oracle初识9.23 DQL DML DCL

Lesson 9

   

    Subquery
Syntax:

      
单行子查询（返回值一个）

      
select last_name,salary

      
from employees

      
where salary >

                    
(select salary

                    
from employees

                    
where last_name = 'Abel')

      
注：必须使用单行运算符(> <
等)子查询的位置(放在运算符右边)、写法(放在小括号内)、返回值必须是单行；另外注意组函数在子查询中的应用，语法与在主查询中的一样；

    

      
多行子查询

          
ALL/ANY 知道 ALL 和 ANY 在比较关系中的范围；

          

      
空值子查询

Lesson 10

      

      
Set 运算符

        
UNION 相当于A B的并集；(A B重复的部分输出一次)(默认升序排序)

        
例: select employee_id,job_id

            
from employees

            
union

            
select employee_id,job_id

            
from job_history

            
注:select
后跟的列数、顺序和返回值类型必须相同;当columns在表中有不存在时，可以用TO_CHAR或其它函数(与相对应的columns的返回值)来匹配；

               
例：SELECT location_id,department_name "Department",TO_CHAR(NULL)
"Warehouselocation"  FROM
departments

                
UNION

                
SELECT location_id,TO_CHAR(NULL) "Department",state province

                
FROM locations;

        
UNION ALL (A B重复的部分输出2次)

        
例: select employee_id,job_id

            
from employees

            
union all

            
select employee_id,job_id

            
from job_history

        
INTERSET (交集)

        
例: select employee_id,job_id

            
from employees