exists在sql中的妙用

最近有两处功能都用到了exists， 觉得它很好地用子查询的方式实现了对主表数据的过滤

1. 支持历史记录id查询主id

那就用子查询过滤一下，主记录对应的历史记录中要存在当前id

        <if test="!searchHistoryFlag">
            and cbi.is_last_submit = 1
            <if test="searchSubmitId != null and searchSubmitId != ''">
                and exists (
                    select
                        1
                    from
                        bendcompete_info t
                    where
                        t.vendor_id = cbi.vendor_id
                        and t.submit_user_mobile = cbi.submit_user_mobile
                        and t.submit_year = cbi.submit_year
                        and t.submit_month = cbi.submit_month
                        and t.submit_id = #{searchSubmitId}
                )
            </if>
        </if>
        <if test="searchHistoryFlag">
            <if test="searchSubmitId != null and searchSubmitId != ''">
                and cbi.submit_id = #{searchSubmitId}
            </if>
        </if>

2. 相同会员品牌下取最新月的数据

由于年和月是分开存储的，为了比较月份谁大谁小，那就把年乘以12跟月份相加，统一按月份进行比较
月份可以理解为12进制的数据

        select rc.* 
        from  bendcompete_prize_receive_config rc
        where rc.prize_status = 1
           and not exists (
                select
                    1
                from
                    bendcompete_prize_receive_config rc2
                where
                    rc2.member_brand_id = rc.member_brand_id
                    and rc2.submit_year * 12 + rc2.submit_month > (rc.submit_year * 12 + rc.submit_month)
            )