using select statement in then clause of case - mysql

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