实现方式一般有三种: 1. 继承关系树每个具体类对应一张表(不介绍) 2. 继承关系树的根类对应一张表 3. 继承关系树的每个类对应一张表 先介绍关系: DayEmployee和MonthEmploy是Employee的子类,并且Company和Employee是一对多关系: 具体代码如下: Compan
实现方式一般有三种:
1. 继承关系树每个具体类对应一张表(不介绍)
2. 继承关系树的根类对应一张表
3. 继承关系树的每个类对应一张表
先介绍关系:
DayEmployee和MonthEmploy是Employee的子类,并且Company和Employee是一对多关系:
具体代码如下:
Company.java
<code class=" hljs java"><span class="hljs-keyword">import</span> java.util.HashSet; <span class="hljs-keyword">import</span> java.util.Set; <span class="hljs-keyword">public</span> <span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Company</span> {</span> <span class="hljs-keyword">private</span> Integer id; <span class="hljs-keyword">private</span> String name; <span class="hljs-keyword">private</span> Set<Employee> employees = <span class="hljs-keyword">new</span> HashSet<Employee>(); <span class="hljs-keyword">public</span> <span class="hljs-title">Company</span>() { <span class="hljs-keyword">super</span>(); } <span class="hljs-keyword">public</span> <span class="hljs-title">Company</span>(String name) { <span class="hljs-keyword">super</span>(); <span class="hljs-keyword">this</span>.name = name; } <span class="hljs-keyword">public</span> Integer <span class="hljs-title">getId</span>() { <span class="hljs-keyword">return</span> id; } <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">setId</span>(Integer id) { <span class="hljs-keyword">this</span>.id = id; } <span class="hljs-keyword">public</span> String <span class="hljs-title">getName</span>() { <span class="hljs-keyword">return</span> name; } <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">setName</span>(String name) { <span class="hljs-keyword">this</span>.name = name; } <span class="hljs-keyword">public</span> Set<Employee> <span class="hljs-title">getEmployees</span>() { <span class="hljs-keyword">return</span> employees; } <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">setEmployees</span>(Set<Employee> employees) { <span class="hljs-keyword">this</span>.employees = employees; } }</code>
Employee.java
<code class=" hljs cs"> <span class="hljs-keyword">public</span> <span class="hljs-keyword">class</span> Employee { <span class="hljs-keyword">private</span> Integer id; <span class="hljs-keyword">private</span> String name; <span class="hljs-keyword">private</span> Integer age; <span class="hljs-keyword">private</span> Company company; <span class="hljs-keyword">public</span> Company <span class="hljs-title">getCompany</span>() { <span class="hljs-keyword">return</span> company; } <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">setCompany</span>(Company company) { <span class="hljs-keyword">this</span>.company = company; } <span class="hljs-keyword">public</span> Integer <span class="hljs-title">getId</span>() { <span class="hljs-keyword">return</span> id; } <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">setId</span>(Integer id) { <span class="hljs-keyword">this</span>.id = id; } <span class="hljs-keyword">public</span> String <span class="hljs-title">getName</span>() { <span class="hljs-keyword">return</span> name; } <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">setName</span>(String name) { <span class="hljs-keyword">this</span>.name = name; } <span class="hljs-keyword">public</span> Integer <span class="hljs-title">getAge</span>() { <span class="hljs-keyword">return</span> age; } <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">setAge</span>(Integer age) { <span class="hljs-keyword">this</span>.age = age; } } </code>
DayEmployee.java
<code class=" hljs java"><span class="hljs-keyword">public</span> <span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">DayEmployee</span> <span class="hljs-keyword">extends</span> <span class="hljs-title">Employee</span>{</span> <span class="hljs-keyword">private</span> Double dayMoney; <span class="hljs-keyword">public</span> Double <span class="hljs-title">getDayMoney</span>() { <span class="hljs-keyword">return</span> dayMoney; } <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">setDayMoney</span>(Double dayMoney) { <span class="hljs-keyword">this</span>.dayMoney = dayMoney; } } </code>
MonthEmploy.java
<code class=" hljs java"> <span class="hljs-keyword">public</span> <span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">MonthEmploy</span> <span class="hljs-keyword">extends</span> <span class="hljs-title">Employee</span>{</span> <span class="hljs-keyword">private</span> Double monthMoney; <span class="hljs-keyword">public</span> Double <span class="hljs-title">getMonthMoney</span>() { <span class="hljs-keyword">return</span> monthMoney; } <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">setMonthMoney</span>(Double monthMoney) { <span class="hljs-keyword">this</span>.monthMoney = monthMoney; } } </code>
