MP多表关联查询

我们关联user表和product表，两个表如下：

• user表

  

• product表

  

现在我们要关联两个表查询出product的全部信息已经对应的用户名字

先写sql语句

然后创建vo

package com.hyn.mybatisplus.entity;

import lombok.Data;

@Data
public class ProductVo {
    private Integer category;
    private Integer count;
    private String description;
    private Integer userId;
    private String userName;
}

usermapper创建新方法和JPA类似

package com.hyn.mybatisplus.mapper;

import com.baomidou.mybatisplus.core.mapper.BaseMapper;
import com.hyn.mybatisplus.entity.ProductVo;
import com.hyn.mybatisplus.entity.User;
import org.apache.ibatis.annotations.Mapper;
import org.apache.ibatis.annotations.Select;
import org.springframework.stereotype.Repository;

import java.util.List;

@Repository
@Mapper
public interface UserMapper extends BaseMapper<User> {
    @Select("select p.*,u.name userName from user u ,product p where u.id=p.user_id and u.id = #{id}")
    List<ProductVo> productList(Integer id);
}

最后测试一下

@Test
void product(){
    mapper.productList(13).forEach(System.out::println);
}

​ 可见关联表查询以及封装数据成功，如果vo和表里的字段不对应可以在sql语句中改别名。

多表关联查询多表关联在xml中写sql，where条件可用queryWrapper实现dao中插入分页对象和查询条件(Page page, @Param(Constants.WRAPPER) QueryWrapper queryWrapper)，xml中使用${ew.customSqlSegment}代替where
1.sql

SELECT w.works_id worksId, w.user_id userId, w.user_id_type userIdType, w.title, w.content, 
	w.media_type mediaType, w.media_ids mediaIds, w.first_image_id firstImageId, w.works_tag 
	worksTag, w.lon_map lonMap, w.lat_map latMap, w.location, w.district_id districtId, w.like_num likeNum, 
	w.favorite_num favoriteNum, w.comment_num commentNum, m.image_id imageId, p.nick_name nickName, w.create_time createTime 
FROM fmzy_biz.works w 
	LEFT JOIN fmzy_main.user_image m on w.user_id = m.user_id 
	LEFT JOIN fmzy_main.user_person p on w.user_id = p.user_id 
WHERE 
	m.image_type = 'U_TX' 
AND p.test_flag = 0 AND w.show_flag = 1 
AND ( p.nick_name LIKE '%1%' OR w.title LIKE '%1%' OR w.content LIKE '%1%' ) 
ORDER BY w.works_id DESC LIMIT 0,20

2.java

 QueryWrapper<WorksInfoVO> queryWrapper = new QueryWrapper<WorksInfoVO>()
                .eq("m.image_type", "U_TX")
                .eq("p.test_flag", 0)
                .eq("w.show_flag", 1)
                .in(!CollectionUtils.isEmpty(districtIds), "w.district_id", districtIds)
                .eq(StringUtils.isNotBlank(mediaType), "w.media_type", mediaType)
                .eq(StringUtils.isNotBlank(worksTag), " w.works_tag", worksTag)
                .eq(StringUtils.isNotBlank(userIdType), "w.user_id_type", userIdType)
                .and(qw->qw.like(StringUtils.isNotBlank(content),"p.nick_name",content)
                        .or()
                        .like(StringUtils.isNotBlank(content),"w.title",content)
                        .or()
                        .like(StringUtils.isNotBlank(content),"w.content",content)).orderByDesc("w.works_id");
        Page<WorksInfoVO> page = new Page<>(pageNo, pageSize);
        IPage<WorksInfoVO> ipage = baseMapper.getFindWorksPage(page,queryWrapper);

3.dao

    IPage<WorksInfoVO> getFindWorksPage(Page<WorksInfoVO> page, @Param(Constants.WRAPPER) QueryWrapper<WorksInfoVO> queryWrapper);

