Problems with MYSQL Error 150 during table creation - mysql

Question

I looked up other posts about this problem (there are a lot of them), but I have to say nothing worked so far for me. The only thing did not try was to disable foreign check.
My Problem:
When I try to create two simple tables I got the error 150 and I can't figure out why.
My Code so far:
drop table if exists SENSOR;
drop table if exists SENSOR_DATEN;
create table SENSOR (
SENSOR_ID int not null auto_increment,
SENSOR_NAME varchar(100),
SENSOR_POSX float not null,
SENSOR_POSY float not null,
SENSOR_EINHEIT varchar(100) not null,
primary key (SENSOR_ID)
);
commit;
create table SENSOR_DATEN (
SENSOR_DATEN_ID int not null auto_increment,
SENSOR_ID int not null,
SENSOR_DATEN_WERT varchar(100),
SENSOR_TYP varchar(100) not null,
primary key (SENSOR_DATEN_ID),
foreign key(SENSOR_ID) references SENSOR(SENSOR_ID),
foreign key(SENSOR_TYP) references SENSOR(SENSOR_EINHEIT)
);
commit;
My main problem is to understand the Problem. I got a way bigger database which is nearly identical but there are no problems. Here is the Code for the Database which works fine:
drop table if exists AUSGELIEHEN_FILM;
drop table if exists AUSGELIEHEN_SPIEL;
drop table if exists KUNDE;
drop table if exists FILM;
drop table if exists SPIEL;
create table KUNDE (
KUNDE_ID int not null auto_increment,
KUNDE_VNAME varchar(100),
KUNDE_NAME varchar(100),
KUNDE_ADRESSE varchar(100),
KUNDE_PLZ int not null,
primary key (KUNDE_ID)
);
create table FILM (
FILM_ID int not null auto_increment,
FILM_NAME varchar(100),
FILM_GENRE varchar(50),
FILM_FSK int not null,
FILM_BEWERTUNG int not null,
FILM_ANZAHL int not null,
FILM_MEDIUM varchar(50),
primary key (FILM_ID)
);
create table SPIEL (
SPIEL_ID int not null auto_increment,
SPIEL_NAME varchar(100),
SPIEL_GENRE varchar(50),
SPIEL_FSK int not null,
SPIEL_BEWERTUNG int not null,
SPIEL_ANZAHL int not null,
SPIEL_PLATFORM varchar(50),
primary key (SPIEL_ID)
);
create table AUSGELIEHEN_FILM (
AUSGELIEHEN_FILM_ID int not null auto_increment,
KUNDE_ID int not null,
FILM_ID int not null,
AUSGELIEHEN_ANZAHL int not null,
primary key(AUSGELIEHEN_FILM_ID),
foreign key(KUNDE_ID) references KUNDE(KUNDE_ID),
foreign key(FILM_ID) references FILM(FILM_ID)
);
create table AUSGELIEHEN_SPIEL(
AUSGELIEHEN_SPIEL_ID int not null auto_increment,
KUNDE_ID int not null,
SPIEL_ID int not null,
AUSGELIEHEN_ANZAHL int not null,
primary key(AUSGELIEHEN_SPIEL_ID),
foreign key(KUNDE_ID) references KUNDE(KUNDE_ID),
foreign key(SPIEL_ID) references SPIEL(SPIEL_ID)
);
commit;

Answer 1

Your table SENSOR_DATEN declares a foreign key that references SENSOR's column SENSOR_EINHEIT. This column is not defined as being unique, and thus can not have a foreign key pointing to it. Hence, your failure.
You could amend the declaration of SENSOR to make the column unique:
create table SENSOR (
SENSOR_ID int not null auto_increment,
SENSOR_NAME varchar(100),
SENSOR_POSX float not null,
SENSOR_POSY float not null,
SENSOR_EINHEIT varchar(100) not null,
primary key (SENSOR_ID),
unique (SENSOR_EINHEIT)
);
Or alternatively, apply it post-factum to an existing table:
alter table SENSOR add constraint SENSOR_EINHEIT_UNQ unique(SENSOR_EINHEIT);