Company.hbm.xml
<code class=" hljs xml"><span class="hljs-pi"><?xml version="1.0"?></span> <span class="hljs-doctype"><!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"></span> <span class="hljs-tag"><<span class="hljs-title">hibernate-mapping</span> <span class="hljs-attribute">package</span>=<span class="hljs-value">"com.dongecs.test1.pojo"</span>></span> <span class="hljs-tag"><<span class="hljs-title">class</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"Company"</span> <span class="hljs-attribute">table</span>=<span class="hljs-value">"COMPANY"</span>></span> <span class="hljs-tag"><<span class="hljs-title">id</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"id"</span> <span class="hljs-attribute">type</span>=<span class="hljs-value">"java.lang.Integer"</span>></span> <span class="hljs-tag"><<span class="hljs-title">column</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"ID"</span> /></span> <span class="hljs-tag"><<span class="hljs-title">generator</span> <span class="hljs-attribute">class</span>=<span class="hljs-value">"native"</span> /></span> <span class="hljs-tag"></<span class="hljs-title">id</span>></span> <span class="hljs-tag"><<span class="hljs-title">property</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"name"</span> <span class="hljs-attribute">type</span>=<span class="hljs-value">"java.lang.String"</span>></span> <span class="hljs-tag"><<span class="hljs-title">column</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"NAME"</span> /></span> <span class="hljs-tag"></<span class="hljs-title">property</span>></span> <span class="hljs-tag"><<span class="hljs-title">set</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"employees"</span> <span class="hljs-attribute">table</span>=<span class="hljs-value">"EMPLOYEE"</span> <span class="hljs-attribute">inverse</span>=<span class="hljs-value">"true"</span> <span class="hljs-attribute">lazy</span>=<span class="hljs-value">"true"</span>></span> <span class="hljs-tag"><<span class="hljs-title">key</span>></span> <span class="hljs-tag"><<span class="hljs-title">column</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"COMPANY"</span> /></span> <span class="hljs-tag"></<span class="hljs-title">key</span>></span> <span class="hljs-tag"><<span class="hljs-title">one-to-many</span> <span class="hljs-attribute">class</span>=<span class="hljs-value">"Employee"</span> /></span> <span class="hljs-tag"></<span class="hljs-title">set</span>></span> <span class="hljs-tag"></<span class="hljs-title">class</span>></span> <span class="hljs-tag"></<span class="hljs-title">hibernate-mapping</span>></span></code>
继承关系树的根类对应一张表对应的Employee.hbm.xml的写法
Employee.hbm.xml
<code class=" hljs xml"><span class="hljs-pi"><?xml version="1.0"?></span> <span class="hljs-doctype"><!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"></span> <span class="hljs-tag"><<span class="hljs-title">hibernate-mapping</span> <span class="hljs-attribute">package</span>=<span class="hljs-value">"com.dongecs.test1.pojo"</span>></span> <span class="hljs-comment"><!-- 利用discriminator-value默认 empType 的值为emp --></span> <span class="hljs-tag"><<span class="hljs-title">class</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"Employee"</span> <span class="hljs-attribute">table</span>=<span class="hljs-value">"EMPLOYEE"</span> <span class="hljs-attribute">discriminator-value</span>=<span class="hljs-value">"emp"</span>></span> <span class="hljs-tag"><<span class="hljs-title">id</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"id"</span> <span class="hljs-attribute">type</span>=<span class="hljs-value">"java.lang.Integer"</span>></span> <span class="hljs-tag"><<span class="hljs-title">column</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"ID"</span> /></span> <span class="hljs-tag"><<span class="hljs-title">generator</span> <span class="hljs-attribute">class</span>=<span class="hljs-value">"native"</span> /></span> <span class="hljs-tag"></<span class="hljs-title">id</span>></span> <span class="hljs-comment"><!