4.xml

 <select id="getFindWorksPage" resultType="bw.fmzy.svc.web.main.vo.WorksInfoVO">
        SELECT w.works_id       worksId,
               w.user_id        userId,
               w.user_id_type   userIdType,
               w.title,
               w.content,
               w.media_type     mediaType,
               w.media_ids      mediaIds,
               w.first_image_id firstImageId,
               w.works_tag      worksTag,
               w.lon_map        lonMap,
               w.lat_map        latMap,
               w.location,
               w.district_id    districtId,
               w.like_num       likeNum,
               w.favorite_num   favoriteNum,
               w.comment_num    commentNum,
               m.image_id       imageId,
               p.nick_name      nickName,
               w.create_time    createTime
        FROM fmzy_biz.works w
                 LEFT JOIN fmzy_main.user_image m on w.user_id = m.user_id
                 LEFT JOIN fmzy_main.user_person p on w.user_id = p.user_id
            ${ew.customSqlSegment}
    </select>

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1.dao层接口使用Select注解写SQL：

重点：@Param(“ew”) Wrapper参数是必须，因为${ew.customSqlSegment} 底层其实就是where 条件，所以为了保证Wrapper不为空，service层代码中的Wrapper至少需要有一个条件：1 = 1

@Override
    @Select("select a.code as code , b.name as name , b.barcode as barcode ,  a.ware_code as wareCode , c.name as wareName , a.qty as qty , a.oprice as oprice , a.total as total , " +
            " a.id as id , a.create_by as createBy , a.create_date as createDate , a.update_by as updateBy , a.update_date as updateDate , a.status as status , a.remarks as remarks  " +
            "from sku_stock a , goods b , warehouse c " +
            "${ew.customSqlSegment} and a.code = b.code and a.ware_code = c.code")
    IPage<SkuStock> selectPage(IPage<SkuStock> page, @Param("ew")Wrapper<SkuStock> queryWrapper);

2.service层代码示例：

service父类封装的findPage方法：

/**
     * 封装findPage
     * @param entity
     * @param search  Map中的key：";"为保留关键字，拆分数组，仅支持最大长度2的数组，
     *                下标0：QueryWrapper查询条件中的列名(支持多表关联查询的表别名 + 列名方式，需要dao层接口支持)
     *                下标1： QueryWrapper中不同的查询条件，eq：等于，ge：大于等..... todo：请自行完善Mybatis eq、ne、gt、lt、ge、le等
     *                Map中的value：QueryWrapper需要查询的值
     * @param args  QueryWrapper中order by 排序数组
     * @return
     */
    public IPage<T> findPage(T entity , Map<String , Object> search , String... args){
        long current = 1L;
        long size = 10L;
        if (EmptyUtil.isNoEmpty(ReflexUtil.getFieldValue(entity , "page")) && (long) ReflexUtil.getFieldValue(entity , "page") != 0){
            current = (long) ReflexUtil.getFieldValue(entity , "page");
        }
        if (EmptyUtil.isNoEmpty(ReflexUtil.getFieldValue(entity , "limit")) && (long) ReflexUtil.getFieldValue(entity , "limit") != 0){
            size = (long) ReflexUtil.getFieldValue(entity , "limit");
        }

        QueryWrapper<T> queryWrapper;
        if (EmptyUtil.isNoEmpty(search)){
            queryWrapper = new QueryWrapper<>();
            for (Map.Entry<String , Object> entry:search.entrySet()
                 ) {
                String[] key = entry.getKey().split(";");
                if (key.length > 1){
                    if (key[1].equals("eq")){
                        queryWrapper.eq(key[0] , entry.getValue());
                    }else if (key[1].equals("ge")){
                        queryWrapper.ge(key[0] , entry.getValue());
                    }else if (key[1].equals("lt")){
                        queryWrapper.lt(key[0] , entry.getValue());
                    }
                }else {
                    queryWrapper.like(entry.getKey() , entry.getValue());
                }
            }
        }else {
            queryWrapper = new QueryWrapper<>(entity);
        }
        queryWrapper.orderByAsc(args);
        return super.page(new Page<T>(current , size) , queryWrapper);
    }

service实现类方法：

public IPage<SkuStock> findPage(SkuStock entity, String... args) {
        Map<String , Object> search = null;
        search = new HashedMap();
        search.put("1;eq" , "1");
        if (EmptyUtil.isNoEmpty(entity.getCode())
                || EmptyUtil.isNoEmpty(entity.getWareCode())
        ){
            if (EmptyUtil.isNoEmpty(entity.getCode())){
                search.put("code" , entity.getCode());
            }
            if (EmptyUtil.isNoEmpty(entity.getWareCode())){
                search.put("ware_code" , entity.getWareCode());
            }
        }else {
            long limit = entity.getLimit();
            long page = entity.getPage();
            entity = new SkuStock();
            entity.setLimit(limit);
            entity.setPage(page);
        }
        return super.findPage(entity , search , args);
    }

