2, ..., fieldn=valuen [WHERE CONDITION]

将表 emp 中 ename 为 “lisa” 的薪水（sal）从 3000 改为 4000

mysql> update emp set sal=4000 where ename='lisa';
Query OK, 1 row affected (0.01 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from emp;
+--------+------------+---------+--------+
| ename  | hiredate   | sal     | deptno |
+--------+------------+---------+--------+
| zzx1   | 2000-01-01 | 2000.00 |      1 |
| lisa   | 2003-02-01 | 4000.00 |      2 |
| bjguan | 2004-04-02 | 4000.00 |      1 |
| dony   | NULL       | 1000.00 |   NULL |
+--------+------------+---------+--------+
4 rows in set (0.00 sec)

update 命令可以同时更新多个表中的数据

UPDATE t1, t2, ..., tn set t1.field1=expr1, t2.field2=expr2, ..., tn.fieldn=exprn [WHERE CONDITION]

同时更新表 emp 中的字段 sal 和表 dept 中的字段 deptname

mysql> select * from emp;
+--------+------------+---------+--------+
| ename  | hiredate   | sal     | deptno |
+--------+------------+---------+--------+
| zzx1   | 2000-01-01 |  100.00 |      1 |
| lisa   | 2003-02-01 |  200.00 |      2 |
| bjguan | 2004-04-02 |  100.00 |      1 |
| dony   | 2005-02-05 | 2000.00 |      4 |
+--------+------------+---------+--------+
4 rows in set (0.00 sec)

mysql> select * from dept;
+--------+----------+
| deptno | deptname |
+--------+----------+
|      1 | tech     |
|      2 | sale     |
|      5 | fin      |
+--------+----------+
3 rows in set (0.00 sec)

mysql> update emp a, dept b set a.sal=a.sal*b.deptno, b.deptname=a.ename where a.deptno=b.deptno;
Query OK, 3 rows affected (0.01 sec)
Rows matched: 5  Changed: 3  Warnings: 0

mysql> select * from emp;
+--------+------------+---------+--------+
| ename  | hiredate   | sal     | deptno |
+--------+------------+---------+--------+
| zzx1   | 2000-01-01 |  100.00 |      1 |
| lisa   | 2003-02-01 |  400.00 |      2 |
| bjguan | 2004-04-02 |  100.00 |      1 |
| dony   | 2005-02-05 | 2000.00 |      4 |
+--------+------------+---------+--------+
4 rows in set (0.00 sec)

mysql> select * from dept;
+--------+----------+
| deptno | deptname |
+--------+----------+
|      1 | zzx1     |
|      2 | lisa     |
|      5 | fin      |
+--------+----------+
3 rows in set (0.00 sec)

删除记录

DELETE FROM tablename [where CONDITION]

将 emp 中 ename 为 “dony” 的记录全部删除

mysql> delete from emp where ename='dony';
Query OK, 1 row affected (0.01 sec)

mysql> select * from emp;
+--------+------------+--------+--------+
| ename  | hiredate   | sal    | deptno |
+--------+------------+--------+--------+
| zzx1   | 2000-01-01 | 100.00 |      1 |
| lisa   | 2003-02-01 | 400.00 |      2 |
| bjguan | 2004-04-02 | 100.00 |      1 |
+--------+------------+--------+--------+
3 rows in set (0.00 sec)

可以一次删除多个表的数据

DELETE t1, t2, ..., tn FROM t1, t2, ..., tn [WHERE CONDITION]

如果 from 后面的表名用别名，则 delete 后面也要用相应的别名，否则会提示语法错误

同时删除表 emp 和 dept 中 deptno 为 3 的记录

mysql> select * from emp;
+--------+------------+---------+--------+
| ename  | hiredate   | sal     | deptno |
+--------+------------+---------+--------+
| zzx1   | 2000-01-01 |  100.00 |      1 |
| lisa   | 2003-02-01 |  200.00 |      2 |
| bjguan | 2004-04-02 |  100.00 |      1 |
| bzshen | 2005-04-01 |  300.00 |      3 |
| dony   | 2005-02-05 | 2000.00 |      4 |
+--------+------------+---------+--------+
5 rows in set (0.00 sec)

mysql> select * from dept;
+--------+----------+
| deptno | deptname |
+--------+----------+
|      1 | tach     |
|      2 | sale     |
|      3 | hr       |
|      5 | fin      |
+--------+----------+
4 rows in set (0.00 sec)

