MySQL快速入门（二）Java进阶学习第十六天

文档版本 开发工具 测试平台 工程名字 日期 作者 备注 V1.0 2016.05.10 lutianfei none SQL多表操作 外键约束 作用：保证数据的完整性。 定义外键约束 可以直接在create语句中定义外键 foreign key 当前表名 (字段名) references 目标表名 (目标表的主键) 创

文档版本	开发工具	测试平台	工程名字	日期	作者	备注
V1.0				2016.05.10	lutianfei	none

---

SQL多表操作

外键约束

• 作用：保证数据的完整性。

• 定义外键约束

  • 可以直接在create语句中定义外键
    
    • foreign key 当前表名 (字段名) references 目标表名 (目标表的主键)

• 创建完语句后，可以直接使用修改语句定义

  • alter table 表名 add foreign key 当前表名 (字段名) references 目标表名 (目标表的主键);

• 作用：保证数据的完整性。

• 例子

  • alter table emp add foreign key emp(dno) references dept(did); //把dno作为did的外键，且did必须是主键

• 有一个部门的表，还有一个员工表，

<code class=" hljs sql">    <span class="hljs-operator"><span class="hljs-keyword">create</span> <span class="hljs-keyword">database</span> day16;</span>
    use day16;
    <span class="hljs-operator"><span class="hljs-keyword">create</span> <span class="hljs-keyword">table</span> dept(
        did <span class="hljs-keyword">int</span> <span class="hljs-keyword">primary</span> <span class="hljs-keyword">key</span> auto_increment,
        dname <span class="hljs-keyword">varchar</span>(<span class="hljs-number">30</span>)
    );</span>

    <span class="hljs-operator"><span class="hljs-keyword">create</span> <span class="hljs-keyword">table</span> emp(
        eid <span class="hljs-keyword">int</span> <span class="hljs-keyword">primary</span> <span class="hljs-keyword">key</span> auto_increment,
        ename <span class="hljs-keyword">varchar</span>(<span class="hljs-number">20</span>),
        salaly <span class="hljs-keyword">double</span>,
        dno <span class="hljs-keyword">int</span>
    );</span>

    <span class="hljs-operator"><span class="hljs-keyword">insert</span> <span class="hljs-keyword">into</span> dept <span class="hljs-keyword">values</span>(<span class="hljs-keyword">null</span>,<span class="hljs-string">'研发部'</span>);</span>
    <span class="hljs-operator"><span class="hljs-keyword">insert</span> <span class="hljs-keyword">into</span> dept <span class="hljs-keyword">values</span>(<span class="hljs-keyword">null</span>,<span class="hljs-string">'销售部'</span>);</span>
    <span class="hljs-operator"><span class="hljs-keyword">insert</span> <span class="hljs-keyword">into</span> dept <span class="hljs-keyword">values</span>(<span class="hljs-keyword">null</span>,<span class="hljs-string">'人事部'</span>);</span>
    <span class="hljs-operator"><span class="hljs-keyword">insert</span> <span class="hljs-keyword">into</span> dept <span class="hljs-keyword">values</span>(<span class="hljs-keyword">null</span>,<span class="hljs-string">'扯淡部'</span>);</span>
    <span class="hljs-operator"><span class="hljs-keyword">insert</span> <span class="hljs-keyword">into</span> dept <span class="hljs-keyword">values</span>(<span class="hljs-keyword">null</span>,<span class="hljs-string">'牛宝宝部'</span>);</span>

