can we have CASE expression/case result as a join table name in oracle - sql

Question

I have 3 tables say Employee, Permanent_Emp and Contract_Emp
SELECT E.EMP_NO,
E.NAME,
JET.EMP_TYPE,
JET.DATE_JOINED
FROM Employee E
LEFT OUTER JOIN
/* Here Join Table Name(JET) it can be Permanent_Emp or Contract_Emp
which i want as a result of my case expression. */
ON (some condition here) ORDER BY E.EMP_NO DESC
case expression:
CASE
WHEN (E.EMP_TYPE_CODE >10 )
THEN
Permanent_Emp JET
ELSE
Contract_Emp JET
END
Note: table and column names are just for an example to understand requirement.
how can i have join table name from a case expression?

Answer 1

Something like this (although without a description of your tables, the exact join conditions or any sample data its hard to give a more precise answer):
SELECT E.EMP_NO,
E.NAME,
COALESCE( P.EMP_TYPE, C.EMP_TYPE ) AS EMP_TYPE
COALESCE( P.DATE_JOINED, C.DATE_JOINED ) AS DATE_JOINED
FROM Employee E
LEFT OUTER JOIN
Permanent_Emp P
ON ( E.EMP_TYPE_CODE > 10 AND E.EMP_NO = P.EMP_NO )
LEFT OUTER JOIN
Contract_Emp C
ON ( E.EMP_TYPE_CODE <= 10 AND E.EMP_NO = C.EMP_NO )
ORDER BY
E.EMP_NO DESC

Answer 2

use your case in select and join both tables
as
SELECT case when 1 then a.column
when 2 then b.column
end
from table c
join table a
on 1=1
join table2 b
on 1=1
but you cant use case while joining. its better to join both tables and in select use case statement with conditions as per your requirement

Answer 3

There is no way to conditionally add tables to a query in static SQL. If the relevant columns in Permanent_Emp and Contract_Emp are roughly equivalent, you could use a union in a sub-query.
SELECT *
FROM employee e
JOIN
(SELECT employee_id, relevant_column, 'P' AS source_indicator
FROM permanent_emp
UNION ALL
SELECT employee_id, relevant_column, 'C' AS source_indicator
FROM contract_emp) se
ON e.employee_id = se.employee_id
AND ( (e.emp_type_code > 10 AND source_indicator = 'P')
OR (e.emp_type_code <= 10 AND source_indicator = 'C'))

Answer 4

Using Alan's query as a starting point you can still use a case statement, just move it to the join condition:
SELECT *
FROM employee e
JOIN (
SELECT employee_id
, relevant_column
, 'P' AS source_indicator
FROM permanent_emp
UNION ALL
SELECT employee_id
, relevant_column
, 'C' AS source_indicator
FROM contract_emp
) se
ON se.employee_id = e.employee_id
and se.source_indicator = case when e.emp_type_code > 10
then 'P'
else 'C'
end
The only difference between this query and Allan's is the use of a case statement instead of an or statement.