mysql> delete a,b from emp a,dept b where a.deptno=b.deptno and a.deptno=3;
Query OK, 2 rows affected (0.01 sec)

mysql> select * from emp;
+--------+------------+---------+--------+
| ename  | hiredate   | sal     | deptno |
+--------+------------+---------+--------+
| zzx1   | 2000-01-01 |  100.00 |      1 |
| lisa   | 2003-02-01 |  200.00 |      2 |
| bjguan | 2004-04-02 |  100.00 |      1 |
| dony   | 2005-02-05 | 2000.00 |      4 |
+--------+------------+---------+--------+
4 rows in set (0.00 sec)

mysql> select * from dept;
+--------+----------+
| deptno | deptname |
+--------+----------+
|      1 | tach     |
|      2 | sale     |
|      5 | fin      |
+--------+----------+
3 rows in set (0.00 sec)

查询记录

SELECT * FROM tablename [where CONDITION]

显示 emp 表中的全部记录

mysql> select * from emp;
+--------+------------+---------+--------+
| ename  | hiredate   | sal     | deptno |
+--------+------------+---------+--------+
| zzx    | 2000-01-01 | 2000.00 |      1 |
| lisa   | 2003-02-01 | 4000.00 |      2 |
| bjguan | 2004-04-02 | 5000.00 |      1 |
+--------+------------+---------+--------+
3 rows in set (0.00 sec)

mysql> select ename,hiredate,sal,deptno from emp;
+--------+------------+---------+--------+
| ename  | hiredate   | sal     | deptno |
+--------+------------+---------+--------+
| zzx    | 2000-01-01 | 2000.00 |      1 |
| lisa   | 2003-02-01 | 4000.00 |      2 |
| bjguan | 2004-04-02 | 5000.00 |      1 |
+--------+------------+---------+--------+
3 rows in set (0.00 sec)

查询不重复的记录

将表中的记录去掉重复后显示出来，可以用distinct关键字来实现：

mysql> select ename,hiredate,sal,deptno from emp;
+--------+------------+---------+--------+
| ename  | hiredate   | sal     | deptno |
+--------+------------+---------+--------+
| zzx    | 2000-01-01 | 2000.00 |      1 |
| lisa   | 2003-02-01 | 4000.00 |      2 |
| bjguan | 2004-04-02 | 5000.00 |      1 |
+--------+------------+---------+--------+
3 rows in set (0.00 sec)

mysql> select distinct deptno from emp;
+--------+
| deptno |
+--------+
|      1 |
|      2 |
+--------+
2 rows in set (0.00 sec)

条件查询

查询所有 deptno 为 1 的记录

mysql> select * from emp;
+--------+------------+---------+--------+
| ename  | hiredate   | sal     | deptno |
+--------+------------+---------+--------+
| zzx    | 2000-01-01 | 2000.00 |      1 |
| lisa   | 2003-02-01 | 4000.00 |      2 |
| bjguan | 2004-04-02 | 5000.00 |      1 |
+--------+------------+---------+--------+
3 rows in set (0.00 sec)

mysql> select * from emp where deptno=1;
+--------+------------+---------+--------+
| ename  | hiredate   | sal     | deptno |
+--------+------------+---------+--------+
| zzx    | 2000-01-01 | 2000.00 |      1 |
| bjguan | 2004-04-02 | 5000.00 |      1 |
+--------+------------+---------+--------+
2 rows in set (0.00 sec)

多字段查询

mysql> select * from emp where deptno=1 and sal<3000;
+-------+------------+---------+--------+
| ename | hiredate   | sal     | deptno |
+-------+------------+---------+--------+
| zzx   | 2000-01-01 | 2000.00 |      1 |
+-------+------------+---------+--------+
1 row in set (0.00 sec)

排序和限制

SELECT * FROM tablename [WHERE CONDITION] [ORDER BY field1 [DESC | ASC], field2 [DESC | ASC], ..., fieldn [DESC | ASC]]

DESC：降序排列?????ASC：升序排列

把 emp 表中的记录按照工资高低进行显示

mysql> select * from emp order by sal;
+--------+------------+---------+--------+
| ename  | hiredate   | sal     | deptno |
+--------+------------+---------+--------+
| zzx    | 2000-01-01 | 2000.00 |      1 |
| dony   | 2005-02-05 | 2000.00 |      4 |
| lisa   | 2003-02-01 | 4000.00 |      2 |
| bjguan | 2004-04-02 | 5000.00 |      1 |
+--------+------------+---------+--------+
4 rows in set (0.00 sec)

