Question

I am trying check if a value is null if so the select null else cast to numeric, but it throws an error. This is actually part of an insert statement

INSERT into someTable(name,created,power) 
SELECT 'xyz',now(),
case when :power ='null' then NULL else cast(:power as numeric) end from abc 

error that I get is

Error: ERROR: invalid input syntax for type numeric: "null"

:power is a variable that can be given any value using java code. If I give a value of null it give an error.

In code I get the following error from the java stack trace

org.postgresql.util.PSQLException: ERROR: cannot cast type bytea to numeric

Answer 1

Error

SELECT CASE WHEN 'null' = 'null' THEN NULL ELSE cast('null' AS numeric) END 

No error

DO
$$
DECLARE
   power text := 'null';
BEGIN
   PERFORM CASE WHEN power = 'null' THEN null ELSE cast(power AS numeric) END;
END
$$

Explanation

If you build a query string, the expression cast('null' AS numeric) or simply 'null'::numeric always raises an exception, even in an ELSE block that is never executed, because it is invalid input syntax and the exception is raised during the syntax check (like the error message implies), not during execution.

A CASE statement like you display only makes sense with a parameter or variable not with literals. The second instance of the literal has no connection to the first instance whatsoever after the query string has been assembled.

For dynamic SQL like that, you need to check the value before you build the query string. Or you use a function or prepared statement and pass the value as parameter. That would work, too.

More advice after comment:

In your particular case you could check the value in the app and build a query string like this:

INSERT INTO tbl(name, abc_id, created, power) 
SELECT 'xyz'
     , abc_id
     , now()
     , <insert_value_of_power_or_NULL_here> -- automatically converted to numeric
FROM   abc;

You may be interested in a different approach to INSERT data from a file conditionally.
Use COPY for files local to the server or psql’s meta-command \copy for files local to the client.