Error Code: 1215. Cannot add foreign key constraint in MySQL - mysql

Question

I have an error code which is Error Code: 1215. Cannot add foreign key constraint in MySQL. My code is
DROP TABLE IF EXISTS Formed;
DROP TABLE IF EXISTS Album;
DROP TABLE IF EXISTS Band;
DROP TABLE IF EXISTS Customers;
DROP TABLE IF EXISTS Track;
CREATE TABLE Formed(
FormedID int NOT NULL,
YearFormed int,
CountryFormed varchar(50),
CityFormed varchar(50),
BandMembers varchar(400),
PRIMARY KEY(FormedID))
ENGINE=InnoDB;
CREATE TABLE Track (
TrackID int NOT NULL,
AlbumID int NOT NULL,
Songs varchar (100),
TrackNumber varchar (20),
Title varchar (20),
TrackDuration varchar (4),
PRIMARY KEY (TrackID),
FOREIGN KEY (AlbumID) REFERENCES Album(AlbumID)ON DELETE SET NULL ON UPDATE CASCADE)
ENGINE=InnoDB;
CREATE TABLE Album(
AlbumID int NOT NULL,
TrackID int NOT NULL,
BandID int NOT NULL,
Price varchar(5),
PublicationDate varchar(11),
Title varchar(30),
Genre varchar (36),
PRIMARY KEY(AlbumID),
FOREIGN KEY (TrackID) REFERENCES Track(TrackID)ON DELETE SET NULL ON UPDATE CASCADE,
FOREIGN KEY (BandID) REFERENCES Band(BandID)ON DELETE SET NULL ON UPDATE CASCADE)
ENGINE=InnoDB;
CREATE TABLE Band(
BandID int NOT NULL,
AlbumID int NOT NULL,
RecordLabel varchar(50),
PRIMARY KEY(BandID),
FOREIGN KEY (AlbumID) REFERENCES Album(AlbumID)ON DELETE SET NULL ON UPDATE CASCADE)
ENGINE=InnoDB;
CREATE TABLE Customers (
CustomerID int NOT NULL,
CName varchar (20),
CPhone int (11),
CEmail varchar (50),
CPPaid varchar (50),
CPDate date,
PRIMARY KEY (CustomerID))
ENGINE=InnoDB;
It creates the first table named Formed, but it's giving me the error when it tries and creates the second table and I don't know why. I know a bit about MySQL and I'm teaching myself about Foreign Keys. I have had a look online on why I could be getting this error but couldn't find anything useful.

Answer 1

From the error message, it seems like you are trying to create Track table before Album table and hence, it's failing as it is not able to find parent table (and column) for foreign key constraint.
I would suggest creating the tables with Primary keys only and then, apply foreign key constraints once all 3 tables are created, e.g.:
ALTER TABLE `Track`
ADD FOREIGN KEY (AlbumID) REFERENCES Album(AlbumID)ON DELETE SET NULL ON UPDATE CASCADE);

Answer 2

As a rule All reference fields in child tables MUST HAVE index defined on them as like in parent table. They must also follow some other constraints.
As per documentation on foreign key constraints:
REFERENCES parent_tbl_name (index_col_name,...)
Define an INDEX on relevant child columns. And make sure that child column definitions must match with those of their parent column definitions.
Code change suggested:
CREATE TABLE Formed( ... ) -- no change suggested
CREATE TABLE Customers ( ... ) -- no change suggested
CREATE TABLE Band(
...
AlbumID int NOT NULL,
...
KEY (AlbumID), -- <---- add this line
...
);
CREATE TABLE Album(
...
TrackID int NOT NULL,
BandID int NOT NULL,
...
KEY (TrakID), -- <---- add this line
KEY (BandID), -- <---- add this line
...
-- FOREIGN KEY (TrackID) -- <-- add this using ALTER
-- REFERENCES Track(TrackID) -- after creating
-- ON DELETE SET NULL ON UPDATE CASCADE, -- Track table
);
ALTER TABLE Band ADD
FOREIGN KEY ( AlbumID )
REFERENCES Album( AlbumID )
ON DELETE SET NULL ON UPDATE CASCADE;
CREATE TABLE Track ( ... ) -- no changes suggested
ALTER TABLE Album ADD
FOREIGN KEY ( TrackID )
REFERENCES Track( TrackID );
ON DELETE SET NULL ON UPDATE CASCADE;
Refer to:
MySQL Using FOREIGN KEY Constraints
[CONSTRAINT [symbol]] FOREIGN KEY
[index_name] (index_col_name, ...)
REFERENCES tbl_name (index_col_name,...)
[ON DELETE reference_option]
[ON UPDATE reference_option]
reference_option:
RESTRICT | CASCADE | SET NULL | NO ACTION