--用于识别此类的类别 --></span> <span class="hljs-tag"><<span class="hljs-title">discriminator</span> <span class="hljs-attribute">column</span>=<span class="hljs-value">"empType"</span> <span class="hljs-attribute">type</span>=<span class="hljs-value">"string"</span>/></span> <span class="hljs-tag"><<span class="hljs-title">property</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"name"</span> <span class="hljs-attribute">type</span>=<span class="hljs-value">"java.lang.String"</span>></span> <span class="hljs-tag"><<span class="hljs-title">column</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"NAME"</span> /></span> <span class="hljs-tag"></<span class="hljs-title">property</span>></span> <span class="hljs-tag"><<span class="hljs-title">property</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"age"</span> <span class="hljs-attribute">type</span>=<span class="hljs-value">"java.lang.Integer"</span>></span> <span class="hljs-tag"><<span class="hljs-title">column</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"AGE"</span> /></span> <span class="hljs-tag"></<span class="hljs-title">property</span>></span> <span class="hljs-tag"><<span class="hljs-title">many-to-one</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"company"</span> <span class="hljs-attribute">class</span>=<span class="hljs-value">"Company"</span> <span class="hljs-attribute">fetch</span>=<span class="hljs-value">"join"</span>></span> <span class="hljs-tag"><<span class="hljs-title">column</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"COMPANY"</span> /></span> <span class="hljs-tag"></<span class="hljs-title">many-to-one</span>></span> <span class="hljs-comment"><!-- 若为DayEmployee,那么将empType的值设为DE --></span> <span class="hljs-tag"><<span class="hljs-title">subclass</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"DayEmployee"</span> <span class="hljs-attribute">discriminator-value</span>=<span class="hljs-value">"DE"</span>></span> <span class="hljs-tag"><<span class="hljs-title">property</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"dayMoney"</span> <span class="hljs-attribute">type</span>=<span class="hljs-value">"double"</span>></span><span class="hljs-tag"></<span class="hljs-title">property</span>></span> <span class="hljs-tag"></<span class="hljs-title">subclass</span>></span> <span class="hljs-comment"><!-- 若为MonthEmploy,那么将empType的值设为ME --></span> <span class="hljs-tag"><<span class="hljs-title">subclass</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"MonthEmploy"</span> <span class="hljs-attribute">discriminator-value</span>=<span class="hljs-value">"ME"</span>></span> <span class="hljs-tag"><<span class="hljs-title">property</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"monthMoney"</span> <span class="hljs-attribute">type</span>=<span class="hljs-value">"double"</span>></span><span class="hljs-tag"></<span class="hljs-title">property</span>></span> <span class="hljs-tag"></<span class="hljs-title">subclass</span>></span> <span class="hljs-tag"></<span class="hljs-title">class</span>></span> <span class="hljs-tag"></<span class="hljs-title">hibernate-mapping</span>></span> </code>
利用上述代码作如下测试:
<code class=" hljs java"><span class="hljs-keyword">public</span> <span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">EmployeeTest</span> {</span> SessionFactory sessionFactory = <span class="hljs-keyword">null</span>; Session session = <span class="hljs-keyword">null</span>; Transaction transaction = <span class="hljs-keyword">null</span>; <span class="hljs-annotation">@Before</span> <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">init</span>() { Configuration configuration = <span class="hljs-keyword">new</span> Configuration().configure(); ServiceRegistry serviceRegistry = <span class="hljs-keyword">new</span> ServiceRegistryBuilder().applySettings(configuration.getProperties()) .buildServiceRegistry(); sessionFactory = configuration.buildSessionFactory(serviceRegistry); session = sessionFactory.openSession(); transaction = session.beginTransaction(); } <span class="hljs-annotation">@After</span> <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">after</span>() { transaction.commit(); session.close(); sessionFactory.close(); } <span class="hljs-annotation">@Test</span> <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">testEmployee</span>() { Company company = <span class="hljs-keyword">new</span> Company(<span class="hljs-string">"com1"</span>); DayEmployee d1 = <span class="hljs-keyword">new</span> DayEmployee(); d1.setAge(<span class="hljs-number">22</span>); d1.setDayMoney(<span class="hljs-number">50.0</span>); d1.setName(<span class="hljs-string">"d1"</span>); MonthEmploy d2 = <span class="hljs-keyword">new</span> MonthEmploy(); d2.setAge(<span class="hljs-number">30</span>); d2.setMonthMoney(<span class="hljs-number">79.0</span>); d2.setName(<span class="hljs-string">"d2"</span>); Employee employee = <span class="hljs-keyword">new</span> Employee(); employee.setAge(<span class="hljs-number">90</span>); employee.setName(<span class="hljs-string">"employee"</span>); d1.setCompany(company); d2.setCompany(company); employee.setCompany(company); <span class="hljs-comment">//inverse = true 时可以关联, 当相对多出3条update,因此建议使用多的一端关联</span> <span class="hljs-comment">// company.getEmployees().add(employee);</span> <span class="hljs-comment">// company.getEmployees().add(d1);</span> <span class="hljs-comment">// company.getEmployees().add(d2);</span> <span class="hljs-comment">//把一的一段放到最前保存,可以减小3条更新操作</span> session.save(company); session.save(employee); session.save(d2); session.save(d1); } }</code>
