MS SQL If Selected is NULL Run another statement - sql-server

Question

I have two sql statements,
select * from UserTable where role='HOD'
select * from UserTable where role='Supervisor'
I want the results to be in a way such that if the first statement returns nothing, I want the second statement to run and if first statement returns something, second statement do not need to run. Is there a way to do it, be it in a stored procedure or a SQLQuery?

Answer 1

Executing two queries is more expensive than executing a single one that returns both result sets. It would be cheaper to filter the results on the client.
Even if you return all results in a single query, you can differentiate the two cases. For example, HOD always comes before Supervisor. You could use a ranking function like ROW_NUMBER() to assign a value to each row, depending on whether it matches HOD or Supervisor:
with a as (
select * ,row_number() over (partition by Role order by Role) as rn
from UserTable
where Role in ('HOD','Supervisor')
)
select *
from a
where rn=1
Another option is to combine a query that returns HOD with a query that returns Supervisor if HOD doesn't exist:
select *
from UserTable
where Role ='HOD'
UNION ALL
select *
from UserTable
where Role ='Supervisor'
AND NOT EXISTS (SELECT 1
from UserTable
where Role ='Supervisor')
The performance of both queries can be improved if Role is part of an index. The first query becomes equivalent to a simple index seek if the table has an index that covers all the returned fields. If the query returns only eg, ID, UserName, Role :
with a as (
select * ,row_number() over (partition by Role order by Role) as rn
from UserTable
where Role in ('HOD','Supervisor')
)
select ID,UserName, Role
from a
where rn=1
and the table has a covering index:
CREATE INDEX IX_UserTable_Roles ON UserTable (Role,ID,UserName)
The resulting execution plan is a single INDEX SEEK on the index

Answer 2

Try This
IF EXISTS (
SELECT 1 FROM UserTable WHERE ROLE = 'HOD'
)
BEGIN
SELECT * FROM UserTable WHERE ROLE = 'HOD'
END
ELSE BEGIN
SELECT * FROM UserTable WHERE ROLE = 'Supervisor'
END

Answer 3

This should do :
IF EXISTS(SELECT 1 FRom UserTable where role='HOD')
Begin
select * from UserTable where role='HOD'
END
ELSE BEGIN
select * from UserTable where role='Supervisor'
END

Answer 4

If you are using Transact SQL then a good page to read about this is https://learn.microsoft.com/en-us/sql/t-sql/language-elements/if-else-transact-sql
In the Transact SQL instance, the general version of what you are looking for is;
IF Boolean_expression
{ sql_statement | statement_block }
[ ELSE
{ sql_statement | statement_block } ]