mybatis oracle MERGE 利用oracle原理实现saveOrUpdate

   MERGE INTO tableName  a
            USING (select count(1) c from tableName where user_code=#{item.user_code} and meter_code = #{item.meter_code}) b
            ON (b.c >0)
            WHEN MATCHED THEN
            UPDATE
            <trim  prefix="SET" suffixOverrides=",">
                <if test="item.bar_code != null">bar_code=#{item.bar_code},</if>
            </trim>
            WHERE user_code=#{item.user_code} and meter_code = #{item.meter_code}
            WHEN NOT MATCHED THEN
            insert
            <trim prefix="(" suffix=")" suffixOverrides=",">
 
                <if test="item.bar_code != null">bar_code,</if>
         
            </trim>
            <trim prefix="values (" suffix=")" suffixOverrides=",">
               
                <if test="item.bar_code != null">#{item.bar_code},</if>
               
            </trim>
        </foreach>