某天，产品经理给了这么一个需求技术小哥，能不能帮用户添加一个搜索栏，查询包含某个关键字的所有类目。技术小哥稍微想了一下，目前跟类目相关的表有两个，一个是content_category类目表，一个是content_system内容系统表。而用户要查找的关键字是存在content_system表里面，这样一来需要连表查询一下。难度好像不大，也就爽快地答应了。

技术小哥再仔细分析了一下两个表的结构：

CREATE TABLE `content_category` (
  `category_id` int(10) unsigned NOT NULL AUTO_INCREMENT COMMENT '类目编号Id',
  `pid` int(10) unsigned DEFAULT NULL COMMENT '上级编号id',
  `level` tinyint(4) NOT NULL COMMENT '层级',
  `name` varchar(20) NOT NULL COMMENT '名称',
  `description` varchar(200) DEFAULT NULL COMMENT '描述',
  `icon` varchar(50) DEFAULT NULL COMMENT '图标',
  `type` tinyint(3) NOT NULL DEFAULT '1' COMMENT '类型(1:普通,2:热门...)',
  `alias` varchar(20) DEFAULT NULL COMMENT '别名',
  `system_id` int(11) DEFAULT NULL COMMENT '系统编号id',
  `ctime` bigint(20) unsigned NOT NULL COMMENT '创建时间',
  `orders` bigint(255) unsigned NOT NULL COMMENT '排序',
  `attention` bigint(20) unsigned NOT NULL COMMENT '关注度',
  PRIMARY KEY (`category_id`),
  KEY `content_category_orders` (`orders`),
  KEY `content_category_pid` (`pid`),
  KEY `content_category_alias` (`alias`),
  KEY `content_category_level` (`level`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8mb4 COMMENT='内容类目表';

CREATE TABLE `content_system` (
  `system_id` int(10) unsigned NOT NULL AUTO_INCREMENT COMMENT '系统编号id',
  `name` varchar(20) NOT NULL COMMENT '系统名称',
  `code` varchar(20) DEFAULT NULL COMMENT '别名',
  `description` varchar(300) DEFAULT NULL COMMENT '描述',
  `ctime` bigint(20) DEFAULT NULL COMMENT '创建时间',
  `orders` bigint(20) DEFAULT NULL COMMENT '排序',
  PRIMARY KEY (`system_id`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8mb4 COMMENT='内容系统表';

不难看出，两个表都有system_id作为关联，当用户输入一个关键字，例如 code = "news" 时候，系统需要自动搜索出 system_id = 1 的所有类目信息。

于是技术小哥开始了他的工作，首先是定义Mapper.xml

<?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.thomson.content.rpc.mapper.ContentCategoryExtMapper">
    <!-- 定义基础类型 -->
    <resultMap id="BaseResultMap" type="com.thomson.content.dao.model.ContentCategory">
        <!-- 实体类的字段名和数据表的字段名映射 -->
        <id column="category_id" jdbcType="INTEGER" property="categoryId" />
        <result column="pid" jdbcType="INTEGER" property="pid" />
        <result column="level" jdbcType="TINYINT" property="level" />
        <result column="name" jdbcType="VARCHAR" property="name" />
        <result column="description" jdbcType="VARCHAR" property="description" />
        <result column="icon" jdbcType="VARCHAR" property="icon" />
        <result column="type" jdbcType="TINYINT" property="type" />
        <result column="alias" jdbcType="VARCHAR" property="alias" />
        <result column="system_id" jdbcType="INTEGER" property="systemId" />
        <result column="ctime" jdbcType="BIGINT" property="ctime" />
        <result column="orders" jdbcType="BIGINT" property="orders" />
        <result column="attention" jdbcType="BIGINT" property="attention" />
    </resultMap>
    <!--继承基础类型BaseResultMap， association 一对一关联查询 -->
    <resultMap extends="BaseResultMap" id="ClassesResultMap" type="com.thomson.content.dao.model.ContentCategory">
    </resultMap>
　　
    <!-- 这一步是关键的连表查询 -->
    <select id="selectContentCategoryByCode" parameterType="map" resultMap="ClassesResultMap">
        select content_c.* from content_category content_c left join content_system content_s on content_s.code=content_s.code=#{code,jdbcType=VARCHAR}
         where content_s.system_id=content_c.system_id
    </select>

    <!-- 缓存 -->
    <cache type="org.mybatis.caches.ehcache.LoggingEhcache" />

</mapper>

然后是Mapper接口

package com.thomson.content.rpc.mapper;

import com.thomson.content.dao.model.ContentCategory;
import org.apache.ibatis.annotations.Param;

import java.util.List;

/**
 * 类目ExtMapper
 * Created by Thomson on 2022/01/10.
 * 根据content_system 的 code 获取类目
 */
public interface ContentCategoryExtMapper {

    int up(String code);

    int down(String code);

  
    List<ContentCategory> selectContentCategoryByCode(@Param("code") String code);

}

接下来是实现类

import com.thomson.Content.dao.model.ContentArticle;
import com.thomson.Content.rpc.mapper.ContentCategoryExtMapper;
import com.thomson.common.annotation.BaseService;
import com.thomson.common.base.BaseServiceImpl;
import com.thomson.Content.dao.mapper.ContentCategoryMapper;
import com.thomson.Content.dao.model.ContentCategory;
import com.thomson.Content.dao.model.ContentCategoryExample;
import com.thomson.Content.rpc.api.ContentCategoryService;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
import org.springframework.transaction.annotation.Transactional;

import java.util.List;

/**
* ContentCategoryService实现
* Created by Thomson on 2021/01/10.
*/
@Service
@Transactional
@BaseService
public class ContentCategoryServiceImpl extends BaseServiceImpl<ContentCategoryMapper, ContentCategory, ContentCategoryExample> implements ContentCategoryService  {

    private static final Logger LOGGER = LoggerFactory.getLogger(ContentCategoryServiceImpl.class);

    @Autowired
    ContentCategoryExtMapper ContentCategoryExtMapper;

    // @Override
    public List<ContentCategory> selectContentCategoryByCode(String code) {
        return ContentCategoryExtMapper.selectContentCategoryByCode(code);
    }

}

在controll下获取数据

    @ApiOperation(value = "类目列表")
    @RequiresPermissions("content:category:read")
    @RequestMapping(value = "/list", method = RequestMethod.GET)
    @ResponseBody
    public Object list(
            @RequestParam(required = false, defaultValue = "0", value = "offset") int offset,
            @RequestParam(required = false, defaultValue = "10", value = "limit") int limit,
            @RequestParam(required = false, value = "sort") String sort,
            @RequestParam(required = false, value = "order") String order) {

至此，技术小哥获取到了自己想要的数据。顺利完成了产品经理给的任务。