如果排序字段的值一样，则值相同的字段按照第二个排序字段进行排序，依次类推。如果只有一个排序字段，则这些字段相同的记录将会无序排列。

把 emp 表中的记录按照部门编号 deptno 字段排序

mysql> select * from emp order by deptno;
+--------+------------+---------+--------+
| ename  | hiredate   | sal     | deptno |
+--------+------------+---------+--------+
| zzx    | 2000-01-01 | 2000.00 |      1 |
| bjguan | 2004-04-02 | 5000.00 |      1 |
| lisa   | 2003-02-01 | 4000.00 |      2 |
| dony   | 2005-02-05 | 2000.00 |      4 |
+--------+------------+---------+--------+
4 rows in set (0.00 sec)

对于 deptno 相同的前两条记录，如果要按照工资由高到低排序，可以使用以下命令

mysql> select * from emp order by deptno,sal desc;
+--------+------------+---------+--------+
| ename  | hiredate   | sal     | deptno |
+--------+------------+---------+--------+
| bjguan | 2004-04-02 | 5000.00 |      1 |
| zzx    | 2000-01-01 | 2000.00 |      1 |
| lisa   | 2003-02-01 | 4000.00 |      2 |
| dony   | 2005-02-05 | 2000.00 |      4 |
+--------+------------+---------+--------+
4 rows in set (0.00 sec)

对于排序后的记录，如果希望只显示一部分，而不是全部，这时，就可以使用 LIMIT 关键字来实现

SELECT ... [LIMIT offset_start, row_count]

其中 offset_start 表示记录的起始偏移量，row_count 表示显示的行数。

在默认情况下，起始偏移量为 0，只需要写记录行数就可以，这时，实际显示的就是前 n 条记录。例如，显示 emp 表中按照 sal 排序后的前 3 条记录：

mysql> select * from emp order by sal limit 3;
+-------+------------+---------+--------+
| ename | hiredate   | sal     | deptno |
+-------+------------+---------+--------+
| zzx   | 2000-01-01 | 2000.00 |      1 |
| dony  | 2005-02-05 | 2000.00 |      4 |
| lisa  | 2003-02-01 | 4000.00 |      2 |
+-------+------------+---------+--------+
3 rows in set (0.00 sec)

如果要显示 emp 表中按照 sal 排序后从第二条记录开始的 3 条记录，可以使用以下命令：

mysql> select * from emp order by sal limit 1,3;
+--------+------------+---------+--------+
| ename  | hiredate   | sal     | deptno |
+--------+------------+---------+--------+
| dony   | 2005-02-05 | 2000.00 |      4 |
| lisa   | 2003-02-01 | 4000.00 |      2 |
| bjguan | 2004-04-02 | 5000.00 |      1 |
+--------+------------+---------+--------+
3 rows in set (0.00 sec)

聚合

SELECT [field1, field2, ..., fieldn] fun_name
FROM tablename
[WHERE where_contition]
[GROUP BY field1, field2, ..., fieldn
WITH ROLLUP]]
[HAVING where_contition]

参数说明：

• fun_name 表示要做的聚合操作，也就是聚合函数，常用的有 sum（求和）、count（记数）、max（最大值）、min（最小值）、avg（平均值）。
• GROUP BY 关键字表示要进行分类聚合的字段，比如要按照部门分类统计员工数量，部门就应该写在 group by 后面。
• WITH ROLLUP 是可选语法，表明是否对分类聚合后的结果进行再汇总。
• HAVING 关键字表示对分类后的结果再进行条件的过滤。

注意：having 和 where 的区别在于，having 是对聚合后的结果进行条件的过滤，而 where 是在聚合前就对记录进行过滤，如果逻辑允许，尽可能用 where 先过滤记录，这样因为结果集减小，将对聚合的效率大大提高，最后再根据逻辑看是否用 having 进行再过滤。

在 emp 表中统计公司人数

mysql> select count(1) from emp;
+----------+
| count(1) |
+----------+
|        4 |
+----------+
1 row in set (0.00 sec)

在此基础上，要统计各个部门的人数

mysql> select deptno,count(1) from emp group by deptno;
+--------+----------+
| deptno | count(1) |
+--------+----------+
|      1 |        2 |
|      2 |        1 |
|      4 |        1 |
+--------+----------+
3 rows in set (0.00 sec)

