FROM關鍵字找不到預期的地方ERROR

Question

我正在嘗試爲家庭作業寫一個Oracle查詢。該任務是編寫一個查詢以返回所有員工的employee_id，job_id，hire_date和department_id，第二個查詢列出job_hist表中的employee_id，job_id，start_date和department_id，並將結果組合爲單個輸出。確保你在輸出中抑制重複。

代碼，我：

SELECT employee_id AS "Employee ID", job_id AS "Job Id", TO_CHAR(NULL) hire_date "Hire  Date", department_id AS "Department Id", 
FROM employees 
UNION 
SELECT employee_id AS "Employee Id", job_id AS "Job Id", TO_CHAR(NULL) start_date "Start  Date",department_id AS "Department Id", 
FROM job_history; 

錯誤我得到：

ORA-00923: FROM keyword not found where expected 

從我可以在書中告訴，序列大致正確。任何幫助將是偉大的！除去逗號之後

代碼：

SELECT employee_id AS "Employee ID", job_id AS "Job Id", TO_CHAR(NULL) hire_date "Hire  Date", department_id AS "Department Id" 
FROM employees 
UNION 
SELECT employee_id AS "Employee Id", job_id AS "Job Id", TO_CHAR(NULL) start_date "Start  Date",department_id AS "Department Id" 
FROM job_history; 

Answer 1

SELECT employee_id AS "Employee Id", job_id AS "Job Id", TO_CHAR(hire_date,'yyyy-mm-dd') AS "Some date", department_id AS "Department Id" 
FROM employees 
UNION 
SELECT employee_id AS "Employee Id", job_id AS "Job Id", TO_CHAR(start_date,'yyyy-mm-dd') AS "Some date",department_id AS "Department Id" 
FROM job_history; 

這方面的工作？

Answer 2

FROM關鍵字（UNION Syntax）之前取出逗號。

SELECT employee_id AS "Employee ID", 
     job_id AS "Job Id", TO_CHAR(NULL) hire_date "Hire  Date", department_id AS "Department Id" 
FROM employees .... 

Answer 3

Select hire_date, employee_id, TO_DATE(null) start_date, 
     job_id, department_id 
from employees 
Union 
Select TO_DATE(null) hire_date, employee_id, start_date, 
     job_id, department_id 
from job_history; 

Answer 4

SELECT僱員，JOB_ID，聘用日期，部門標識FROM 員工 UNION ALL SELECT僱員，JOB_ID，起始日期，部門標識FROM JOB_HISTORY ORDER BY EMPLOYEE_ID