MySql 以及Mybatis 使用问题汇总

在后台开发中必不可少地使用数据库，先将遇到过的场景罗列，便于日后查询使用：

删除记录

delete from pi_active_power_config where UU_LIMIT=200;

插入记录

insert into pi_active_power_config (uu_limit, lower_limit, point_id) VALUES (100, 1000, 1);

insert into pi_point (name, device_id) VALUES ("test3", 3);

创建外键

alter table PI_ACTIVE_POWER_CONFIG add constraint FK_ACTIVE_POWER_CONFIG foreign key (POINT_ID)

references PI_POINT (ID) on delete restrict on update restrict;

alter table PI_ACTIVE_POWER_CONFIG add constraint FK_ACTIVE_POWER_CONFIG foreign key (POINT_ID)

references PI_POINT (ID) on delete cascade on update cascade;

删除外键

alter table pi_active_power_config drop FOREIGN key FK_ACTIVE_POWER_CONFIG;

新增一列

ALTER table pi_analog_param_config add COLUMN MESSAGE_SEND tinyint default 0 comment '短信订阅，0为非，1为是' after account;

-----------------------------------------查询某个字段是否包含在数据库中-----------------------------------------------

https://blog.csdn.net/TimOut/article/details/74179348

SELECT * FROM information_schema.columns WHERE column_name='column_name';

查询某个表在哪个数据库中：

select * from information_schema.TABLES where TABLE_NAME="safe_start_day"

通常java引用代码需要结合MyBatis 使用来操作数据库，关于多参数的传递需要用到列表：

MySql 以及Mybatis 使用问题汇总

如果只有一个array 或者 list 参数传入，则collection中会默认取list 或者array, 如果有多个则需要用参数注解@Param(“xxx”) 声明参数名称。array 的话取长度为length, list 的话取长度为size(), 需要注意区分。

<select id="selectNewestImageByDevId" resultType="com.imagewall.entity.ImageInfo">
   select a.* from image_info as a,
   (select stcd, max(b.time) as time from image_info as b group by b.stcd) as b
   where a.stcd=b.stcd and a.time = b.time
   <if test="array != null and array.length>0">
   and b.stcd in
<foreach collection="array" index="index" item="item" open="(" separator="," close=")">
#{item}
</foreach>
</if>
</select>

还有一个问题是需要注意使用分组group by 之后，并不一定会得到max(b.time) 这条记录的数据，所以为了获取最新时间的数据需要加上条件来判断时间相等。

传递参数# 和 $ 符号的区别就是，#号会对参数添加单引号，而$则直接拼装SQL：

https://www.cnblogs.com/keyi/p/8509155.html

       SELECT stationid, pno, stcd, stnm, lgtd, ltid, address, createtime, remark, stationName from
       (
       SELECT * FROM `image_point` a LEFT JOIN
       (SELECT stationid as sysStationId, name as stationName from sys_station) b
       on a.stationid=b.sysStationId
       )
       as c

       <where>
           1=1
           <if test="stationIds != null and stationIds.size()>0">
               and c.stationId in
               <foreach collection="stationIds" index="index" item="item" open="(" separator="," close=")">
                   #{item}
               </foreach>
           </if>
           <if test="stcd != null and stcd != ''">
               and c.stcd = #{stcd}
           </if>
           <if test="stnm != null and stnm != ''">
               and c.stnm like "%"#{stnm}"%"
           </if>
       </where>

       <if test="orderByField != null and orderByField != ''">
           ORDER BY ${orderByField}
       </if>

</select>