MP的几种查询方式

对于Mybatis而言，我们可以使用XML或者使用注解来编写我们的SQL语句

<select id="listUserByCondition" resultType="com.yang.entity.User">
    select id, name, phone, email, profession, age, gender, status, create_time
    from tb_user
    <where>
        <if test="name != null and name != ''">
            and name like concat('%', #{name}, '#')
        </if>
        <if test="profession != null and profession != null">
            and profession like concat('%', #{profession}, '%')
        </if>
        <if test="createTimeMin != null and createTimeMin != ''">
            and create_time &gt;= #{createTimeMin}
        </if>
        <if test="createTimeMax != null and createTimeMax">
            and create_time &lt; #{createTimeMax}
        </if>
    </where>
    <if test="orderByColumn != null and orderByColumn != ''">
        order by ${orderByColumn} ${isAsc}
    </if>
    limit ${pageNum} ${pageSize}
</select>

由于业务逻辑可能会发生变化，若该表的结构在后续发生了变化，我们需要对原先编写的sql进行修改，但是由于sql是运行时才能发现，无法及时的发现所有需要修改的地方，可能会造成访问错误等问题。而且该sql的编写比较繁琐。

在MP中，我们可以使用wrapper来构造条件，用page自动进行分页操作，使用MP自带的sql语句，大大减少了我们的代码量，同时可以将经理集中在where条件中。

public PageResVO<User> list1(UserDTO userDTO) {
    LambdaQueryWrapper<User> wrapper = new LambdaQueryWrapper<>();
    String orderBy = StringUtils.isBlank(userDTO.getIsAsc()) ? "" : userDTO.getIsAsc();
    wrapper.like(StringUtils.isNotBlank(userDTO.getName()), User::getName, userDTO.getName())
            .like(StringUtils.isNotBlank(userDTO.getProfession()), User::getProfession, userDTO.getProfession())
            .ge(Objects.nonNull(userDTO.getCreateTimeMin()), User::getCreateTime, userDTO.getCreateTimeMin())
            .lt(Objects.nonNull(userDTO.getCreateTimeMax()), User::getCreateTime, userDTO.getCreateTimeMax())
            .last(StringUtils.isNotBlank(userDTO.getOrderByColumn()),
                    Constant.ORDER_BY_BLANK + StringUtils.camelToUnderline(userDTO.getOrderByColumn()) + " " + orderBy);

    IPage<User> page = this.baseMapper.selectPage(new Page<>(userDTO.getPageNum(), userDTO.getPageSize()), wrapper);
    return PageResVO.getBean(page, page.getRecords());
}

但是在业务中，我们一般的入参是一个DTO，而返回的是一个VO，对于这种情况，有三种不同的写法

XML

Page<UserVO> list(Page<User> page, @Param("dto") UserDTO dto);

<select id="list" resultType="com.yang.vo.UserVO">
    select id, name, phone, email, profession, age, gender, status, create_time
    from tb_user
    <where>
        <if test="dto.name != null and dto.name != ''">
            and name like concat('%', #{dto.name}, '#')
        </if>
        <if test="dto.profession != null and dto.profession != null">
            and profession like concat('%', #{dto.profession}, '%')
        </if>
        <if test="dto.createTimeMin != null and dto.createTimeMin != ''">
            and create_time &gt;= #{dto.createTimeMin}
        </if>
        <if test="dto.createTimeMax != null and dto.createTimeMax">
            and create_time &lt; #{dto.createTimeMax}
        </if>
    </where>
    <if test="dto.orderByColumn != null and dto.orderByColumn != ''">
        order by ${dto.orderByColumn} ${dto.isAsc}
    </if>
</select>

@Override
public PageResVO<UserVO> list1(UserDTO userDTO) {
    Page<UserVO> page = new Page<>(userDTO.getPageNum(), userDTO.getPageSize());
    Page<UserVO> list = this.baseMapper.list(page, userDTO);
    return PageResVO.getBean(list);
}

