<set> 标签可以理解为 SQL update 语句的 set 语句，该语句没有任何属性。该标签可以被用于动态包含更新的列，而不包含不需更新的。比如：
<update id="updateAuthorIfNecessary" parameterType="domain.blog.Author">
update Author
<set>
<if test="username != null">username=#{username},</if>
<if test="password != null">password=#{password},</if>
<if test="email != null">email=#{email},</if>
<if test="bio != null">bio=#{bio}</if>
</set>
where id=#{id}
</update>这里 <set> 标签会动态前置 SET 关键字，而且也会消除任意无关的逗号。
（1）MyBatis 配置文件 mybatis-cfg.xml 内容如下：
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
<properties resource="database.properties"/>
<environments default="MySqlDatabase" >
<environment id="MySqlDatabase" >
<transactionManager type="JDBC" />
<dataSource type="POOLED">
<property name="driver" value="${jdbc.driver}"/>
<property name="url" value="${jdbc.url}"/>
<property name="username" value="${jdbc.username}"/>
<property name="password" value="${jdbc.password}"/>
</dataSource>
</environment>
</environments>
<mappers>
<mapper resource="com/hxstrive/mybatis/dynamic_sql/demo5/UserMapper.xml" />
</mappers>
</configuration>（2）实体 UserBean.java 代码如下：
package com.hxstrive.mybatis.dynamic_sql.demo5;
public class UserBean {
private Integer userId;
private String name;
private String sex;
private Integer age;
// 忽略 getter 和 setter 方法
@Override
public String toString() {
return "UserBean{" +
"userId=" + userId +
", name='" + name + '\'' +
", sex='" + sex + '\'' +
", age=" + age +
'}';
}
}（3）定义 Mapper
a、UserMapper.java
package com.hxstrive.mybatis.dynamic_sql.demo5;
public interface UserMapper {
void updateUser(UserBean userBean);
void updateUser2(UserBean userBean);
}b、UserMapper.xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.hxstrive.mybatis.dynamic_sql.demo5.UserMapper">
<update id="updateUser">
update `user`
<set>
<if test="name != null and name != ''">`name`=#{name},</if>
<if test="sex != null and sex != ''">`sex`=#{sex},</if>
<if test="age != null and age > 0">`age`=#{age}</if>
</set>
where `user_id`=#{userId}
</update>
<!-- 利用 trim 实现 set 同等效果 -->
<update id="updateUser2">
update `user`
<trim prefix="set" suffixOverrides=",">
<if test="name != null and name != ''">`name`=#{name},</if>
<if test="sex != null and sex != ''">`sex`=#{sex},</if>
<if test="age != null and age > 0">`age`=#{age}</if>
</trim>
where `user_id`=#{userId}
</update>
</mapper>（4）客户端代码
package com.hxstrive.mybatis.dynamic_sql.demo5;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import java.io.InputStream;
public class DynamicSqlDemo {
public static void main(String[] args) throws Exception {
String cfgName = "com/hxstrive/mybatis/dynamic_sql/demo5/mybatis-cfg.xml";
InputStream input = Resources.getResourceAsStream(cfgName);
SqlSessionFactoryBuilder factoryBuilder = new SqlSessionFactoryBuilder();
SqlSessionFactory sqlFactory = factoryBuilder.build(input);
System.out.println("\n\n示例1：");
updateUser(sqlFactory);
System.out.println("\n\n示例2：");
updateUser2(sqlFactory);
}
private static void updateUser(SqlSessionFactory sqlFactory) {
UserBean userBean = new UserBean();
userBean.setUserId(1);
userBean.setName("爱丽丝");
userBean.setAge(27);
SqlSession sqlSession = sqlFactory.openSession(true);
UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
userMapper.updateUser(userBean);
}
private static void updateUser2(SqlSessionFactory sqlFactory) {
UserBean userBean = new UserBean();
userBean.setUserId(1);
userBean.setName("赫仑");
userBean.setSex("男");
SqlSession sqlSession = sqlFactory.openSession(true);
UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
userMapper.updateUser2(userBean);
}
}
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