基础有关问题：两表比较是否相等

--两个表A,B。使用下述语句判断，显示‘不等’select Matchtype= case when (SELECT CHECKSUM_AGG (BINARY_CHECKSUM (*)) FROM A)              =              (SELECT CHECKSUM_AGG (BINARY_CHECKSUM (*)) FROM  B) then 'Equal'        else 'Not Equal'        end--但是，使用下面语句时，显示记录条数相等。这是为什么呢？select count(1) from(select * from A  union  select * from B)finselect count(1) from A

declare @a table(id int,de nchar(500))declare @b table(id int,de varchar(5))insert into @a values(1,null),(2,'bbb'),(2,'bbb')insert into @b values(1,null),(11,null),(2,'bbb')select count(1) from @a--> 3select count(1) from @b--> 3select count(1) from(select * from @a union select * from @b) fin--> 3select Matchtype=       case when (select checksum_agg(binary_checksum(*)) from @a)                   =                 (select checksum_agg(binary_checksum(*)) from @b)             then 'Equal' else 'Not Equal'       end--> Not Equal