Oracle Conditional where clause - oracle

Question

is there any way to write query with following functionality, add where clause as a conditional way,
select e.emp_id, emp.admin_user from employees e
if emp.admin != 'Y'
then
query run with where clause
else
query run without where clause ?

Answer 1

Using a CASE expression in the WHERE clause should do the trick. When you say you don't need the where clause if condition is not met, then all you want is a condition like WHERE 1 = 1, i.e. when condition is not met then return all rows. So, you need to make the not met condition as always TRUE.
For example,
I have an employee table,
SQL> SELECT empno, ename, deptno
2 FROM emp;
EMPNO ENAME DEPTNO
---------- ---------- ----------
7369 SMITH 20
7499 ALLEN 30
7521 WARD 30
7566 JONES 20
7654 MARTIN 30
7698 BLAKE 30
7782 CLARK 10
7788 SCOTT 20
7839 KING 10
7844 TURNER 30
7876 ADAMS 20
7900 JAMES 30
7902 FORD 20
7934 MILLER 10
14 rows selected.
SQL>
I want to select the employee details, if department is 20 then use the where clause else return all the employee details, but filter the department which meets the where condition.
SQL> SELECT empno, ename, deptno
2 FROM emp
3 WHERE ename =
4 CASE
5 WHEN deptno = 20
6 THEN 'SCOTT'
7 ELSE ename
8 END
9 /
EMPNO ENAME DEPTNO
---------- ---------- ----------
7499 ALLEN 30
7521 WARD 30
7654 MARTIN 30
7698 BLAKE 30
7782 CLARK 10
7788 SCOTT 20
7839 KING 10
7844 TURNER 30
7900 JAMES 30
7934 MILLER 10
10 rows selected.
SQL>
So, for department 20, the filter is applied by where clause, and I get only the row for ename SCOTT, for others it returns all the rows.

Answer 2

To keep it simple I would go for union clause in this case, so you can have your where clause as complex as you need. I tried to guess your table structure from above comment, let's see this example:
SQL> create table employees (emp_id number, admin_user number, project_id number);
Table created.
SQL> create table project_accessible_to_user (emp_id number, project_id number);
Table created.
Now make simple union all of two queries one with where condition anoother without it
SQL> select * from employees e where e.admin_user!='Y' and project_id in
(select project_id from project_accessible_to_user where emp_id=e.emp_id)
union all
select * from employees e where (e.admin_user is null or
e.admin_user='Y');
UNION ALL is better from performance point of view as UNION because it means that it is not checking for intersect values so if there are any it will return duplicates. However in this case it is filtered already by condition on admin_user, so these duplicates will not occure.