SQL左连接数据变少

 

      前一段时间，同学写sql碰到一个问题：两段sql连接看似一样，但是条数却不一样。原因是在左连接之后的where条件对数据进行了筛选，使得数据变少了。两段sql如下：

select count(1)
  from (select *
          from KXAPP.I_CASH_LOAN_WHITELIST_C2_V_NEW b
         where b.BG_DT < '20180820'
           and b.EN_DT >= '20180904') gh
  left join (select *
               from KXAPP.I_CASH_LOAN_WISH_SCORE
              where busi_dt = '20180820') hg
    on gh.loan_no = hg.loan_no

select count(1)
  from KXAPP.I_CASH_LOAN_WHITELIST_C2_V_NEW b
  left join KXAPP.I_CASH_LOAN_WISH_SCORE c
    on b.loan_no = c.loan_no
 where b.BG_DT < '20180820'
   and b.EN_DT >= '20180904'
   and busi_dt = '20180820') hg

     乍一看，两段sql好像没有什么区别。第一个是先通过条件取子表，再链接；第二个是先进行链接，在通过条件进行筛选。统计后，发现上边的数据量要比下面的多。

    其实这两个链接是有区别的。第一种链接方式，主表是gh；第二种链接方式，主表是b。第二种链接方式数据量少于第一种链接方式的原因是：链接后添加了where对链接结果进行了筛选。

    下面通过一个例子说明：

表A结构如下：

 

表B结构如下： 

select *
  from (select 1 as a, 2 as b
          from dual
        union all
        select 2 as a, 2 as b from dual) a
  left join (select 3 as a, 2 as b
               from dual
             union all
             select 2 as a, 2 as b from dual) b
    on a.a = b.a
   and b.a = 1;

select *
  from (select 1 as a, 2 as b
          from dual
        union all
        select 2 as a, 2 as b from dual) a
  left join (select 3 as a, 2 as b
               from dual
             union all
             select 2 as a, 2 as b from dual) b
    on a.a = b.a
 where b.a = 1;