SQL 减号运算符

Question

select deptno  
from emp2  
MINUS  
select deptno 
from dpt  
order by deptno;  

上面的查询返回 = No Data Found

鉴于，

select deptno,ename   
from emp2  
MINUS  
select deptno,dname 
from dpt   
order by deptno;  

返回所有 deptno 和 ename 字段值。

您能解释一下为什么尽管在第二个查询中使用了 MINUS 运算符，我还是得到了 deptno 字段的所有值吗？

legend: 
emp is employee table and dpt is department table,
ename is employee name -belonging to emp,
dname is department name -belonging to dpt,
deptno is department no. -common to both

Answer 1

在第一个查询中表之间没有不同的deptno，在第二个查询中你有相同的deptno，但名称不同

这样想:

查询 1:

select deptno  
from emp2  
MINUS  
select deptno 
from dpt  
order by deptno; 

示例:

[1,2,3,4] - [1,2,3,4] = empty

查询 2:

select deptno,ename   
from emp2  
MINUS  
select dept no,dname 
from dpt  
order by deptno; 

例子:

[(1, 'a'),(2, 'b'),(3, 'c'),(4, 'd')] - 
[(1, 'z'),(2, 'x'),(3, 'u'),(4, 'w')] = 
[(1, 'a'),(2, 'b'),(3, 'c'),(4, 'd')]

MINUS

MINUS operator, which returns only unique rows returned by the first query but not by the second