更详细一些，既要统计部门人数，又要统计总人数

mysql> select deptno,count(1) from emp group by deptno with rollup;
+--------+----------+
| deptno | count(1) |
+--------+----------+
|      1 |        2 |
|      2 |        1 |
|      4 |        1 |
|   NULL |        4 |
+--------+----------+
4 rows in set (0.00 sec)

统计人数大于 1 的部门

mysql> select deptno,count(1) from emp group by deptno having count(1)>1;
+--------+----------+
| deptno | count(1) |
+--------+----------+
|      1 |        2 |
+--------+----------+
1 row in set (0.00 sec)

统计公司所有员工薪水总额、最高和最低薪水

mysql> select * from emp;
+--------+------------+---------+--------+
| ename  | hiredate   | sal     | deptno |
+--------+------------+---------+--------+
| zzx    | 2000-01-01 | 2000.00 |      1 |
| lisa   | 2003-02-01 | 4000.00 |      2 |
| bjguan | 2004-04-02 | 5000.00 |      1 |
| dony   | 2005-02-05 | 2000.00 |      4 |
+--------+------------+---------+--------+
4 rows in set (0.00 sec)

mysql> select sum(sal),max(sal),min(sal) from emp;
+----------+----------+----------+
| sum(sal) | max(sal) | min(sal) |
+----------+----------+----------+
| 13000.00 |  5000.00 |  2000.00 |
+----------+----------+----------+
1 row in set (0.00 sec)

表连接

当需要同时显示多个表中的字段时，就可以用表连接来实现这样的功能。从大类上分，表连接分为内连接和外连接，它们之间的最主要区别是，内连接仅选出两张表中互相匹配的记录，而外连接会选出其他不匹配的记录。常用的是内连接。

查询出所有雇员的名字和所在部门名称，因为雇员名称和部门分别存放在表 emp 和 dept 中，因此，需要使用表连接来进行查询

mysql> select * from emp;
+--------+------------+---------+--------+
| ename  | hiredate   | sal     | deptno |
+--------+------------+---------+--------+
| zzx    | 2000-01-01 | 2000.00 |      1 |
| lisa   | 2003-02-01 | 4000.00 |      2 |
| bjguan | 2004-04-02 | 5000.00 |      1 |
| dony   | 2005-02-05 | 2000.00 |      4 |
+--------+------------+---------+--------+
4 rows in set (0.00 sec)

mysql> select * from dept;
+--------+----------+
| deptno | deptname |
+--------+----------+
|      1 | tach     |
|      2 | sale     |
|      5 | fin      |
+--------+----------+
3 rows in set (0.01 sec)

mysql> select ename,deptname from emp,dept where emp.deptno=dept.deptno;
+--------+----------+
| ename  | deptname |
+--------+----------+
| zzx    | tach     |
| bjguan | tach     |
| lisa   | sale     |
+--------+----------+
3 rows in set (0.00 sec)

外连接又分为左连接和右连接：

• 左连接：包含所有的左边表中的记录甚至是右边表中没有和它匹配的记录。
• 右连接：包含所有的右边表中的记录甚至是左边表中没有和它匹配的记录。

查询 emp 中所有用户名和所在部门名称

mysql> select * from emp;
+--------+------------+---------+--------+
| ename  | hiredate   | sal     | deptno |
+--------+------------+---------+--------+
| zzx    | 2000-01-01 | 2000.00 |      1 |
| lisa   | 2003-02-01 | 4000.00 |      2 |
| bjguan | 2004-04-02 | 5000.00 |      1 |
| dony   | 2005-02-05 | 2000.00 |      4 |
+--------+------------+---------+--------+
4 rows in set (0.00 sec)

mysql> select * from dept;
+--------+----------+
| deptno | deptname |
+--------+----------+
|      1 | tach     |
|      2 | sale     |
|      5 | fin      |
+--------+----------+
3 rows in set (0.00 sec)

mysql> select ename,deptname from emp left join dept on emp.deptno=dept.deptno;
+--------+----------+
| ename  | deptname |
+--------+----------+
| zzx    | tach     |
| bjguan | tach     |
| lisa   | sale     |
| dony   | NULL     |
+--------+----------+
4 rows in set (0.00 sec)

右连接和左连接类似，两者之间可以互相转化，上面的例子可以改写为如下的右连接

mysql> select ename,deptname from dept right join emp on dept.deptno=emp.deptno;
+--------+----------+
| ename  | deptname |
+--------+----------+
| zzx    | tach     |
| bjguan | tach     |
| lisa   | sale     |
| dony   | NULL     |
+--------+----------+
4 rows in set (0.00 sec)