3.反射工具类：

package org.bluedream.comm.utils;

import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

/**
 * @ClassName ReflexUtil
 * @Description TODO
 * @Author foxsand
 * @Data 2021-06-09 15:17
 * @Version
 */
public class ReflexUtil {

    /**
     * 返回 entity 对象的所有属性，包含父类
     * @param obj
     * @return
     */
    public static List<Field> getObjectFields(Object obj){
        Class clazz = obj.getClass();
        List<Field> fieldList = new ArrayList<>() ;
        while (clazz != null) {//当父类为null的时候说明到达了最上层的父类(Object类).
            fieldList.addAll(Arrays.asList(clazz .getDeclaredFields()));
            clazz = clazz.getSuperclass(); //得到父类,然后赋给自己
        }
        return fieldList;
    }

    public static List<Field> getObjectFields(Class<?> clazz){
        List<Field> fieldList = new ArrayList<>() ;
        while (clazz != null){
            fieldList.addAll(Arrays.asList(clazz .getDeclaredFields()));
            clazz = clazz.getSuperclass(); //得到父类,然后赋给自己
        }
        return fieldList;
    }

    /**
     * 判断 Class entity 是否存在名称为 fieldName 的属性
     * @param fieldName
     * @param entity
     * @return
     */
    public static Boolean isField(String fieldName , Object entity){
        List<Field> fieldList = getObjectFields(entity);
        for (Field f1:fieldList
        ) {
            if (fieldName.equals(f1.getName()))
                return true;
        }
        return false;
    }

    /**
     * 返回 entity 对象中的所有方法，包含父类
     * @param entity
     * @return
     */
    public static List<Method> getObjectMethods(Object entity){
        Class<?> clazz = entity.getClass();
        List<Method> methods = new ArrayList<>();
        while (clazz != null && clazz != Object.class) {//当父类为null的时候说明到达了最上层的父类(Object类).
            methods.addAll(Arrays.asList(clazz .getDeclaredMethods()));
            clazz = clazz.getSuperclass(); //得到父类,然后赋给自己
        }
        return methods;
    }

    public static List<Method> getObjectMethods(Class<?> clazz){
        List<Method> methods = new ArrayList<>();
        while (clazz != null && clazz != Object.class) {//当父类为null的时候说明到达了最上层的父类(Object类).
            methods.addAll(Arrays.asList(clazz .getDeclaredMethods()));
            clazz = clazz.getSuperclass(); //得到父类,然后赋给自己
        }
        return methods;
    }

    /**
     * 判断 Class entity 是否存在名称为 methodName 的方法
     * @param methodName
     * @param entity
     * @return
     */
    public static Boolean isMethod(String methodName , Object entity){
        List<Method> methods = getObjectMethods(entity);
        for (Method m1:methods
        ) {
            if (methodName.equals(m1.getName()))
                return true;
        }
        return false;
    }

    /**
     * 循环向上转型, 获取对象的 DeclaredMethod
     * @param obj
     * @param methodName
     * @param parameterTypes  方法参数类型
     * @return
     */
    public static Method getDeclaredMethod(Object obj , String methodName , Class<?>...parameterTypes) {
        for (Class<?> clazz = obj.getClass(); clazz != Object.class && clazz != null; clazz = clazz.getSuperclass()) {
            try {
                return clazz.getDeclaredMethod(methodName, parameterTypes);
            } catch (Exception e) {
                // 这里甚么都不要做！并且这里的异常必须这样写，不能抛出去。
                // 如果这里的异常打印或者往外抛，则就不会执行clazz=clazz.getSuperclass(),最后就不会进入到父类中了
            }
        }
        return null;
    }

    public static Object invoke(Object object, String methodName, Class<?>[] parameterTypes,
                                Object[] parameters){
        Method method = getDeclaredMethod(object, methodName, parameterTypes);
        try {
            if (method != null){
                method.setAccessible(true);
                // 调用object 的 method 所代表的方法，其方法的参数是 parameters
                return method.invoke(object, parameters);
            }
        }catch (Exception e1){
            e1.printStackTrace();
        }
        return null;
    }

