集操作符要拼凑字段列表：
SQL> create table retired_employees (
2 EMPLOYEE_IDNUMBER(6),
3 FIRST_NAME VARCHAR2(20),
4 LAST_NAMEVARCHAR2(25),
5 EMAILVARCHAR2(25),
6 retired_date date,
7 job_id VARCHAR2(10),
8SALARYNUMBER(8,2),
9MANAGER_ID NUMBER(6),
10DEPARTMENT_ID NUMBER(4) ) ;
Table created.
SQL> insert into retired_employeesvalues(
2304,
3'Alexandera',
4 'Runold',
5 'ARUNOLD',
6to_date('03-JAN-11','DD-MON-YY'),
7'IT_PROG',
86000,
9149,
1060) ;
1 row created.
selectjob_id, hire_date ,nullas "retired_date"
from employees
union
selectjob_id, null ,retired_date
from retired_employees;
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