Mybatis一些使用小技巧(随时更新)

1.1对1实体类映射可以不使用association

如图所示 User实体类中关联着部门表

而在数据库中是一个字段 office_id
我们需要使用resultMap来进行映射

<resultMap id="userResult" type="User">
		<id property="id" column="id" />
		<result property="loginName" column="loginName" />
		<result property="password" column="password" />
		<result property="loginFlag" column="login_flag"/>
		<result property="office.id" column="office.id" /> //同理插入与更新都可以直接使用.属性名
		<result property="office.name" column="office.name" />
		<result property="office.parentIds" column="office.parentIds" />
	</resultMap>

查询语句举例 查询出一个用户的所有信息(包括office信息)

<select id="get" resultMap="userResult">
		SELECT
			a.id,a.login_name AS "loginName",
			a.office_id AS "office.id",    //起别名直接与ResultMap的属性名映射
			o.name AS "office.name",
			o.parent_id AS "office.parent.id" 
		FROM sys_user a
		LEFT JOIN sys_office o ON o.id = a.office_id   //连接查询
		WHERE a.id = #{id}
	</select>

2.使用标签判断多数据源执行不同的语句

<select id="findListByOffice" resultMap="userResult">
		SELECT
			<include refid="userColumns"/>
		FROM sys_user a
		<include refid="userJoins"/>

		WHERE a.del_flag = #{DEL_FLAG_NORMAL}

		<if test="office != null and office.id != null and office.id != ''">
			AND o.id = #{office.id}
		</if>
		<if test="office == null">
			AND (o.id = ''  or o.id is null)
		</if>

		<if test="loginName != null and loginName != ''">
			AND a.login_name like
			//多数据源判断
					<if test="_databaseId == 'postgre'">'%'||#{loginName}||'%'
					<if test="_databaseId == 'oracle'">'%'||#{loginName}||'%'
					<if test="_databaseId == 'mysql'">CONCAT('%', #{loginName}, '%')
					<if test="_databaseId == 'mssql'">'%'+#{loginName}+'%'
		</if>
		<!-- 排序 -->
		ORDER BY  a.name
	</select>

3.collection标签的嵌套查询来映射自定义List

 返回给前端必须为List对象的时候 
 可以自己创建一个不存在于数据库的实体类属性赋值后返回给前端
 插入、更新时 正常使用String类型的一串id或信息即可 
 该情景常见于1张表里有个字段存放多个id

使用collection标签的嵌套查询
此方法可能要实体类中加入注解 @JsonIgnoreProperties(value = { “handler”})

<resultMap id="MonitorModelResultMap" type="com.jeeplus.modules.system.entity.MonitorModel">
		<id property="id" column="id"/>
		<result property="monitorFactorids" column="monitor_factor_ids"/>
		<collection property="monitorFactors" select="findInFactor"  column="monitor_factor_ids" ofType="MonitorFactor"> // 映射到这个属性时候 先执行select里的sql 参数为column 解决集放入这个属性
		</collection>
	</resultMap>
	//嵌套查询如下 查询出因子信息 会在映射时自动执行这个sql 此需要写入dao接口中
	
	<select id="findInFactor" parameterType="string"  resultType="com.jeeplus.modules.system.entity.MonitorFactor">
		select f.id,f.name,f.fcode from monitor_factor f
		where find_in_set(f.id,#{ids})!=0
	</select>

4.使用和来减少sql语句的冗余

  数据字段查询量很大+多个sql反复使用的时候
 可以将select字段放入标签里

<sql id="userColumns">
    	a.id,
    	a.office_id AS "office.id",
    	a.login_name AS "loginName",
    	a.password,
		a.name,
		a.email,
		a.phone,
		a.mobile,
		a.login_ip,
		a.login_date,
		a.remarks,
		a.login_flag,
		a.photo,
		a.qrcode,
		a.sign,
		a.create_by AS "createBy.id",
		a.create_date,
		a.update_by AS "updateBy.id",
		a.update_date,
		a.del_flag,
    	o.name AS "office.name",
    	o.parent_id AS "office.parent.id",
    	o.parent_ids AS "office.parentIds",
    	oa.id AS "office.area.id",
    	oa.name AS "office.area.name",
    	oa.parent_id AS "office.area.parent.id",
    	oa.parent_ids AS "office.area.parentIds"
    </sql>
    <sql id="userJoins">
		LEFT JOIN sys_office o ON o.id = a.office_id
		LEFT JOIN sys_area oa ON oa.id = o.area_id
    </sql>

 使用 调用写好的

<select id="get" resultMap="userResult">
		SELECT
			<include refid="userColumns"/>,
		FROM sys_user a
		   <include refid="userJoins"/>
		WHERE a.id = #{id}
	</select>

5.在mybatis中 in与not in 条件必须为集合

    如题  记得传List然后遍历即可

<select id="siteForON" parameterType="map" resultType="com.jeeplus.modules.siteinfo.entity.SiteInfo">
		select s.id, o.name from site_info s
		left join sys_office o on o.id=s.office_id
		where o.type='6'  and
		s.id not in
		<foreach collection="strings" item="item" index="index" open="(" close=")" separator=",">
			#{item}
		</foreach>
	</select>