会得出如下结果:
继承关系树的每个类对应一张表Employee.hbm.xml的写法
<code class=" hljs xml"><span class="hljs-pi"><?xml version="1.0"?></span> <span class="hljs-doctype"><!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"></span> <span class="hljs-tag"><<span class="hljs-title">hibernate-mapping</span> <span class="hljs-attribute">package</span>=<span class="hljs-value">"com.dongecs.test1.pojo"</span>></span> <span class="hljs-tag"><<span class="hljs-title">class</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"Employee"</span> <span class="hljs-attribute">table</span>=<span class="hljs-value">"EMPLOYEE"</span>></span> <span class="hljs-tag"><<span class="hljs-title">id</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"id"</span> <span class="hljs-attribute">type</span>=<span class="hljs-value">"java.lang.Integer"</span>></span> <span class="hljs-tag"><<span class="hljs-title">column</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"ID"</span> /></span> <span class="hljs-tag"><<span class="hljs-title">generator</span> <span class="hljs-attribute">class</span>=<span class="hljs-value">"native"</span> /></span> <span class="hljs-tag"></<span class="hljs-title">id</span>></span> <span class="hljs-tag"><<span class="hljs-title">property</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"name"</span> <span class="hljs-attribute">type</span>=<span class="hljs-value">"java.lang.String"</span>></span> <span class="hljs-tag"><<span class="hljs-title">column</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"NAME"</span> /></span> <span class="hljs-tag"></<span class="hljs-title">property</span>></span> <span class="hljs-tag"><<span class="hljs-title">property</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"age"</span> <span class="hljs-attribute">type</span>=<span class="hljs-value">"java.lang.Integer"</span>></span> <span class="hljs-tag"><<span class="hljs-title">column</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"AGE"</span> /></span> <span class="hljs-tag"></<span class="hljs-title">property</span>></span> <span class="hljs-tag"><<span class="hljs-title">many-to-one</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"company"</span> <span class="hljs-attribute">class</span>=<span class="hljs-value">"Company"</span> <span class="hljs-attribute">fetch</span>=<span class="hljs-value">"join"</span>></span> <span class="hljs-tag"><<span class="hljs-title">column</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"COMPANY"</span> /></span> <span class="hljs-tag"></<span class="hljs-title">many-to-one</span>></span> <span class="hljs-comment"><!-- 利用joined-subclass表签外键关联MonthEmploy --></span> <span class="hljs-tag"><<span class="hljs-title">joined-subclass</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"MonthEmploy"</span> <span class="hljs-attribute">table</span>=<span class="hljs-value">"MONEMPLOY"</span>></span> <span class="hljs-tag"><<span class="hljs-title">key</span> <span class="hljs-attribute">column</span>=<span class="hljs-value">"EMPLOYEE_ID"</span>></span><span class="hljs-tag"></<span class="hljs-title">key</span>></span> <span class="hljs-tag"><<span class="hljs-title">property</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"monthMoney"</span> <span class="hljs-attribute">type</span>=<span class="hljs-value">"double"</span>></span><span class="hljs-tag"></<span class="hljs-title">property</span>></span> <span class="hljs-tag"></<span class="hljs-title">joined-subclass</span>></span> <span class="hljs-tag"><<span class="hljs-title">joined-subclass</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"DayEmployee"</span> <span class="hljs-attribute">table</span>=<span class="hljs-value">"DAYEMPLOY"</span>></span> <span class="hljs-tag"><<span class="hljs-title">key</span> <span class="hljs-attribute">column</span>=<span class="hljs-value">"EMPLOYEE_ID"</span>></span><span class="hljs-tag"></<span class="hljs-title">key</span>></span> <span class="hljs-tag"><<span class="hljs-title">property</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"dayMoney"</span> <span class="hljs-attribute">type</span>=<span class="hljs-value">"double"</span>></span><span class="hljs-tag"></<span class="hljs-title">property</span>></span> <span class="hljs-tag"></<span class="hljs-title">joined-subclass</span>></span> <span class="hljs-tag"></<span class="hljs-title">class</span>></span> <span class="hljs-tag"></<span class="hljs-title">hibernate-mapping</span>></span></code>