    <span class="hljs-operator"><span class="hljs-keyword">insert</span> <span class="hljs-keyword">into</span> emp <span class="hljs-keyword">values</span>(<span class="hljs-keyword">null</span>,<span class="hljs-string">'班长'</span>,<span class="hljs-number">10000</span>,<span class="hljs-number">1</span>);</span>
    <span class="hljs-operator"><span class="hljs-keyword">insert</span> <span class="hljs-keyword">into</span> emp <span class="hljs-keyword">values</span>(<span class="hljs-keyword">null</span>,<span class="hljs-string">'美美'</span>,<span class="hljs-number">10000</span>,<span class="hljs-number">2</span>);</span>
    <span class="hljs-operator"><span class="hljs-keyword">insert</span> <span class="hljs-keyword">into</span> emp <span class="hljs-keyword">values</span>(<span class="hljs-keyword">null</span>,<span class="hljs-string">'小凤'</span>,<span class="hljs-number">12000</span>,<span class="hljs-number">3</span>);</span>
    <span class="hljs-operator"><span class="hljs-keyword">insert</span> <span class="hljs-keyword">into</span> emp <span class="hljs-keyword">values</span>(<span class="hljs-keyword">null</span>,<span class="hljs-string">'如花'</span>,<span class="hljs-number">14000</span>,<span class="hljs-number">2</span>);</span>
    <span class="hljs-operator"><span class="hljs-keyword">insert</span> <span class="hljs-keyword">into</span> emp <span class="hljs-keyword">values</span>(<span class="hljs-keyword">null</span>,<span class="hljs-string">'芙蓉'</span>,<span class="hljs-number">11000</span>,<span class="hljs-number">1</span>);</span>
    <span class="hljs-operator"><span class="hljs-keyword">insert</span> <span class="hljs-keyword">into</span> emp <span class="hljs-keyword">values</span>(<span class="hljs-keyword">null</span>,<span class="hljs-string">'东东'</span>,<span class="hljs-number">800</span>,<span class="hljs-keyword">null</span>);</span>
    <span class="hljs-operator"><span class="hljs-keyword">insert</span> <span class="hljs-keyword">into</span> emp <span class="hljs-keyword">values</span>(<span class="hljs-keyword">null</span>,<span class="hljs-string">'波波'</span>,<span class="hljs-number">1000</span>,<span class="hljs-keyword">null</span>);</span>

<span class="hljs-operator"><span class="hljs-keyword">update</span> emp <span class="hljs-keyword">set</span> salaly=<span class="hljs-number">2500</span> <span class="hljs-keyword">where</span> eid = <span class="hljs-number">5</span>;</span>
</code>

---

数据库的设计

• 多表设计中三种实体关系
  

一对多表的设计

• 一对多:在多方需要添加一个字段，并且和一方主键的类型必须是相同的。

  • 把该字段作为外键指向一方的主键。

• 建表原则（都是在多方表中维护）：

  • 在多方表中添加一个字段，把该字段作为外键，并且指向一方表中的主键。
  • 将关系的属性及非多方的主标识加入到多方表
  • 多方表的外键是非多方实体的主标识

• 画图举例部门与员工的关系
  

多对多表的设计

• 在生活中一个学生可以选择多门课程，每一门课程也可以被多个学生所选择。这些例子在数据库设计中可以使用多对多来完成。

• 建表原则：

  • 需要创建第三张表，该表中至少有两个字段，作为外键分别指向多对多的双方的主键。
  • 联系的属性及两个实体的主标识形成关系表。
  • 关系表的主键为两个实体主标识的组合
  • 拆开两个一对多的关系，中间创建一个中间表，至少有两个字段。作为外键指向两个多对多关系表的主键。
    

一对一表的设计（很少用）

• 在生活中，1对1的应用比较少，因为完全可以作为一张表出现。但是有可能公司的业务原因，而需要设计1对1的表结构。例如：一个公司对应一个地址，一个地址也只能对应一个公司。
• 建表原则：
  
  • 主键对应
    
    • 将两个表的主键进行关联
  • 唯一外键对应
    
    • 在任何一方加一个外键，但是需要设置成唯一（unique），指向另一方的主键

• 简单购物的网站表结构设计
  
  • 包含哪些实体？ 用户 订单 商品 分类

多表查询 —— 笛卡尔积

• 笛卡尔积的概念：（了解）
  

• 查询的语法
  select * from 表A,表B; 返回的结果就是笛卡尔积。

• 多表查询时，如果不指定表之间的连接条件，则连接变成笛卡尔乘积操作，笛卡尔积的效果如本页所示。

• 进行笛卡尔积后，查询结果中存在大量无意义的数据，示例中只有A表中的A_ID和B表中A_ID一致的行才有意义，这样我们通过加上WHERE过滤条件得到想要的数据。这种横跨多表的查询操作一般用连接完成。

