前面章节的例子已经方便地处理了一个臭名昭著的动态 SQL 问题。如果我们有下面这样一个示例：
<select id="findActiveBlogLike" parameterType="Blog" resultType="Blog">
SELECT * FROM BLOG
WHERE
<if test="state != null">
state = #{state}
</if>
<if test="title != null">
AND title like #{title}
</if>
<if test="author != null and author.name != null">
AND title like #{author.name}
</if>
</select>如果这些条件都没有匹配上将会发生什么？这条 SQL 结束时就会成这样：
SELECT * FROM BLOG WHERE
这会导致查询失败。如果仅仅第二个条件匹配是什么样的？这条 SQL 结束时就会是这样：
SELECT * FROM BLOG WHERE AND title like "someTitle"
这个查询也会失败。这个问题不能简单的用条件来解决，如果你从来没有这样写过，那么你以后也不会这样来写。
MyBatis 有一个简单的处理，这在 90%的情况下都会有用。而在不能使用的地方，你可以自定义处理方式。加上一个简单的改变，所有事情都会顺利进行：
<select id="findActiveBlogLike" parameterType="Blog" resultType="Blog">
SELECT * FROM BLOG
<where>
<if test="state != null">
state = #{state}
</if>
<if test="title != null">
AND title like #{title}
</if>
<if test="author != null and author.name != null">
AND title like #{author.name}
</if>
</where>
</select><where> 元素知道如果由被包含的标记（如：<if>标签）返回了任意内容，则<where>标签仅仅插入“WHERE”字符串。如果 <where> 标签包含的标签的内容以“AND”或“ OR”开头的内容，那么 MyBatis 会将 where 后面紧接的 <if> 标签的内容前面的 AND 和 OR 移除。
（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/demo3/UserMapper.xml" />
</mappers>
</configuration>（2）Java 实体Bean UserBean.java，代码如下：
package com.hxstrive.mybatis.dynamic_sql.demo3;
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 文件
a、UserMapper.java
package com.hxstrive.mybatis.dynamic_sql.demo3;
import org.apache.ibatis.annotations.Param;
import java.util.List;
public interface UserMapper {
List<UserBean> getUserList(@Param("username") String username, @Param("userId") int userId);
}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.demo3.UserMapper">
<!-- 映射结果 -->
<resultMap id="RESULT_MAP" type="com.hxstrive.mybatis.dynamic_sql.demo3.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`
<where>
<if test="username != null and username != ''">
and `name` like #{username}
</if>
<if test="userId > 0">
and `user_id`=#{userId}
</if>
</where>
</select>
</mapper>（4）客户端代码如下：
package com.hxstrive.mybatis.dynamic_sql.demo3;
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/demo3/mybatis-cfg.xml";
InputStream input = Resources.getResourceAsStream(cfgName);
SqlSessionFactoryBuilder factoryBuilder = new SqlSessionFactoryBuilder();
SqlSessionFactory sqlFactory = factoryBuilder.build(input);
getUserList(sqlFactory);
}
private static void getUserList(SqlSessionFactory sqlFactory) {
SqlSession sqlSession = sqlFactory.openSession(true);
UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
List<UserBean> userBeanList = userMapper.getUserList(null, 1);
for(UserBean userBean : userBeanList) {
System.out.println(userBean);
}
}
}运行客户端代码，输出结果如下：
2020-09-10 22:07:51,251 DEBUG [org.apache.ibatis.logging.LogFactory] - Logging initialized using 'class org.apache.ibatis.logging.slf4j.Slf4jImpl' adapter.
2020-09-10 22:07:51,292 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] - PooledDataSource forcefully closed/removed all connections.
2020-09-10 22:07:51,292 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] - PooledDataSource forcefully closed/removed all connections.
2020-09-10 22:07:51,292 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] - PooledDataSource forcefully closed/removed all connections.
2020-09-10 22:07:51,292 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] - PooledDataSource forcefully closed/removed all connections.
2020-09-10 22:07:51,511 DEBUG [org.apache.ibatis.transaction.jdbc.JdbcTransaction] - Opening JDBC Connection
2020-09-10 22:07:51,959 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] - Created connection 1297149880.
2020-09-10 22:07:51,962 DEBUG [com.hxstrive.mybatis.dynamic_sql.demo3.UserMapper.getUserList] - ooo Using Connection [com.mysql.jdbc.JDBC4Connection@4d50efb8]
2020-09-10 22:07:51,963 DEBUG [com.hxstrive.mybatis.dynamic_sql.demo3.UserMapper.getUserList] - ==> Preparing: select `user_id`, `name`, `age`, `sex` from `user` WHERE `user_id`=?
2020-09-10 22:07:52,023 DEBUG [com.hxstrive.mybatis.dynamic_sql.demo3.UserMapper.getUserList] - ==> Parameters: 1(Integer)
2020-09-10 22:07:52,063 DEBUG [com.hxstrive.mybatis.dynamic_sql.demo3.UserMapper.getUserList] - <== Total: 1
UserBean{userId=1, name='赫仑', sex='男', age=27}上面示例中，执行 getUserList 时，参数 username=null，userId=1，将返回如下 SQL 语句：
select `user_id`, `name`, `age`, `sex` from `user` WHERE `user_id`=?
按照上面 Mapper xml 的定义，如果 userId 大于 0，则输出 “and `user_id`=#{userId}”，与其他 SQL 语句拼接在一起，则应该如下：
select `user_id`, `name`, `age`, `sex` from `user`
where
and `user_id`=#{userId}上面 SQL 是有问题的。但是，MySQL 自动将 “where and `user_id`=#{userId}”之前的 and 去除掉了。也就导致上面示例运行成功了！！
