Mybatis20_MyBatis映射文件深入(动态SQL)6

一、动态sql语句

　1、动态sql语句概述

　　Mybatis的映射文件中，前面我们的SQL都是比较简单的，有些时候业务逻辑复杂时，我们的 SQL是动态变化的，此时在前面的学习中我们的 SQL 就不能满足要求了。

　2、环境搭建

　　UserMapper.java

package com.itheima.mapper;

import com.itheima.domain.User;
import java.util.List;

public interface UserMapper {

    public List<User> findByCondition(User user);

}

　　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.itheima.mapper.UserMapper">

    <!--查询操作-->
    <select id="findByCondition" parameterType="user" resultType="user">
        select * from user where id=#{id} and username=#{username} and password=#{password}
    </select>

</mapper>

　　log4j.properties

### direct log messages to stdout ###
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.Target=System.out
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%d{ABSOLUTE} %5p %c{1}:%L - %m%n

### direct messages to file mylog.log ###
log4j.appender.file=org.apache.log4j.FileAppender
log4j.appender.file.File=c:/mylog.log
log4j.appender.file.layout=org.apache.log4j.PatternLayout
log4j.appender.file.layout.ConversionPattern=%d{ABSOLUTE} %5p %c{1}:%L - %m%n

### set log levels - for more verbose logging change 'info' to 'debug' ###

log4j.rootLogger=debug, stdout

　　MapperTest.java

package com.itheima.test;

import com.itheima.domain.User;
import com.itheima.mapper.UserMapper;
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 org.junit.Test;
import java.io.IOException;
import java.io.InputStream;
import java.util.List;

public class MapperTest {

    @Test
    public void test1() throws IOException {
        InputStream resourceAsStream = Resources.getResourceAsStream("sqlMapConfig.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();
        UserMapper mapper = sqlSession.getMapper(UserMapper.class);

        //模拟条件user
        User condition = new User();
        condition.setId(1);
        condition.setUsername("zhangsan");
        condition.setPassword("123");

        List<User> userList = mapper.findByCondition(condition);
        System.out.println(userList);
    }
}

　　Run test1方法，检查结果：[User{id=1, username='zhangsan', password='123'}]

　　当查询条件id、username和password都存在时控制台打印的sql语句如下：

　3、动态sql之<if>

　　我们根据实体类的不同取值，使用不同的 SQL语句来进行查询。比如在 id如果不为空时可以根据id查询，如果username 不为空时还要加入用户名作为条件。这种情况在我们的多条件组合查询中经常会碰到。

<select id="findByCondition" parameterType="user" resultType="user">
    select * from User
    <where>
        <if test="id!=0">
            and id=#{id}
        </if>
        <if test="username!=null">
            and username=#{username}
        </if>
    </where>
</select>

　　当查询条件id和username都存在时，控制台打印的sql语句如下：