mapper.xml文件

<?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.xxx.dao.CityHospitalDao">
    <!-- 根据某字段查询所有 -->
    <select id="getHospitalByType" resultType="com.xx.entity.CityHospitalEntity" parameterType="String">
       select * from hospital
  <!-- 非空判断，无数据查询所有 -->
　　<if test="type != null and type != '' "> 
　　where type = #{type} 
　　</if> 
　　</select>
 </mapper>

 dao接口：

import java.util.List;
import org.apache.ibatis.annotations.Mapper;
import org.apache.ibatis.annotations.Param;
import com.baomidou.mybatisplus.core.mapper.BaseMapper;
import com.xxx.city.entity.CityHospitalEntity;
@Mapper
public interface CityHospitalDao extends BaseMapper<CityHospitalDao>{
    
     List<CityHospitalEntity> getHospitalByType(@Param("type") String type);
}