测试代码:
<code class=" hljs java"><span class="hljs-keyword">public</span> <span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">EmployeeTest</span> {</span> SessionFactory sessionFactory = <span class="hljs-keyword">null</span>; Session session = <span class="hljs-keyword">null</span>; Transaction transaction = <span class="hljs-keyword">null</span>; <span class="hljs-annotation">@Before</span> <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">init</span>() { Configuration configuration = <span class="hljs-keyword">new</span> Configuration().configure(); ServiceRegistry serviceRegistry = <span class="hljs-keyword">new</span> ServiceRegistryBuilder().applySettings(configuration.getProperties()) .buildServiceRegistry(); sessionFactory = configuration.buildSessionFactory(serviceRegistry); session = sessionFactory.openSession(); transaction = session.beginTransaction(); } <span class="hljs-annotation">@After</span> <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">after</span>() { transaction.commit(); session.close(); sessionFactory.close(); } <span class="hljs-annotation">@Test</span> <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">testEmployee</span>() { Company company = <span class="hljs-keyword">new</span> Company(<span class="hljs-string">"com1"</span>); DayEmployee d1 = <span class="hljs-keyword">new</span> DayEmployee(); d1.setAge(<span class="hljs-number">22</span>); d1.setDayMoney(<span class="hljs-number">50.0</span>); d1.setName(<span class="hljs-string">"d1"</span>); MonthEmploy d2 = <span class="hljs-keyword">new</span> MonthEmploy(); d2.setAge(<span class="hljs-number">30</span>); d2.setMonthMoney(<span class="hljs-number">79.0</span>); d2.setName(<span class="hljs-string">"d2"</span>); Employee employee = <span class="hljs-keyword">new</span> Employee(); employee.setAge(<span class="hljs-number">90</span>); employee.setName(<span class="hljs-string">"employee"</span>); d1.setCompany(company); d2.setCompany(company); employee.setCompany(company); <span class="hljs-comment">//inverse = true 时可以关联, 当相对多出3条update,因此建议使用多的一端关联</span> <span class="hljs-comment">// company.getEmployees().add(employee);</span> <span class="hljs-comment">// company.getEmployees().add(d1);</span> <span class="hljs-comment">// company.getEmployees().add(d2);</span> <span class="hljs-comment">//把一的一段放到最前保存,可以减小3条更新操作</span> session.save(company); session.save(employee); session.save(d2); session.save(d1); } <span class="hljs-annotation">@Test</span> <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">findFromEmployeeTable</span>(){ String hql = <span class="hljs-string">"from Employee"</span>; <span class="hljs-comment">//通过左外链接找</span> List<Employee> list = session.createQuery(hql).list(); <span class="hljs-keyword">for</span> (Employee emp : list){ <span class="hljs-keyword">if</span> (emp <span class="hljs-keyword">instanceof</span> DayEmployee){ System.out.println(<span class="hljs-string">"day"</span>); } <span class="hljs-keyword">else</span> <span class="hljs-keyword">if</span> (emp <span class="hljs-keyword">instanceof</span> MonthEmploy){ System.out.println(<span class="hljs-string">"mon"</span>); } <span class="hljs-keyword">else</span> { System.out.println(<span class="hljs-string">"emp"</span>); } } } <span class="hljs-annotation">@Test</span> <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">findFromDayEmployeeTable</span>(){ String hql = <span class="hljs-string">"from DayEmployee"</span>; <span class="hljs-comment">//通过内连接找</span> List<Employee> list = session.createQuery(hql).list(); System.out.println(list.size()); } } </code>
得出如下结果:
结论:
1.使用根类对应一张表
好处:只需创建一张表,不用外键关联
缺点:如果子类属性多,会导致另一子类有过多的空值,而且不能设置一些字段的not null约束,因此不能保证数据库的完整性
2.每个类对应一张表(外键关联)
好处:如果某个类的属性发生变化,只需修改和这个类对应的表
缺点:利用外键关联查询
ref:
孙卫琴.精通Hibernate:Java对象持久化技术详解

MySQL은 초보자가 데이터베이스 기술을 배우는 데 적합합니다. 1. MySQL 서버 및 클라이언트 도구를 설치하십시오. 2. SELECT와 같은 기본 SQL 쿼리를 이해하십시오. 3. 마스터 데이터 작업 : 데이터를 만들고, 삽입, 업데이트 및 삭제합니다. 4. 고급 기술 배우기 : 하위 쿼리 및 창 함수. 5. 디버깅 및 최적화 : 구문 확인, 인덱스 사용, 선택*을 피하고 제한을 사용하십시오.