• 如果一张表的记录为m，另一张表的记录是n，两个表做交叉连接后，查询结果的数量为m*n条。

多表查询—内链接

• 两个表之间是有联系的，通过一个外键关联
• 内连接分成两种
  
  • 普通内连接
    
    • 前提条件：需要有外键的。
    • 提交关键字 inner join … on
    • select * from dept inner join emp on dept.did = emp.dno;
  • 隐式内连接（用的是最多的）
    
    • 可以不使用inner join … on关键字
    • select * from dept,emp where dept.did = emp.dno;

多表查询—外链接

• 左外链接（看左表，把左表所有的数据全部查询出来）

  • 前提条件：需要有外键的。
  • 语法： 使用关键字 left [outer] join … on
    
    • select * from dept left outer join emp on dept.did = emp.dno;

• 右外链接（看右表，把右表所有的数据全部查询出来）

  • 前提条件：需要有外键的。
  • 语法： 使用关键字 right [outer] join … on
    
    • select * from dept right join emp on dept.did = emp.dno;

• 外链接与内连接的关系

多表查询—子查询

• 在sql语言中，select…from…语为一个查询块，将一个查询块嵌套在另一个查询块中作为条件称为嵌套查询,也称为子查询。

• 外层的查询块称为父查询，内层的查询块称为子查询。

• 语法：select * from table where 条件 > (select * from table where 条件)

  • >any 大于结果的最小值
  • >all 大于结果的最大值

基础练习

<code class=" hljs sql"><span class="hljs-operator"><span class="hljs-keyword">create</span> <span class="hljs-keyword">table</span> dept(
    did <span class="hljs-keyword">int</span> <span class="hljs-keyword">primary</span> <span class="hljs-keyword">key</span> auto_increment,
    dname <span class="hljs-keyword">varchar</span>(<span class="hljs-number">30</span>)
);</span>

<span class="hljs-operator"><span class="hljs-keyword">create</span> <span class="hljs-keyword">table</span> emp(
    eid <span class="hljs-keyword">int</span> <span class="hljs-keyword">primary</span> <span class="hljs-keyword">key</span> auto_increment,
    ename <span class="hljs-keyword">varchar</span>(<span class="hljs-number">20</span>),
    salaly <span class="hljs-keyword">double</span>,
    dno <span class="hljs-keyword">int</span>
);</span>


//查看所有人所属的部门名称和员工名称?
    //方法一：
    <span class="hljs-operator"><span class="hljs-keyword">select</span> dept.dname,emp.ename <span class="hljs-keyword">from</span> dept,emp <span class="hljs-keyword">where</span> dept.did = emp.dno;</span>

    //方法二：
    <span class="hljs-operator"><span class="hljs-keyword">select</span> d.dname,e.ename <span class="hljs-keyword">from</span> dept d,emp e <span class="hljs-keyword">where</span> d.did = e.dno;</span>


//统计每个部门的人数(按照部门名称统计，分组group by  count)
    <span class="hljs-operator"><span class="hljs-keyword">select</span> d.dname,<span class="hljs-aggregate">count</span>(*) <span class="hljs-keyword">from</span> dept d,emp e <span class="hljs-keyword">where</span> d.did = e.dno <span class="hljs-keyword">group</span> <span class="hljs-keyword">by</span> d.dname;</span>


//统计部门的平均工资（按部门名称统计 ，分组group by  avg）
    <span class="hljs-operator"><span class="hljs-keyword">select</span> d.dname,<span class="hljs-aggregate">avg</span>(salaly) <span class="hljs-keyword">from</span> dept d,emp e <span class="hljs-keyword">where</span> d.did = e.dno <span class="hljs-keyword">group</span> <span class="hljs-keyword">by</span> d.dname;</span>


