FROM Schlüsselwort nicht gefunden, wo erwartet ERROR

Question

Ich versuche eine Oracle-Abfrage für eine Hausaufgabe zu schreiben. Die Aufgabe besteht darin, eine Abfrage zu schreiben, um die employee_id, job_id, hire_date und department_id aller Mitarbeiter zurückzugeben. Eine zweite Abfrage listet employee_id, job_id, start_date und department_id aus der Tabelle job_hist auf und kombiniert die Ergebnisse als eine einzige Ausgabe. Stellen Sie sicher, dass Duplikate in der Ausgabe unterdrückt werden.

-Code Ich habe:

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; 

Fehler Ich erhalte:

ORA-00923: FROM keyword not found where expected 

Von dem, was ich in dem Buch sagen kann, wobei die Reihenfolge richtig aussieht. Jede Hilfe wäre großartig!

Code nach dem Komma zu entfernen:

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; 

Ist das Arbeits?

Answer 2

das Komma entfernen, bevor Schlüsselwort FROM (UNION Syntax).

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, hire_date, department_id FROM Mitarbeiter UNION ALL SELECT employee_id, job_id, start_date, department_id VON job_history ORDER BY employee_id