UPDATE tableName
    SET orderId = CASE id 
        WHEN 1 THEN 3 
        WHEN 2 THEN 4 
        WHEN 3 THEN 5 
    END
WHERE id IN (1,2,3)

这句sql 的意思是，更新orderId 字段，如果id=1 则orderId 的值为3，如果id=2 则orderId 的值为4……
where部分不影响代码的执行，但是会提高sql执行的效率。确保sql语句仅执行需要修改的行数，这里只有3条数据进行更新，而where子句确保只有3行数据执行。

如果更新多个值的话，只需要稍加修改：

多字段：
UPDATE categories 
    SET orderId = CASE id 
        WHEN 1 THEN 3 
        WHEN 2 THEN 4 
        WHEN 3 THEN 5 
    END, 
    title = CASE id 
        WHEN 1 THEN 'New Title 1'
        WHEN 2 THEN 'New Title 2'
        WHEN 3 THEN 'New Title 3'
    END
WHERE id IN (1,2,3)

单个字段修改：

<update id="BatchUpdate" parameterType="map">
		update activity_gift_status
		<set>
			<trim prefix="ref_activity_type=case" suffix="end,">
				<foreach collection="list" item="item" index="index">
					<if test="item.refActivityType !=null">
						when ref_activity_id=#{item.refActivityId} then #{item.refActivityType}
					</if>
				</foreach>
			</trim>
		</set>
		where
		activity_gift_status_id in
		<foreach collection="list" item="item" index="index" separator="," open="(" close=")">
			#{item.activityGiftStatusId}
		</foreach>
	</update>

多条件修改：

 <update id="updateByBatch" parameterType="list">
        update message_notice_task
        <trim prefix="set" suffixOverrides=",">
            <trim prefix=" call_back_status = case" suffix="end,">
                <foreach collection="list" item="item" index="index">
                    <if test="item.callBackStatus !=null">
                        when  task_id =  #{item.taskId} then   #{item.callBackStatus}
                    </if>
                </foreach>
            </trim>
            <trim prefix="reason = case" suffix="end,">
                <foreach collection="list" item="item" index="index">
                    <if test="item.reason !=null">
                        when  task_id =  #{item.taskId} then #{item.reason}
                    </if>
                </foreach>
            </trim>
            <trim prefix="reason_text = case" suffix="end,">
                <foreach collection="list" item="item" index="index">
                    <if test="item.reasonText !=null">
                        when  task_id =  #{item.taskId} then #{item.reasonText}
                    </if>
                </foreach>
            </trim>
        </trim>
        <where>
            AND task_id in
            <foreach collection="list" item="item" index="index" separator="," open="(" close=")">
                #{item.taskId}
            </foreach>
        </where>
    </update>

mybatis批量更新

1.常见的批量更新(生成多条执行语句)

<update id="updateBatch"  parameterType="java.util.List">  
    <foreach collection="list" item="item" index="index" open="" close="" separator=";">
        update course
        <set>
            name=${item.name}
        </set>
        where id = ${item.id}
    </foreach>      
</update>

这种写法需要给数据库添加配置allowMultiQueries=true

2.一条语句执行批量更新操作

<update id="updateBatch" parameterType="java.util.List">
    update mydata_table 
    set  status=
    <foreach collection="list" item="item" index="index" 
        separator=" " open="case ID" close="end">
        when #{item.id} then #{item.status}
    </foreach>
    where id in
    <foreach collection="list" index="index" item="item" 
        separator="," open="(" close=")">
        #{item.id,jdbcType=BIGINT}
    </foreach>
 </update>

生成的sql如下：

    update mydata_table 
    set status = 
    --此处应该是<foreach>展开值
    case id
        when id = #{item.id} then #{item.status}
        ...
    end
    where id in (...);

第一种方法是用传统方式生成的多条SQL语句，执行效率相对较低。第二种方法是巧妙使用case when的语法完成的一条语句实现的批量更新的操作，执行效率相对较快。使用起来也比较方便。

参考文献

1. https://blog.csdn.net/xyjawq1/article/details/74129316
2. https://blog.csdn.net/qq_40558766/article/details/89966773

Mybatis批量更新实现

<update id="updateDeviceStatusBatch">
        update s_camera_device
        <trim prefix="set" suffixOverrides=",">
            <trim prefix="status =case" suffix="end,">
                <foreach collection="devices" item="item" index="index">
                    <if test="item != null">
                        when device_no = #{item.deviceNo}
                        then #{item.status}
                    </if>
                </foreach>
            </trim>
        </trim>
        where device_no in
        <foreach collection="devices" index="index" item="item" separator="," open="(" close=")">
            #{item.deviceNo}
        </foreach>
    </update>

根据设备号更新设备当前状态

该语法使用 case when 函数来解决批量更新数据

执行SQL

update s_camera_device set status = case when device_no = 1001 then 1  
 when device_no = 1002 then 1  
 when device_no = 1003 then 0 end   
 where device_no in (1001,1002,1003)