//统计部门的平均工资大于公司平均工资的部门（子查询）
    * 公司的平均工资
        <span class="hljs-operator"><span class="hljs-keyword">select</span> <span class="hljs-aggregate">avg</span>(salaly) <span class="hljs-keyword">from</span> emp;</span>
    * 部门的平均工资
        <span class="hljs-operator"><span class="hljs-keyword">select</span> d.dname,<span class="hljs-aggregate">avg</span>(e.salaly) <span class="hljs-keyword">as</span> sa <span class="hljs-keyword">from</span> dept d,emp e <span class="hljs-keyword">where</span> d.did = e.dno <span class="hljs-keyword">group</span> <span class="hljs-keyword">by</span> d.dname <span class="hljs-keyword">having</span> sa > (<span class="hljs-keyword">select</span> <span class="hljs-aggregate">avg</span>(salaly) <span class="hljs-keyword">from</span> emp);</span></code>

应用练习

• 表结构

<code class=" hljs sql"><span class="hljs-operator"><span class="hljs-keyword">DROP</span> <span class="hljs-keyword">TABLE</span> dept;</span>
<span class="hljs-comment">--部门表</span>
<span class="hljs-operator"><span class="hljs-keyword">CREATE</span> <span class="hljs-keyword">TABLE</span> dept(
    deptno <span class="hljs-keyword">int</span>  <span class="hljs-keyword">PRIMARY</span> <span class="hljs-keyword">KEY</span>,
    dname <span class="hljs-keyword">VARCHAR</span>(<span class="hljs-number">14</span>) , --部门名称
    loc <span class="hljs-keyword">VARCHAR</span>(<span class="hljs-number">13</span>)  ---部门地址
    ) ;</span>

<span class="hljs-operator"><span class="hljs-keyword">CREATE</span> <span class="hljs-keyword">TABLE</span> dept(
    deptno <span class="hljs-keyword">int</span>  <span class="hljs-keyword">PRIMARY</span> <span class="hljs-keyword">KEY</span>,
    dname <span class="hljs-keyword">VARCHAR</span>(<span class="hljs-number">14</span>) , 
    loc <span class="hljs-keyword">VARCHAR</span>(<span class="hljs-number">13</span>) 
    ) ;</span>


    <span class="hljs-operator"><span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> DEPT <span class="hljs-keyword">VALUES</span> (<span class="hljs-number">10</span>,<span class="hljs-string">'ACCOUNTING'</span>,<span class="hljs-string">'NEW YORK'</span>);</span>
    <span class="hljs-operator"><span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> DEPT <span class="hljs-keyword">VALUES</span> (<span class="hljs-number">20</span>,<span class="hljs-string">'RESEARCH'</span>,<span class="hljs-string">'DALLAS'</span>);</span>
    <span class="hljs-operator"><span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> DEPT <span class="hljs-keyword">VALUES</span> (<span class="hljs-number">30</span>,<span class="hljs-string">'SALES'</span>,<span class="hljs-string">'CHICAGO'</span>);</span>
    <span class="hljs-operator"><span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> DEPT <span class="hljs-keyword">VALUES</span> (<span class="hljs-number">40</span>,<span class="hljs-string">'OPERATIONS'</span>,<span class="hljs-string">'BOSTON'</span>);</span>



