How to raise error from PostgreSql SQL statement if some condition is met

I’m looking for a way to raise error from sql select if some condition is met.

Tried code below to got error shown in comment.

How to fix ?

 

Andrus

 

    CREATE OR REPLACE FUNCTION "exec"(text)

      RETURNS text AS

    $BODY$

        BEGIN

          EXECUTE $1;

          RETURN $1;

        END;

    $BODY$

      LANGUAGE plpgsql VOLATILE;

     

    -- ERROR:  syntax error at or near "raise"

    -- LINE 1: raise 'test'

   

    select exec('raise ''test'' ') where true -- in real application true is replaced by some condition