在case的then子句中使用select语句

Question

I'm trying to include select statement in the then of case statement but the output is not as expected. I know there is different method to do this but can it be done the way i'm trying to do.

Using the following example data:

create table example(name varchar(10));

insert into example values
('abc'),('bcd'),('xyz');

I have tried this query (here is the fiddle ):

select 
case when ((select * from example where name='abc')>=1)
then (select * from example where name='abc')
else (select count(*) from example)
end
from example

But it outputs

3
3
3

Expected output if name='abc' exist

name
abc

if not the count(*)

Thanks in advance

Answer 1

Your subquery in the example is (select * from example where name='abc') which is a result set, not a scalar value. Currently it "works" because it is comparing the only column in the table to the value 1 but if you had more than one column in the table it would error out. Perhaps you intended (select count(*) from example where name='abc') ?

Similarly, the THEN clause in a case can only be used to provide a single column value. In order to do this, perhaps you meant the following:

select 
    case when exists (select * from example where name='abc')
              then (select name from example where name='abc')
         else (select count(*) from example)
    end
from example

But even here you will get three rows and there is no correlation between the rows in example and the result set, so I am not really sure what you're trying to do. I imagine there is a higher purpose though so I will leave it at that.

Answer 2

This should do the trick

select distinct
case when ((select count(name) from example where name='abc')>=1)
then (select * from example where name='abc')
else (select count(*) from example)
end
from example

Let me know if it works.

Answer 3

Point 1 :
For the query, you are trying, the from example in the last will cause to loop through all the records and fetch all the records. To restrict that, you have to remove that.

Point 2 :

You can't combine multi row select * in a true condition with a single row count(*) in a false condition. You should limit to select a single row.

Example :

select 
  case when ( select count(*) from example where name='abc' ) >= 1
            then ( select * from example where name='abc' limit 1 )
            else ( select count(*) from example )
        end as name

Answer 4

No need to bother with the complex queries.

SELECT COUNT(*) AS ct 
FROM example 
GROUP BY name = 'abc' 
ORDER BY name = 'abc' DESC 
LIMIT 1;

If you really want to use CASE just for the sake of using it:

SELECT 
   CASE name 
      WHEN 'abc' THEN 'abc' 
      ELSE 'others' 
   END AS name, COUNT(*) AS ct
FROM example 
GROUP BY name = 'abc' 
ORDER BY name = 'abc' DESC 
LIMIT 1;

Answer 5

Try below query, which will work even you enter a second duplicate row as value 'abc'. Mostly above suggested queries will not work as you enter this duplicate row while as per your query condition (>=1), there can be multiple rows for name as 'abc'.

SELECT 
CASE WHEN b.cnt>=1 
THEN a.name 
ELSE (SELECT COUNT(*) FROM EXAMPLE) 
END 
FROM (SELECT DISTINCT NAME FROM EXAMPLE WHERE NAME='abc') a 
JOIN (SELECT NAME,COUNT(*) AS cnt FROM EXAMPLE WHERE NAME='abc') b 
ON a.name=b.name