6.动态SQL

zhanglei 2022年06月02日 431次浏览

直接使用if 标签时,由于只要满足条件就直接拼接,所以很容易引起sql语法错误。

以where 标签为例,where标签里面包含一个或者多个if标签,至少一个test条件成立时,就会在sql语句后面加 where 关键字并拼接语句,test均不成立,则不添加 where 关键字,并且会自动删除紧跟在 where 关键字后面的 and 以及 or 使得 sql 语句正确。

mybatis-config.xml

<?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>
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <!--配置数据源:创建Connection对象-->
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.cj.jdbc.Driver"/>
                <property name="url"
                          value="jdbc:mysql://localhost:3306/bjpowernode?useUnicode=true&amp;characterEncoding=UTF-8"/>
                <property name="username" value="root"/>
                <property name="password" value="admin"/>
            </dataSource>
        </environment>
    </environments>
    <!--指定其他mapper文件的位置
    找到其他mapper文件的目的是找到sql语句

    使用注意:
        resource="mapper.文件的路径,用/分割"-->

    <mappers>
        <mapper resource="Dao/StudentDao.xml"/>
    </mappers>
</configuration>

StudentDao

package Dao;

import domain.Student;

public interface StudentDao {
    //where标签
    List<Student> selectWhere(Student st);
}

StudentDao.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">

<!--绑定接口-->
<mapper namespace="Dao.StudentDao">

    <!--动态sql
        使用if标签时,由于只要满足条件就直接拼接
        所以很容易引起sql语法错误。

        where标签里面包含一个或多个if标签
        至少一个test条件成立时,就在sql语句后加where关键字并拼接语句
        test均不成立,则不添加where关键字
        并且会自动删除紧跟在where关键字后面的and 和 or使得sql语句正确
    -->

    <select id="selectWhere" resultType="domain.Student">
        select name,age from student

        <where>
            <if test="age > 0">or age = #{age}</if>
            <if test="name !=null">or name =#{name}</if>
        </where>
    </select>


</mapper>

实体类Student

package domain;

public class Student {
    private Integer id;
    private String name;
    private String email;
    private Integer age;

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "学生实体{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", email='" + email + '\'' +
                ", age=" + age +
                '}';
    }
}

工具类MyBatisUtil

package util;

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.IOException;
import java.io.InputStream;

public class MyBatisUtil {
    //获得SqlSessionFactory
    public static SqlSessionFactory sqlSessionFactory;
    static{
        String config="mybatis-config.xml ";
        try {
            InputStream inputStream = Resources.getResourceAsStream(config);
            sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        } catch (IOException e) {
            e.printStackTrace();
        }

    }
    //获得sqlSession
    public static SqlSession getSqlSession(){
        return sqlSessionFactory.openSession();
    }
}

Test类

import Dao.StudentDao;
import domain.Student;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;


public class MyTest {
    @Test
    //where标签
    public void testSelectWhere(){
        //获取sqlSession
        SqlSession sqlSession = MyBatisUtil.getSqlSession();
        //获取dao实现类
        StudentDao mapper = sqlSession.getMapper(StudentDao.class);
        //调用接口中的方法
        Student st=new Student();
        st.setAge(19);
        st.setName("张磊");
        List<Student> list= mapper.selectWhere(st);
        //lambda表达式,遍历集合
        list.forEach(stu-> System.out.println(stu));
        sqlSession.close();
    }
}

测试结果

学生实体{id=null, name='张磊', email='null', age=23}
学生实体{id=null, name='陆启赛', email='null', age=19}