mysql批量插入防重复

防重复插入大致有以下几种：

insert ignore into: 结合主键或者唯一索引

on duplicate key update

insert into … select … where not exists

replace into

注意：这里主要对<foreach> 中 separator用union all 

<insert id="insertBatch">
    insert into t_activity_obj (activity_id, target_id, target_type, sort)
    <foreach collection="targets" item="item" separator="union all">
    ( select #{activityId}, #{item.targetId}, #{item.targetType}, ifnull(#{item.sort}, 999) 
      from dual
      where not exists (
          select activity_obj_id from t_activity_obj
          where activity_id=#{activityId} and target_id=#{item.targetId} 
          and target_type=#{item.targetType})
    )
    </foreach>
  </insert>