sql - Oracle 语句计数不正确

Question

我有一个 SQL 语句，其中有一个对我来说无法解释的奇怪行为。 也许你可以找到问题所在:

当我使用该语句时

select count(*) from department

我得到了 2755 个结果

使用以下语句

select 
      building1.street, building1.streetno, building1.plz, building1.city, dept1.buildingid
  from 
      department dept1
    left join 
      supporter sup 
    on 
      dept.supporterid = sup.id
    left join 
      building building1
    on 
      sup.buildingid = building1.ibuildingid
  where 
      dept.usepostaladresssupporter = 1 
  union all
  select 
      building2.street, building2.streetno, building2.plz, building2.city, dept2.buildingid
  from 
      building building2
    right join 
      tueks_department dept2 
    on 
      dept2.buildingid = building2.ibuildingid
  where 
      dept2.usepostaladresssupporter = 0

我也得到了 2755 个结果。 但是当我想用左连接组合这两个语句时:

select count(*) from department
  left join
    (
      select 
          building1.street, building1.streetno, building1.plz, building1.city, dept1.buildingid
      from 
          department dept1
        left join 
          supporter sup 
        on 
          dept.supporterid = sup.id
        left join 
          building building1
        on 
          sup.buildingid = building1.ibuildingid
      where 
          dept.usepostaladresssupporter = 1 
      union all
      select 
          building2.street, building2.streetno, building2.plz, building2.city, dept2.buildingid
      from 
          building building2
        right join 
          tueks_department dept2 
        on 
          dept2.buildingid = building2.ibuildingid
      where 
          dept2.usepostaladresssupporter = 0
    ) postadress
  on
     department.buildingid = postadress.buildingid;

我得到了 3648513 个结果。

我的预期是，我只得到 2755 个结果。 哪里错了？

感谢您的帮助!

Answer 1

我假设 buildingid 不是唯一的(根据我的推理，它不可能是唯一的)

想象一下下面的简单表格

表A

create TableA (name VARCHAR(32));
insert into TableA values ('Lieven');
insert into TableA values ('Lieven');      

表B

create TableB (name VARCHAR(32));
insert into TableB values ('Lieven');
insert into TableB values ('Lieven');
insert into TableB values ('AnyOtherValue');

选择语句

select * from TableA a left outer join TableB b on a.name = b.name

由于TableA 的每条记录与TableB 的每条记录相匹配，其中name 相等，这将导致 4 条记录(AnyOtherValue 由于不匹配而被忽略)

• TableA 的第一条记录与“TableB”的三个记录中的两个一起返回
• TableA 的第二条记录与“TableB”的三个记录中的两个一起返回