<span class="hljs-operator"><span class="hljs-keyword">drop</span> <span class="hljs-keyword">table</span> emp;</span>
<span class="hljs-comment">--员工表</span>
<span class="hljs-operator"><span class="hljs-keyword">CREATE</span> <span class="hljs-keyword">TABLE</span> emp(
    empno <span class="hljs-keyword">int</span>  <span class="hljs-keyword">PRIMARY</span> <span class="hljs-keyword">KEY</span>, --员工编号
    ename <span class="hljs-keyword">VARCHAR</span>(<span class="hljs-number">10</span>), ---员工姓名
    job <span class="hljs-keyword">VARCHAR</span>(<span class="hljs-number">9</span>), --员工工作
    mgr <span class="hljs-keyword">int</span>, ----员工直属领导编号
    hiredate <span class="hljs-keyword">DATE</span>, ----入职时间
    sal <span class="hljs-keyword">double</span>, ---工资
    comm <span class="hljs-keyword">double</span>, --奖金
    deptno <span class="hljs-keyword">int</span>  <span class="hljs-keyword">REFERENCES</span> dept);</span>  <span class="hljs-comment">--关联dept表</span>



<span class="hljs-operator"><span class="hljs-keyword">CREATE</span> <span class="hljs-keyword">TABLE</span> emp(
    empno <span class="hljs-keyword">int</span>  <span class="hljs-keyword">PRIMARY</span> <span class="hljs-keyword">KEY</span>,
    ename <span class="hljs-keyword">VARCHAR</span>(<span class="hljs-number">10</span>),
    job <span class="hljs-keyword">VARCHAR</span>(<span class="hljs-number">9</span>),
    mgr <span class="hljs-keyword">int</span>, 
    hiredate <span class="hljs-keyword">DATE</span>, 
    sal <span class="hljs-keyword">double</span>,
    comm <span class="hljs-keyword">double</span>,
    deptno <span class="hljs-keyword">int</span> ,
    <span class="hljs-keyword">foreign</span> <span class="hljs-keyword">key</span> emp(deptno) <span class="hljs-keyword">REFERENCES</span> dept(deptno)
);</span>  