MySQL은 테이블 구조 및 SQL 쿼리를 통해 구조화 된 데이터를 효율적으로 관리하고 외래 키를 통해 테이블 간 관계를 구현합니다. 1. 테이블을 만들 때 데이터 형식을 정의하고 입력하십시오. 2. 외래 키를 사용하여 테이블 간의 관계를 설정하십시오. 3. 인덱싱 및 쿼리 최적화를 통해 성능을 향상시킵니다. 4. 데이터 보안 및 성능 최적화를 보장하기 위해 데이터베이스를 정기적으로 백업 및 모니터링합니다.

MySQL은 웹 개발에 널리 사용되는 오픈 소스 관계형 데이터베이스 관리 시스템입니다. 주요 기능에는 다음이 포함됩니다. 1. 다른 시나리오에 적합한 InnoDB 및 MyISAM과 같은 여러 스토리지 엔진을 지원합니다. 2.로드 밸런싱 및 데이터 백업을 용이하게하기 위해 마스터 슬레이브 복제 기능을 제공합니다. 3. 쿼리 최적화 및 색인 사용을 통해 쿼리 효율성을 향상시킵니다.

SQL은 MySQL 데이터베이스와 상호 작용하여 데이터 첨가, 삭제, 수정, 검사 및 데이터베이스 설계를 실현하는 데 사용됩니다. 1) SQL은 Select, Insert, Update, Delete 문을 통해 데이터 작업을 수행합니다. 2) 데이터베이스 설계 및 관리에 대한 생성, 변경, 삭제 문을 사용하십시오. 3) 복잡한 쿼리 및 데이터 분석은 SQL을 통해 구현되어 비즈니스 의사 결정 효율성을 향상시킵니다.

MySQL의 기본 작업에는 데이터베이스, 테이블 작성 및 SQL을 사용하여 데이터에서 CRUD 작업을 수행하는 것이 포함됩니다. 1. 데이터베이스 생성 : createAbasemy_first_db; 2. 테이블 만들기 : CreateTableBooks (idintauto_incrementprimarykey, titlevarchar (100) notnull, authorvarchar (100) notnull, published_yearint); 3. 데이터 삽입 : InsertIntobooks (Title, Author, Published_year) VA

웹 응용 프로그램에서 MySQL의 주요 역할은 데이터를 저장하고 관리하는 것입니다. 1. MySQL은 사용자 정보, 제품 카탈로그, 트랜잭션 레코드 및 기타 데이터를 효율적으로 처리합니다. 2. SQL 쿼리를 통해 개발자는 데이터베이스에서 정보를 추출하여 동적 컨텐츠를 생성 할 수 있습니다. 3.mysql은 클라이언트-서버 모델을 기반으로 작동하여 허용 가능한 쿼리 속도를 보장합니다.

MySQL 데이터베이스를 구축하는 단계에는 다음이 포함됩니다. 1. 데이터베이스 및 테이블 작성, 2. 데이터 삽입 및 3. 쿼리를 수행하십시오. 먼저 CreateAbase 및 CreateTable 문을 사용하여 데이터베이스 및 테이블을 작성한 다음 InsertInto 문을 사용하여 데이터를 삽입 한 다음 최종적으로 SELECT 문을 사용하여 데이터를 쿼리하십시오.

MySQL은 사용하기 쉽고 강력하기 때문에 초보자에게 적합합니다. 1.MySQL은 관계형 데이터베이스이며 CRUD 작업에 SQL을 사용합니다. 2. 설치가 간단하고 루트 사용자 비밀번호를 구성해야합니다. 3. 삽입, 업데이트, 삭제 및 선택하여 데이터 작업을 수행하십시오. 4. Orderby, Where and Join은 복잡한 쿼리에 사용될 수 있습니다. 5. 디버깅은 구문을 확인하고 쿼리를 분석하기 위해 설명을 사용해야합니다. 6. 최적화 제안에는 인덱스 사용, 올바른 데이터 유형 선택 및 우수한 프로그래밍 습관이 포함됩니다.


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