子查询

某些情况下，当进行查询的时候，需要的条件是另外一个 select 语句的结果，这个时候，就要用到子查询。用于子查询的关键字主要包括 in、not in、=、!=、exists、not exists 等。

从 emp 表中查询出所有部门在 dept 表中的所有记录

mysql> select * from emp
    -> where deptno
    -> in(select deptno from dept);
+--------+------------+---------+--------+
| ename  | hiredate   | sal     | deptno |
+--------+------------+---------+--------+
| zzx    | 2000-01-01 | 2000.00 |      1 |
| bjguan | 2004-04-02 | 5000.00 |      1 |
| lisa   | 2003-02-01 | 4000.00 |      2 |
+--------+------------+---------+--------+
3 rows in set (0.00 sec)

如果子查询记录数唯一，还可以用 = 代替 in

mysql> select * from emp
    -> where deptno = (select deptno from dept);
ERROR 1242 (21000): Subquery returns more than 1 row
mysql> select * from emp
    -> where deptno = (select deptno from dept limit 1);
+--------+------------+---------+--------+
| ename  | hiredate   | sal     | deptno |
+--------+------------+---------+--------+
| zzx    | 2000-01-01 | 2000.00 |      1 |
| bjguan | 2004-04-02 | 5000.00 |      1 |
+--------+------------+---------+--------+
2 rows in set (0.00 sec)

某些情况下，子查询可以转化为表连接

mysql> select * from emp where deptno in(select deptno from dept);
+--------+------------+---------+--------+
| ename  | hiredate   | sal     | deptno |
+--------+------------+---------+--------+
| zzx    | 2000-01-01 | 2000.00 |      1 |
| bjguan | 2004-04-02 | 5000.00 |      1 |
| lisa   | 2003-02-01 | 4000.00 |      2 |
+--------+------------+---------+--------+
3 rows in set (0.00 sec)

mysql> select emp.* from emp, dept where emp.deptno=dept.deptno;
+--------+------------+---------+--------+
| ename  | hiredate   | sal     | deptno |
+--------+------------+---------+--------+
| zzx    | 2000-01-01 | 2000.00 |      1 |
| bjguan | 2004-04-02 | 5000.00 |      1 |
| lisa   | 2003-02-01 | 4000.00 |      2 |
+--------+------------+---------+--------+
3 rows in set (0.00 sec)

注意：子查询和表连接之间的转换主要应用在两个方面。

• MySQL 4.1 以前的版本不支持子查询，需要用表连接来实现子查询的功能.
• 表连接在很多情况下用于优化子查询。

记录联合

将两个表的数据按照一定的查询条件查询出来后，将结果合并到一起显示出来，这个时候，就需要用 union 和 union all 关键字来实现这样的功能

SELECT * FROM t1
UNION | UNION ALL
SELECT * FROM t2
...
UNION | UNION ALL
SELECT * FROM tn;

UNION 和 UNION ALL 的主要区别是 UNION ALL 是把结果集直接合并在一起，而 UNION 是将 UNION ALL 后的结果进行一次 DISTINCT，去除重复记录后的结果。

将 emp 和 dept 表中的部门编号的集合显示出来

mysql> select * from emp;
+--------+------------+---------+--------+
| ename  | hiredate   | sal     | deptno |
+--------+------------+---------+--------+
| zzx    | 2000-01-01 | 2000.00 |      1 |
| lisa   | 2003-02-01 | 4000.00 |      2 |
| bjguan | 2004-04-02 | 5000.00 |      1 |
| dony   | 2005-02-05 | 2000.00 |      4 |
+--------+------------+---------+--------+
4 rows in set (0.00 sec)

mysql> select * from dept;
+--------+----------+
| deptno | deptname |
+--------+----------+
|      1 | tach     |
|      2 | sale     |
|      5 | fin      |
+--------+----------+
3 rows in set (0.00 sec)

mysql> select deptno from emp
    -> union all
    -> select deptno from dept;
+--------+
| deptno |
+--------+
|      1 |
|      2 |
|      1 |
|      4 |
|      1 |
|      2 |
|      5 |
+--------+
7 rows in set (0.00 sec)

将结果去掉重复记录后显示如下

mysql> select deptno from emp
    -> union
    -> select deptno from dept;
+--------+
| deptno |
+--------+
|      1 |
|      2 |
|      4 |
|      5 |
+--------+
4 rows in set (0.00 sec)