动态SQL之 where 标签
where和if一般结合使用：
- 1.若where标签中的 if 条件都不满足，则where标签没有任何功能，即不会添加where关键字
- 2.若where标签中的 if 条件满足，则where标签会自动添加where关键字，并将条件最前方多余的 and去掉
- 注意：where标签不能去掉条件最后多余的and
DynamicSQLMapper接口
public interface DynamicSQLMapper {/*** 根据条件来查询员工信息* @param emp* @return*/List<Emp> getEmpByCondition(Emp emp);
}
MyBatis的映射文件DynamicSQLMapper.xml
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""https://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.atguigu.mybatis.mapper.DynamicSQLMapper"><!--List<Emp> getEmpByCondition(Emp emp);--><select id="getEmpByCondition" resultType="Emp">select * from t_emp<where><if test="empName != null and empName != ''">emp_name = #{empName}</if><if test="age != null and age != ''">and age = #{age}</if><if test="gender != null and gender != ''">and gender = #{gender}</if></where></select></mapper>
测试
public class DynamicMapperTest {@Testpublic void testGetEmpByCondition(){SqlSession sqlSession = SqlSessionUtil.getSqlSession();DynamicSQLMapper mapper = sqlSession.getMapper(DynamicSQLMapper.class);Emp emp = new Emp(null,"张三",20,"男");List<Emp> list = mapper.getEmpByCondition(emp);list.forEach(System.out::println);}
}
