在 MyBatis 动态 SQL 中所做的最通用的事情是包含部分 where 字句的条件。比如：
<select id="findActiveBlogWithTitleLike" parameterType="Blog" resultType="Blog">
SELECT * FROM BLOG
WHERE state="ACTIVE"
<if test="title != null">
AND title like #{title}
</if>
</select>这条语句会提供一个可选的文本查找功能。如果你没有传递 title，那么所有激活的（state="ACTIVE"）博客都会被返回。但是如果你传递了 title，那么就会查找相近的 title 的博客。
假若我们想可选地搜索 title 和 author 呢？首先，要改变语句的名称让它有意义。然后简单加入另外的一个条件。
<select id="findActiveBlogLike" parameterType="Blog" resultType="Blog">
SELECT * FROM BLOG WHERE state="ACTIVE"
<if test="title != null">
AND title like #{title}
</if>
<if test="author != null and author.name != null">
AND title like #{author.name}
</if>
</select>下面示例将演示动态添加 where 语句和根据参数可选择的添加多个 where 子条件。
（1）MyBatis 配置文件 mybatis-cfg.xml 文件内容如下：
<?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/demo1/UserMapper.xml" />
</mappers>
</configuration>（2）定义 JavaBean UserBean.java，代码如下：
package com.hxstrive.mybatis.dynamic_sql.demo1;
public class UserBean {
private Integer userId;
private String name;
private String sex;
private Integer age;
// 忽略 getter 和 setter
@Override
public String toString() {
return "UserBean{" +
"userId=" + userId +
", name='" + name + '\'' +
", sex='" + sex + '\'' +
", age=" + age +
'}';
}
}（3）定义 Mapper 接口和 XML 文件：
a、UserMapper.java
package com.hxstrive.mybatis.dynamic_sql.demo1;
import org.apache.ibatis.annotations.Param;
import java.util.List;
public interface UserMapper {
List<UserBean> getUserList(@Param("userId") int userId);
List<UserBean> getUserList2(@Param("username") String username, @Param("age") int age);
}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.demo1.UserMapper">
<!-- 映射结果 -->
<resultMap id="RESULT_MAP" type="com.hxstrive.mybatis.dynamic_sql.demo1.UserBean">
<id column="user_id" jdbcType="INTEGER" property="userId" />
<result column="name" jdbcType="VARCHAR" property="name" />
<result column="sex" jdbcType="VARCHAR" property="sex" />
<result column="age" jdbcType="INTEGER" property="age" />
</resultMap>
<!-- 动态添加 where 条件 -->
<select id="getUserList" resultMap="RESULT_MAP">
select `user_id`, `name`, `age`, `sex` from `user`
<if test="userId > 0">
where `user_id`=#{userId}
</if>
</select>
<!-- 动态添加可选条件（用户名和年龄）-->
<select id="getUserList2" resultMap="RESULT_MAP">
select `user_id`, `name`, `age`, `sex` from `user`
where `user_id` is not null
<if test="username != null and username != ''">
and `name` like #{username}
</if>
<if test="age > 0">
and `age` > #{age}
</if>
</select>
</mapper>（4）客户端代码如下：
package com.hxstrive.mybatis.dynamic_sql.demo1;
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;
import java.util.List;
public class DynamicSqlDemo {
public static void main(String[] args) throws Exception {
String cfgName = "com/hxstrive/mybatis/dynamic_sql/demo1/mybatis-cfg.xml";
InputStream input = Resources.getResourceAsStream(cfgName);
SqlSessionFactoryBuilder factoryBuilder = new SqlSessionFactoryBuilder();
SqlSessionFactory sqlFactory = factoryBuilder.build(input);
SqlSession sqlSession = sqlFactory.openSession(true);
UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
getUserList(userMapper);
getUserList2(userMapper);
}
private static void getUserList(UserMapper userMapper) {
System.out.println("getUserList()：");
List<UserBean> userBeanList = userMapper.getUserList(1);
for(UserBean userBean : userBeanList) {
System.out.println(userBean);
}
}
private static void getUserList2(UserMapper userMapper) {
System.out.println("getUserList2()：");
List<UserBean> userBeanList = userMapper.getUserList2("%张%", -1);
for(UserBean userBean : userBeanList) {
System.out.println(userBean);
}
}
}运行客户端代码，输出结果如下：
2020-09-09 13:10:00,916 DEBUG [org.apache.ibatis.logging.LogFactory] - Logging initialized using 'class org.apache.ibatis.logging.slf4j.Slf4jImpl' adapter.
2020-09-09 13:10:00,972 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] - PooledDataSource forcefully closed/removed all connections.
2020-09-09 13:10:00,976 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] - PooledDataSource forcefully closed/removed all connections.
2020-09-09 13:10:00,976 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] - PooledDataSource forcefully closed/removed all connections.
2020-09-09 13:10:00,976 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] - PooledDataSource forcefully closed/removed all connections.
getUserList()：
2020-09-09 13:10:01,206 DEBUG [org.apache.ibatis.transaction.jdbc.JdbcTransaction] - Opening JDBC Connection
2020-09-09 13:10:01,755 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] - Created connection 1263877414.
2020-09-09 13:10:01,757 DEBUG [com.hxstrive.mybatis.dynamic_sql.demo1.UserMapper.getUserList] - ooo Using Connection [com.mysql.jdbc.JDBC4Connection@4b553d26]
2020-09-09 13:10:01,759 DEBUG [com.hxstrive.mybatis.dynamic_sql.demo1.UserMapper.getUserList] - ==> Preparing: select `user_id`, `name`, `age`, `sex` from `user` where `user_id`=?
2020-09-09 13:10:01,892 DEBUG [com.hxstrive.mybatis.dynamic_sql.demo1.UserMapper.getUserList] - ==> Parameters: 1(Integer)
2020-09-09 13:10:01,937 DEBUG [com.hxstrive.mybatis.dynamic_sql.demo1.UserMapper.getUserList] - <== Total: 1
UserBean{userId=1, name='Helen', sex='男', age=28}
getUserList2()：
2020-09-09 13:10:01,941 DEBUG [com.hxstrive.mybatis.dynamic_sql.demo1.UserMapper.getUserList2] - ooo Using Connection [com.mysql.jdbc.JDBC4Connection@4b553d26]
2020-09-09 13:10:01,942 DEBUG [com.hxstrive.mybatis.dynamic_sql.demo1.UserMapper.getUserList2] - ==> Preparing: select `user_id`, `name`, `age`, `sex` from `user` where `user_id` is not null and `name` like ?
2020-09-09 13:10:01,944 DEBUG [com.hxstrive.mybatis.dynamic_sql.demo1.UserMapper.getUserList2] - ==> Parameters: %张%(String)
2020-09-09 13:10:01,956 DEBUG [com.hxstrive.mybatis.dynamic_sql.demo1.UserMapper.getUserList2] - <== Total: 1
UserBean{userId=2, name='张小凡', sex='男', age=30}仔细查看上面输出的日志信息，其中：
getUserList() 执行的 SQL 语句如下：
select `user_id`, `name`, `age`, `sex` from `user` where `user_id`=?
其中 “where `user_id`=? ” 条件是因为参数 userId 大于0动态添加的。
getUserList2() 执行的 SQL 语句如下：
select `user_id`, `name`, `age`, `sex` from `user` where `user_id` is not null and `name` like ?
其中 “and `name` like ?” 语句是因为传递的 username 参数不为空。
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