missing expression error occured in case statement

Question

I have source table Test contains id column, target table testm consist of id , col2 . Could please assist me to get rid of this error.

test (source)                  

 id                              
---                              
 10                     
 10 
 20 
 20 
 20 
 30 
 30 
 40 

Target

testm 

id  col2                
--  ----                
10  1                 
10  2         
20  1          
20  2          
20  3          
30  1          
30  2          
40  1          

The query:

select id, (case id 
            when 10 then select count(id) from test where id =10
            when 20 then select count(id) from test where id =20
            when 30 then select count(id) from test where id =30
            when 40 then select count(id) from test where id =40
else 0 END ) col2 from test

throws the error:

missing expression

Answer 1

From looking at the desired output I guess that you want to number each occurrence of an id by groups; if this is the case using the row_number analytical function should do what you want:

select id, row_number() over (partition by id order by id) as col2
from test
order by id;

See this sample SQL Fiddle

Given you sample source data this would be the output:

| ID | COL2 |
|----|------|
| 10 |    1 |
| 10 |    2 |
| 20 |    1 |
| 20 |    2 |
| 20 |    3 |
| 30 |    1 |
| 30 |    2 |
| 40 |    1 |