FROMキーワードが見つかりませんでした。ERROR

Question

宿題のためのOracleクエリを作成しようとしています。割り当ては、すべての従業員のemployee_id、job_id、hire_dateおよびdepartment_id、およびjob_histテーブルからemployee_id、job_id、start_dateおよびdepartment_idをリストする2つ目のクエリを返す1つのクエリを作成し、その結果を1つの単一出力として結合することです。出力に重複が表示されないようにしてください。

コードは、私が持っている：

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; 

エラー私は取得しています：私は本の中で伝えることができるものから、

ORA-00923: FROM keyword not found where expected 

は、シーケンスが右に見えます。どんな助けも素晴らしいだろう！コンマを除去した後

コード：

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; 

ですか？

Answer 2

キーワード（UNION Syntax）FROMの前にカンマを削除します。この作業は

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 EMPLOYEE_ID、JOB_ID、入社年月日は、私はちょうどここにコメントします UNIONをALL SELECT EMPLOYEE_ID、JOB_IDは、START_DATEは、EMPLOYEE_ID