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.if标签的使用
根据实体类的不同取值,使用不同的 SQL语句来进行查询
<where>:条件标签,如果有动态条件,则使用该标签代替where关键字
<if>:条件判断标签
java
<if test="条件判断">
拼接的查询条件
</if>
例:多条件查询
StudentMapper.xml:
XML
<select id="selectCondition" resultType="student" parameterType="student">
select * from student
<where>
<if test="id != null">
id = #{id}
</if>
<if test="name != null">
AND name = #{name}
</if>
<if test="id != null">
AND age = #{age}
</if>
</where>
</select>
Mapper接口:
java
//多条件查询
public abstract List<Student> selectCondition(Student stu);
测试类:
java
package Mybatis2;
import Mybatis2.mapper.StudentMapper;
import mybatis1.bean.Student;
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.InputStream;
import java.util.List;
public class test1 {
@Test
public void selectCondition() throws Exception {
InputStream is = Resources.getResourceAsStream("MyBatisConfig.xml");
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(is);
SqlSession sqlSession = sqlSessionFactory.openSession(true);
//获取StudentMapper接口的实现类对象
StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
Student stu = new Student();
stu.setId(2);
stu.setName("李四");
stu.setAge(24);
//调用实现类的方法,接收结果
List<Student> list = mapper.selectCondition(stu);
//处理结果
for (Student student : list) {
System.out.println(student);
}
sqlSession.close();
is.close();
}
}
3.foreach标签的使用
<foreach>:循环遍历标签。适用于多个参数或者的关系
XML
<foreach collection="" open="" close="" item="" separator="">
获取参数
</foreach>
属性:
collection:参数容器类型,(list-集合,array-数组)
open:开始的SQL语句
close:结束的SQL语句
item:参数变量名
separator:分隔符
增加StudentMapper.xml:
XML
<select id="selectByIds" resultType="student" parameterType="list">
<include refid="select"/>
<where>
<foreach collection="list" open="id IN (" close=")" item="id" separator=",">
#{id}
</foreach>
</where>
</select>
Mapper接口:
java
//根据多个id查询
public abstract List<Student> selectByIds(List<Integer> sids);
测试类:
java
@Test
public void selectByIds() throws Exception{
//1.加载核心配置文件
InputStream is = Resources.getResourceAsStream("MybatisConfig.xml");
//2.获取SqlSession工厂对象
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(is);
//3.通过工厂对象获取SqlSession
SqlSession sqlSession = sqlSessionFactory.openSession(true);
//4.获取StudentMapper接口的实现类对象
StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
//5.调用实现类的方法,接收结果
List<Integer> sids = new ArrayList<>();
sids.add(1);
sids.add(2);
List<Student> list = mapper.selectByIds(sids);
for(Student student : list){
System.out.println(student);
}
sqlSession.close();
is.close();
}
4.SQL片段的抽取
我们可以将一些重复性的SQL语句进行抽取,以达到复用的效果
<sql>:抽取SQL语句的标签
<sql id="片段唯一标识">抽取的SQL语句</sql>
<include>:引入SQL片段标签
<include refid="片段唯一标识" />
XML
<?xml version="1.0" encoding="UTF-8" ?>
<!--MyBatis的DTD约束-->
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<!--
mapper:核心根标签
namespace属性:名称空间
-->
<mapper namespace="Mybatis2.mapper.StudentMapper">
<sql id="select">SELECT * FROM student</sql>
<!--select:查询功能的标签
id属性:唯一标识
resultType属性:指定结果映射对象类型
parameterType属性:指定参数映射对象类型-->
<select id="selectAll" resultType="Student">
<include refid="select"/>
</select>
<select id="selectById" resultType="Student" parameterType="java.lang.Integer">
<include refid="select"/> WHERE id = #{id}
</select>
<insert id="insert" parameterType="Student">
INSERT INTO student VALUES (#{id},#{name},#{age})
</insert>
<delete id="delete" parameterType="Student">
DELETE FROM student WHERE id = #{id}
</delete>
<select id="selectCondition" resultType="student" parameterType="student">
<include refid="select"/>
<where>
<if test="id != null">
id = #{id}
</if>
<if test="name != null">
AND name = #{name}
</if>
<if test="id != null">
AND age = #{age}
</if>
</where>
</select>
<select id="selectByIds" resultType="student" parameterType="list">
<include refid="select"/>
<where>
<foreach collection="list" open="id IN (" close=")" item="id" separator=",">
#{id}
</foreach>
</where>
</select>
</mapper>