<span class="hljs-operator"><span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> EMP <span class="hljs-keyword">VALUES</span>(<span class="hljs-number">7369</span>,<span class="hljs-string">'SMITH'</span>,<span class="hljs-string">'CLERK'</span>,<span class="hljs-number">7902</span>,<span class="hljs-string">"1980-12-17"</span>,<span class="hljs-number">800</span>,<span class="hljs-keyword">NULL</span>,<span class="hljs-number">20</span>);</span>
<span class="hljs-operator"><span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> EMP <span class="hljs-keyword">VALUES</span>(<span class="hljs-number">7499</span>,<span class="hljs-string">'ALLEN'</span>,<span class="hljs-string">'SALESMAN'</span>,<span class="hljs-number">7698</span>,<span class="hljs-string">'1981-02-20'</span>,<span class="hljs-number">1600</span>,<span class="hljs-number">300</span>,<span class="hljs-number">30</span>);</span>
<span class="hljs-operator"><span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> EMP <span class="hljs-keyword">VALUES</span>(<span class="hljs-number">7521</span>,<span class="hljs-string">'WARD'</span>,<span class="hljs-string">'SALESMAN'</span>,<span class="hljs-number">7698</span>,<span class="hljs-string">'1981-02-22'</span>,<span class="hljs-number">1250</span>,<span class="hljs-number">500</span>,<span class="hljs-number">30</span>);</span>
<span class="hljs-operator"><span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> EMP <span class="hljs-keyword">VALUES</span>(<span class="hljs-number">7566</span>,<span class="hljs-string">'JONES'</span>,<span class="hljs-string">'MANAGER'</span>,<span class="hljs-number">7839</span>,<span class="hljs-string">'1981-04-02'</span>,<span class="hljs-number">2975</span>,<span class="hljs-keyword">NULL</span>,<span class="hljs-number">20</span>);</span>
<span class="hljs-operator"><span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> EMP <span class="hljs-keyword">VALUES</span>(<span class="hljs-number">7654</span>,<span class="hljs-string">'MARTIN'</span>,<span class="hljs-string">'SALESMAN'</span>,<span class="hljs-number">7698</span>,<span class="hljs-string">'1981-09-28'</span>,<span class="hljs-number">1250</span>,<span class="hljs-number">1400</span>,<span class="hljs-number">30</span>);</span>
<span class="hljs-operator"><span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> EMP <span class="hljs-keyword">VALUES</span>(<span class="hljs-number">7698</span>,<span class="hljs-string">'BLAKE'</span>,<span class="hljs-string">'MANAGER'</span>,<span class="hljs-number">7839</span>,<span class="hljs-string">'1981-05-01'</span>,<span class="hljs-number">2850</span>,<span class="hljs-keyword">NULL</span>,<span class="hljs-number">30</span>);</span>
<span class="hljs-operator"><span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> EMP <span class="hljs-keyword">VALUES</span>(<span class="hljs-number">7782</span>,<span class="hljs-string">'CLARK'</span>,<span class="hljs-string">'MANAGER'</span>,<span class="hljs-number">7839</span>,<span class="hljs-string">'1981-06-09'</span>,<span class="hljs-number">2450</span>,<span class="hljs-keyword">NULL</span>,<span class="hljs-number">10</span>);</span>
<span class="hljs-operator"><span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> EMP <span class="hljs-keyword">VALUES</span>(<span class="hljs-number">7788</span>,<span class="hljs-string">'SCOTT'</span>,<span class="hljs-string">'ANALYST'</span>,<span class="hljs-number">7566</span>,<span class="hljs-string">'1987-07-03'</span>,<span class="hljs-number">3000</span>,<span class="hljs-keyword">NULL</span>,<span class="hljs-number">20</span>);</span>
<span class="hljs-operator"><span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> EMP <span class="hljs-keyword">VALUES</span>(<span class="hljs-number">7839</span>,<span class="hljs-string">'KING'</span>,<span class="hljs-string">'PRESIDENT'</span>,<span class="hljs-keyword">NULL</span>,<span class="hljs-string">'1981-11-17'</span>,<span class="hljs-number">5000</span>,<span class="hljs-keyword">NULL</span>,<span class="hljs-number">10</span>);</span>
<span class="hljs-operator"><span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> EMP <span class="hljs-keyword">VALUES</span>(<span class="hljs-number">7844</span>,<span class="hljs-string">'TURNER'</span>,<span class="hljs-string">'SALESMAN'</span>,<span class="hljs-number">7698</span>,<span class="hljs-string">'1981-09-08'</span>,<span class="hljs-number">1500</span>,<span class="hljs-number">0</span>,<span class="hljs-number">30</span>);</span>
<span class="hljs-operator"><span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> EMP <span class="hljs-keyword">VALUES</span>(<span class="hljs-number">7876</span>,<span class="hljs-string">'ADAMS'</span>,<span class="hljs-string">'CLERK'</span>,<span class="hljs-number">7788</span>,<span class="hljs-string">'1987-07-13'</span>,<span class="hljs-number">1100</span>,<span class="hljs-keyword">NULL</span>,<span class="hljs-number">20</span>);</span>
<span class="hljs-operator"><span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> EMP <span class="hljs-keyword">VALUES</span>(<span class="hljs-number">7900</span>,<span class="hljs-string">'JAMES'</span>,<span class="hljs-string">'CLERK'</span>,<span class="hljs-number">7698</span>,<span class="hljs-string">'1981-12-03'</span>,<span class="hljs-number">950</span>,<span class="hljs-keyword">NULL</span>,<span class="hljs-number">30</span>);</span>
<span class="hljs-operator"><span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> EMP <span class="hljs-keyword">VALUES</span>(<span class="hljs-number">7902</span>,<span class="hljs-string">'FORD'</span>,<span class="hljs-string">'ANALYST'</span>,<span class="hljs-number">7566</span>,<span class="hljs-string">'1981-12-03'</span>,<span class="hljs-number">3000</span>,<span class="hljs-keyword">NULL</span>,<span class="hljs-number">20</span>);</span>
<span class="hljs-operator"><span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> EMP <span class="hljs-keyword">VALUES</span>(<span class="hljs-number">7934</span>,<span class="hljs-string">'MILLER'</span>,<span class="hljs-string">'CLERK'</span>,<span class="hljs-number">7782</span>,<span class="hljs-string">'1981-01-23'</span>,<span class="hljs-number">1300</span>,<span class="hljs-keyword">NULL</span>,<span class="hljs-number">10</span>);</span>