这种方式相对于Mybatis而言简化了分页操作，将分页交给了MP的page

XML+Wrapper

Page<UserVO> list1(Page<UserVO> page, @Param(Constants.WRAPPER) QueryWrapper wrapper);

<select id="list1" resultType="com.yang.vo.UserVO">
    select  name, profession, age, status, create_time
    from tb_user
    <where>
        <if test="ew.sqlSegment != null and ew.sqlSegment != ''">
            ${ew.sqlSegment}
        </if>
    </where>
</select>

@Override
public PageResVO<UserVO> list1(UserDTO userDTO) {
    Page<UserVO> page = new Page<>(userDTO.getPageNum(), userDTO.getPageSize());
    QueryWrapper<UserDTO> wrapper = new QueryWrapper<>();
    String orderBy = StringUtils.isBlank(userDTO.getIsAsc()) ? "" : userDTO.getIsAsc();
    wrapper.like(StringUtils.isNotBlank(userDTO.getName()), "name", userDTO.getName())
            .like(StringUtils.isNotBlank(userDTO.getProfession()), "profession", userDTO.getProfession())
            .ge(Objects.nonNull(userDTO.getCreateTimeMin()), "create_time", userDTO.getCreateTimeMin())
            .lt(Objects.nonNull(userDTO.getCreateTimeMax()), "create_time", userDTO.getCreateTimeMax())
            .last(StringUtils.isNotBlank(userDTO.getOrderByColumn()),
                    Constant.ORDER_BY_BLANK + StringUtils.camelToUnderline(userDTO.getOrderByColumn()) + " " + orderBy);
    Page<UserVO> list = this.baseMapper.list1(page, wrapper);
    return PageResVO.getBean(list);

该方法是xml负责需要查询的列，而wrapper负责查询条件的编写，不过在MP中我们可以使用wrapper.select() 列出需要的。

QueryWrapper<UserDTO> wrapper = new QueryWrapper<>();
wrapper.select("name", "profession", "create_time");

由于wrapper条件中的列都是写死的，而且都是魔法值，不满足代码规范和后续修改维护，我们可以自定义一个方法来根据方法引用获取方法名。

参考：通过方法引用获取方法名_夜光下丶的博客-CSDN博客

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wrapper

使用wrapper查询的结果为我们的entity实体类，所以我们还需要进行一次类型转换

public PageResVO<UserVO> list1(UserDTO userDTO) {
    LambdaQueryWrapper<User> wrapper = new LambdaQueryWrapper<>();
    String orderBy = StringUtils.isBlank(userDTO.getIsAsc()) ? "" : userDTO.getIsAsc();
    wrapper.like(StringUtils.isNotBlank(userDTO.getName()), User::getName, userDTO.getName())
            .like(StringUtils.isNotBlank(userDTO.getProfession()), User::getProfession, userDTO.getProfession())
            .ge(Objects.nonNull(userDTO.getCreateTimeMin()), User::getCreateTime, userDTO.getCreateTimeMin())
            .lt(Objects.nonNull(userDTO.getCreateTimeMax()), User::getCreateTime, userDTO.getCreateTimeMax())
            .last(StringUtils.isNotBlank(userDTO.getOrderByColumn()),
                    Constant.ORDER_BY_BLANK + com.baomidou.mybatisplus.core.toolkit.StringUtils.camelToUnderline(userDTO.getOrderByColumn()) + " " + orderBy);
    IPage<User> page = this.baseMapper.selectPage(new Page<>(userDTO.getPageNum(), userDTO.getPageSize()), wrapper);
    IPage<UserVO> userVOs = page.convert(u ->{
        UserVO vo = new UserVO();
        BeanUtils.copyProperties(u, vo);
        return vo;
    })
    return PageResVO.getBean(userVOs, userVOs.getRecords());
}

类型转换可以使用page自带的转换方法 convert 或者可以使用mapStruct

mapStruct的使用参考：MapStruct_夜光下丶的博客-CSDN博客