Condition in sql query - sql

Question

I want to insert in sql query something like that:
Select * from Users where id=[if #userId>3 then #userId else "donnt use this condition"] and Name=[switch #userId
case 1:"Alex"
case 2:"John"
default:"donnt use this condition"];
How can i do it?
yet another similar question
When showAll is false it works ok but when showAll is true it returns nothing. Why and how to make it working right? IsClosed column has a bit type.
Select * from orders where IsClosed=CASE WHEN #showAll='false' THEN 'false' ELSE NULL END;

Answer 1

This will perform horribly:
Select *
from Users
where (#userid > 3 AND id = #userId)
OR (#userId BETWEEN 1 AND 2 AND name = CASE
WHEN #userId = 1 THEN 'Alex'
ELSE 'John'
END)
The best performing option is dynamic SQL:
SQL Server 2005+
DECLARE #SQL NVARCHAR(4000)
SET #SQL = 'SELECT u.*
FROM USERS u
WHERE 1 = 1 '
SET#SQL = #SQL + CASE
WHEN #userId > 3 THEN ' AND u.id = #userId '
ELSE ''
END
SET#SQL = #SQL + CASE #userId
WHEN 1 THEN ' AND u.name = ''Alex'' '
WHEN 2 THEN ' AND u.name = ''John'' '
ELSE ''
END
BEGIN
EXEC sp_executesql #SQL, N'#userId INT', #userId
END
For more info on SQL Server's dynamic SQL support, read "The Curse and Blessings of Dynamic SQL"

Answer 2

Select *
from Users
where id = CASE WHEN #userId>3 THEN #userId ELSE NULL END
OR name = CASE WHEN #userId = 1 THEN 'Alex' WHEN #UserId = 2 THEN 'John' ELSE NULL END

Answer 3

Please try this:
select *
from Users
where id = (case when #userId > 3 then #userId
else id end)
and Name = (case cast(#userId as varchar)
when '1' then 'Alex'
when '2' then 'John'
else Name end)
Or I think this will perform better:
select aa.*
from (select *
, case when #userId > 3 then #userId
else id end as UserID
, case cast(#userId as varchar)
when '1' then 'Alex'
when '2' then 'John'
else Name end as UserName
from Users) aa
where aa.id = aa.UserID
and aa.Name = aa.UserName
You might want to define each field that you only need on your select, instead of using asterisk(*).