SQL query based on a column in parent table - Parent child relationship - sql

Question

I have the following three TABLES(ACCOUNTS,CUSTOMER,EMPLOYEE) and I would like to join them based on the columns AGENT_CODE & AGENT_TYPE and achieve the below.
What should be the best way to join these tables when AGENT_CODE can be same in CUSTOMER & EMPLOYEE table?
I have this query which is giving me wrong results
SELECT ac.AGENT_CODE,
ac.WORKING_AREA,
ac.AGENT_TYPE,
CONCAT(c.FIRST_NAME,c.LASTNAME_NAME),
e.EMP_NAME
FROM ACCOUNTS ac,
CUSTOMER c,
EMPLOYEE e
WHERE ac.AGENT_CODE = e.AGENT_CODE
OR ac.AGENT_CODE = c.AGENT_CODE
GETTING_WRONG_RESULTS_WITH_THE_ABOVE_QUERY
+------------+--------------------+------------+--------------+--------------+
| AGENT_CODE | WORKING_AREA | AGENT_TYPE | CUSTOMER_NAME| EMP_NAME |
+------------+--------------------+------------+--------------+--------------+
| A007 | Bangalore | CUSTOMER |Walter Holmes |Walter Holmes |
| A007 | London | EMPLOYEE |Walter Holmes |Peter Sam |
| A008 | New York | CUSTOMER |Micheal Junior|Micheal Junior|
| A007 | Bangalore | EMPLOYEE |Walter Holmes |John Tyler |
| A010 | Chennai | CUSTOMER |Micheal |Micheal |
| A007 | San Jose | EMPLOYEE |Walter Holmes |Albert |
+------------+--------------------+------------+--------------+--------------+
Expecting Result
+------------+--------------------+------------+--------------+
| AGENT_CODE | WORKING_AREA | AGENT_TYPE | AGENT_NAME |
+------------+--------------------+------------+--------------+
| A007 | Bangalore | CUSTOMER |Walter Holmes |
| A003 | London | EMPLOYEE |Peter Sam |
| A008 | New York | CUSTOMER |Micheal Junior|
| A011 | Bangalore | EMPLOYEE |John Tyler |
| A010 | Chennai | CUSTOMER |Micheal |
| A012 | San Jose | EMPLOYEE |Albert |
+------------+--------------------+------------+--------------+
ACCOUNTS(AGENT_CODE -PrimaryKey)
+------------+--------------------+------------+
| AGENT_CODE | WORKING_AREA | AGENT_TYPE |
+------------+--------------------+------------+
| A007 | Bangalore | CUSTOMER |
| A003 | London | EMPLOYEE |
| A008 | New York | CUSTOMER |
| A011 | Bangalore | EMPLOYEE |
| A010 | Chennai | CUSTOMER |
| A012 | San Jose | EMPLOYEE |
| A005 | Brisban | EMPLOYEE |
+------------+--------------------+------------+
CUSTOMER(AGENT_CODE -ForeignKey)
+-----------+-------------+-------------+------------+
|CUST_CODE | FIRST_NAME | LAST_NAME | AGENT_CODE |
+-----------+-------------+-------------+------------+
| C00013 | Walter | Holmes | A007 |
| C00001 | Micheal | Junior | A008 |
| C00020 | Albert | Skyler | A010 |
+-----------+-------------+-------------+------------+
EMPLOYEES(AGENT_CODE -ForeignKey)
EMP_NAME EMP_CODE AGENT_CODE
---------- --------------- ----------
Peter Sam C00054 A003
John Tyler C00023 A011
White Bolt C00043 A012

Answer 1

If you want to combine the result, you may want to UNION your result.
SELECT a.AGENT_CODE, a.WORKING_AREA, a.AGENT_TYPE, c.FIRST_NAME || ' ' || c.LAST_NAME AS AGENT_NAME
FROM ACCOUNTS a
JOIN CUSTOMER c ON c.AGENT_CODE = a.AGENT_CODE
UNION
SELECT a.AGENT_CODE, a.WORKING_AREA, a.AGENT_TYPE, e.EMP_NAME
FROM ACCOUNTS a
JOIN CUSTOMER e ON e.AGENT_CODE = a.AGENT_CODE