3
a)
```SQL 
CREATE OR REPLACE TRIGGER InsertLagerplatz

BEFORE INSERT ON LAGER

FOR EACH ROW

DECLARE

    anzahl INTEGER DEFAULT 0;

    nurZweiLPs EXCEPTION;

BEGIN

    if :new.anummer is NOT NULL

    THEN

        select count(*) into anzahl

        from LAGER

        where ANUMMER= :new.anummer;

        IF anzahl >=2 THEN

            RAISE nurZweiLPs;

        END IF;

    END IF;

EXCEPTION

    when  nurZweiLPs

    THEN raise_application_error(-20500, 'Es gibt bereits zwei Lagerplaetze');

END;
```

b)
```SQL 

```