<span class="hljs-operator"><span class="hljs-keyword">CREATE</span> <span class="hljs-keyword">TABLE</span> salgrade( 
    grade <span class="hljs-keyword">int</span>,--等级
    losal <span class="hljs-keyword">double</span>, --最低工资
    hisal <span class="hljs-keyword">double</span> );</span> <span class="hljs-comment">--最高工次</span>


<span class="hljs-operator"><span class="hljs-keyword">CREATE</span> <span class="hljs-keyword">TABLE</span> salgrade( 
    grade <span class="hljs-keyword">int</span>,
    losal <span class="hljs-keyword">double</span>, 
    hisal <span class="hljs-keyword">double</span> );</span>
<span class="hljs-operator"><span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> SALGRADE <span class="hljs-keyword">VALUES</span> (<span class="hljs-number">1</span>,<span class="hljs-number">700</span>,<span class="hljs-number">1200</span>);</span>
<span class="hljs-operator"><span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> SALGRADE <span class="hljs-keyword">VALUES</span> (<span class="hljs-number">2</span>,<span class="hljs-number">1201</span>,<span class="hljs-number">1400</span>);</span>
<span class="hljs-operator"><span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> SALGRADE <span class="hljs-keyword">VALUES</span> (<span class="hljs-number">3</span>,<span class="hljs-number">1401</span>,<span class="hljs-number">2000</span>);</span>
<span class="hljs-operator"><span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> SALGRADE <span class="hljs-keyword">VALUES</span> (<span class="hljs-number">4</span>,<span class="hljs-number">2001</span>,<span class="hljs-number">3000</span>);</span>
<span class="hljs-operator"><span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> SALGRADE <span class="hljs-keyword">VALUES</span> (<span class="hljs-number">5</span>,<span class="hljs-number">3001</span>,<span class="hljs-number">9999</span>);</span></code>

• 1、返回拥有员工的部门名、部门号。

  • select d.deptno,d.dname from dept d,emp e where d.deptno = e.deptno group by d.deptno;

• 2、工资水平多于smith的员工信息。

  • select * from emp e where e.sal > (select em.sal from emp em where em.ename = ‘smith’ );

• 3、返回员工和所属经理的姓名。

  • select e.ename,em.ename from emp e,emp em where e.mgr = em.empno and em.job = ‘MANAGER’;

• 4、返回雇员的雇佣日期早于其经理雇佣日期的员工及其经理姓名。

  • select e.ename,em.ename from emp e,emp em where e.mgr = em.empno and em.job = ‘MANAGER’ and e.hiredate

• 5、返回员工姓名及其所在的部门名称。

  • select d.dname,e.ename from dept d,emp e where d.deptno = e.deptno;

• 6、返回从事clerk工作的员工姓名和所在部门名称。

  • select d.dname,e.ename,e.job from dept d,emp e where d.deptno = e.deptno and job = ‘clerk’;

• 7、返回部门号及其本部门的最低工资。

  • select d.deptno,min(e.sal) from dept d,emp e where d.deptno = e.deptno group by d.deptno;

• 8、返回销售部(sales)所有员工的姓名。

  • select * from dept d,emp e where d.deptno = e.deptno and d.dname = ‘sales’;

• 9、返回工资水平多于平均工资的员工。

  • select * from emp e where e.sal > (select avg(sal) from emp)；

• 10、返回与SCOTT从事相同工作的员工。

  • select * from emp e where e.job = (select job from emp em where em.ename = ‘SCOTT’);

• 11、返回与30部门员工工资水平相同的员工姓名与工资。

  • select * from emp e where e.sal in (select e.sal from dept d,emp e where d.deptno = e.deptno and d.deptno = 30);