    /**
     * 循环向上转型, 获取对象的 DeclaredField
     *
     * @param object
     *            : 子类对象
     * @param fieldName
     *            : 父类中的属性名
     * @return 父类中的属性对象
     */

    public static Field getDeclaredField(Object object, String fieldName) {
        Field field = null;

        Class<?> clazz = object.getClass();

        for (; clazz != Object.class && clazz != null; clazz = clazz.getSuperclass()) {
            try {
                field = clazz.getDeclaredField(fieldName);
                return field;
            } catch (Exception e) {
                // 这里甚么都不要做！并且这里的异常必须这样写，不能抛出去。
                // 如果这里的异常打印或者往外抛，则就不会执行clazz = clazz.getSuperclass(),最后就不会进入到父类中了

            }
        }

        return null;
    }

    /**
     * 直接设置对象属性值, 忽略 private/protected 修饰符, 也不经过 setter
     *
     * @param object
     *            : 子类对象
     * @param fieldName
     *            : 父类中的属性名
     * @param value
     *            : 将要设置的值
     */

    public static void setFieldValue(Object object, String fieldName, Object value) {

        // 根据 对象和属性名通过反射 调用上面的方法获取 Field对象
        Field field = getDeclaredField(object, fieldName);

        if (field != null){
            // 抑制Java对其的检查
            field.setAccessible(true);
            try {
                // 将 object 中 field 所代表的值 设置为 value
                field.set(object, value);
            } catch (IllegalArgumentException e) {
                e.printStackTrace();
            } catch (IllegalAccessException e) {
                e.printStackTrace();
            }
        }
    }

    /**
     * 直接读取对象的属性值, 忽略 private/protected 修饰符, 也不经过 getter
     *
     * @param object
     *            : 子类对象
     * @param fieldName
     *            : 父类中的属性名
     * @return : 父类中的属性值
     */

    public static Object getFieldValue(Object object, String fieldName) {
        // 根据 对象和属性名通过反射 调用上面的方法获取 Field对象
        Field field = getDeclaredField(object, fieldName);

        if (field != null){
            // 抑制Java对其的检查
            field.setAccessible(true);
            try {
                // 获取 object 中 field 所代表的属性值
                return field.get(object);

            } catch (Exception e) {
                e.printStackTrace();
            }
        }
        return null;
    }


}

4.判空工具类：

package org.bluedream.comm.utils;

import java.util.Collection;
import java.util.Map;

public class EmptyUtil {
    //Suppress default constructor for noninstantiability
    private EmptyUtil(){
        throw new AssertionError();
    }

    public static boolean isEmpty(Object object){
        if (object == null){
            return true;
        }
        if (object instanceof int[]){
            return ((int[]) object).length == 0;
        }
        if (object instanceof double[]){
            return ((double[]) object).length == 0;
        }
        if (object instanceof long[]){
            return ((long[]) object).length == 0;
        }
        if (object instanceof byte[]){
            return ((byte[]) object).length == 0;
        }
        if (object instanceof short[]){
            return ((short[]) object).length == 0;
        }
        if (object instanceof float[]){
            return ((float[]) object).length == 0;
        }
        if (object instanceof char[]){
            return ((char[]) object).length == 0;
        }
        if (object instanceof Object[]){
            return ((Object[]) object).length == 0;
        }
        if (object instanceof CharSequence) {
            return ((CharSequence) object).length() == 0;
        }
        if (object instanceof Collection ){
            return ((Collection) object).isEmpty();
        }
        if (object instanceof Map){
            return ((Map) object).isEmpty();
        }
        return false;
    }

    public static boolean isNoEmpty(Object object){
        return !isEmpty(object);
    }

}