Question

I have a request, if operator types in nothing ,I will retrive all users. but if operator multiple intput some usernames, I only retirve the users whos’ names are inputed, I am tring to use subquery like below

I am using subquery(table b) as IN condition to get users’ information from table a, and it is working well.

SELECT * FROM a WHERE UserName IN (SELECT UserName FROM b)

But, If table b has nothing, I need to select all Users from a, I don’t know how to do it.
Before I can do this only for one user, if typeInUserName = NULL, WHERE UserName = UserName will retirve all users for me.

SELECT * FROM a WHERE UserName = NVL(typeInUserName,UserName)

Answer 1

create table a (username varchar2(20));

insert into a values('gaurav');
insert into a values('rahul');
insert into a values('rohan');
insert into a values('niharika');

create table b (username varchar2(20));

 insert into b values('gaurav');
 insert into b values('rahul');

case 1 :when there is not records in table B

Delete from  b;

WITH cnt_b as 
(
 select count(*) cnt from b
)
select a.username from a ,cnt_b where  1= CASE WHEN cnt_b.cnt=0 THEN 1 ELSE 0 END
UNION 
select a.username from a,cnt_b,b where a.username =b.username and cnt_b.cnt>0;

USERNAME            

---

gaurav              
niharika            
rahul               
rohan               

4 rows selected.

Case 2 When there are two matching records in table B

 insert into b values('gaurav');
 insert into b values('rahul');

WITH cnt_b as 
(
 select count(*) cnt from b
)
select a.username from a ,cnt_b where  1= CASE WHEN cnt_b.cnt=0 THEN 1 ELSE 0 END
UNION 
select a.username from a,cnt_b,b where a.username =b.username and cnt_b.cnt>0;


USERNAME            
--------------------
gaurav              
rahul               

2 rows selected.

Case3 When there are records in table B ,but no matching records.

delete from  b

insert into b values('rajat');
insert into b values('garima');

with cnt_b as 
(
 select count(*) cnt from b
)
select a.username from a ,cnt_b where  1= CASE WHEN cnt_b.cnt=0 THEN 1 ELSE 0 END
UNION 
select a.username from a,cnt_b,b where a.username =b.username and cnt_b.cnt>0

no rows selected.

Cheers GS