Select data where condition matches and if none, then select all? - sql

Question

select * from (values
('dept1','user1'),
('dept2','user2'),
('dept3','user3'),
('dept4','user4')
)table1([department],[user])
where [user] = #id
scenario1:
#id = 'user1'
dept1
scenario2:
#id = 'user5'
dept1
dept2
dept3
dept4
this is what it looks like from a noobish query
declare #id varchar(12) = 'user1'
declare #var int = (select count(*) from table1 where [user] = #id)
select * from table1 where [user] = #id or #var = 0

Answer 1

DECLARE #id VARCHAR(5) = 'user1';
--DECLARE #id VARCHAR(5) = 'user5';
WITH UsersAndDepartments
AS ( SELECT *
FROM ( VALUES ( 'dept1', 'user1'), ( 'dept2', 'user2'),
( 'dept3', 'user3'), ( 'dept4', 'user4') ) x ( [department], [user] )
)
SELECT *
FROM UsersAndDepartments ud1
WHERE ud1.[user] =
CASE
WHEN EXISTS ( SELECT 1 FROM UsersAndDepartments ud2 WHERE ud2.[user] = #id ) THEN #id
ELSE ud1.[user]
END
The above just checks on user column if any row exists for an id, else matches on all.

Answer 2

declare #tab table (id int , value varchar(10))
declare #id int = 4
insert into #tab
select 1,'Ajay'
union all
select 2,'Ajay1'
union all
select 3,'Ajay2'
union all
select 4,'Ajay3'
union all
select 5,'Ajay4'
select * from #tab
where id = case when exists (select * from #tab where id = #id) then #id else id end

Answer 3

I would do this with a simple OR, not a CASE expression in the WHERE.
In general, you want to avoid CASE expressions in the WHERE clause for several reasons:
The logic can almost be written concisely using basic boolean operations.
Adding additional constructs (in addition to AND, OR, and NOT) just makes the logic harder for people to follow.
It pretty much kills any optimization paths.
I would suggest:
with table1 as
select v.*
from (values ('dept1', 'user1'),
('dept2', 'user2'),
('dept3', 'user3'),
('dept4', 'user4')
) v([department], [user])
)
select t1.*
from table1 t1
where t1.[user] = #id or
not exists (select 1 from table1 t1 where t1.user = #id);

Answer 4

You may have to do a check first something like this
Declare #RowCount int
Select #RowCount = (select count(*) from [Table] Where [Column] = 'xxx')
If #RowCount > 0
begin
Select 1 -- put code here if records
end
else
begin
Select 2 -- put code here if no records
end

Answer 5

you can try this:
DECLARE #id varchar(12) = 'user1'
IF EXISTS(SELECT COUNT(*) FROM table1 WHERE [user] = #id)
BEGIN
SELECT * FROM table1 WHERE [user] = #id
END
ELSE
BEGIN
SELECT * FROM table1
END
you can also read more about "EXISTS" syntax on:
https://learn.microsoft.com/en-us/sql/t-sql/language-elements/exists-transact-sql?view=sql-server-2017

Answer 6

You can have a slightly better execution plan if you separate the "if exist" logic from the actual query:
DECLARE #id varchar(10) = 'user5'
DECLARE #table TABLE ([department] varchar(10), [user] varchar(10))
insert into #table values
('dept1','user1'),
('dept2','user2'),
('dept3','user3'),
('dept4','user4')
DECLARE #exists BIT =
(SELECT 1 FROM #table WHERE [user] = #id)
SELECT department FROM #table
WHERE [user] = CASE #exists WHEN 1 THEN #id ELSE [user] END