How to raise error from PostgreSql SQL statement if some condition is met - Mailing list pgsql-general

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

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