MySQL error: Cannot add foreign key constraint?

Question

There always be the error:"Cannot add foreign key constraint" when I create my last table.

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System: Mac OS X 10.9
DB : MySQL 5.6.14
DBM : Sequel Pro

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CREATE TABLE users (
uid INT AUTO_INCREMENT PRIMARY KEY,
uname VARCHAR(20) NOT NULL,
uemail VARCHAR(20) NOT NULL,
ucity VARCHAR(20),
upassw VARCHAR(20) NOT NULL
);

CREATE TABLE songs(
sid INT AUTO_INCREMENT PRIMARY KEY,
sname VARCHAR(20) NOT NULL,
srldate DATE NOT NULL
);



CREATE TABLE albums (
albid INT AUTO_INCREMENT PRIMARY KEY,
albname VARCHAR(20) NOT NULL,
albrldate DATE NOT NULL,
albrltime TIME NOT NULL
);

CREATE TABLE artists (
aid INT AUTO_INCREMENT PRIMARY KEY,
aname VARCHAR(20) NOT NULL
);

CREATE TABLE genres (
gid INT AUTO_INCREMENT PRIMARY KEY,
gname VARCHAR(20) NOT NULL
);

CREATE TABLE playlists (
uid INT NOT NULL,
sid INT NOT NULL,
plname VARCHAR(20) NOT NULL,
plmdate DATE NOT NULL,
plmtime TIME NOT NULL,
PRIMARY KEY(uid, sid, plname, plmdate, plmtime),
FOREIGN KEY(uid) REFERENCES users(uid),
FOREIGN KEY(sid) REFERENCES songs(sid)
);

CREATE TABLE u_like_a (
aid INT NOT NULL,
uid INT NOT NULL,
PRIMARY KEY(aid, uid),
FOREIGN KEY(aid) REFERENCES artists(aid),
FOREIGN KEY(uid) REFERENCES users(uid)
);

CREATE TABLE be_fan (
aid INT NOT NULL,
uid INT NOT NULL,
PRIMARY KEY(aid, uid),
FOREIGN KEY(aid) REFERENCES artists(aid),
FOREIGN KEY(uid) REFERENCES users(uid)
);

CREATE TABLE follow (
uid INT NOT NULL,
to_uid INT NOT NULL,
PRIMARY KEY(uid, to_uid),
FOREIGN KEY(uid) REFERENCES users(uid),
FOREIGN KEY(to_uid) REFERENCES users(uid)
);

CREATE TABLE u_like_g (
gid INT NOT NULL,
uid INT NOT NULL,
PRIMARY KEY(gid, uid),
FOREIGN KEY(gid) REFERENCES genres(gid),
FOREIGN KEY(uid) REFERENCES users(uid)
);

CREATE TABLE u_share_pl(
uid INT NOT NULL,
from_uid INT NOT NULL,
plname VARCHAR(20) NOT NULL,
plmdate DATE NOT NULL,
plmtime TIME NOT NULL,
PRIMARY KEY(uid, from_uid, plname, plmdate, plmtime),
FOREIGN KEY(uid) REFERENCES users(uid),
FOREIGN KEY(from_uid) REFERENCES users(uid),
FOREIGN KEY(plname) REFERENCES playlists(plname),
FOREIGN KEY(plmdate) REFERENCES playlists(plmdate),
FOREIGN KEY(plmtime) REFERENCES playlists(plmtime)
); #####--->  This is the last table. 

ERROR comes from here. I really don't why. I have check the type for all attributes. The type and name of attributes have no problem. But the mysql always say "Cannot add foreign key constraint"

Answer 1

Here is you reference wrong forgien REFERENCES users(from_uid) in last table.

FOREIGN KEY(from_uid) REFERENCES users(from_uid)

from_uid not belong to users

This should be

FOREIGN KEY(from_uid) REFERENCES users(uid)

your playLists table has primary key combination of four columns, so you should supply all these four columns as forieng key in u_share_pl table.

Another composite key as a reference should be a single constraint like

FOREIGN KEY(from_uid,sid,plname,plmdate,plmtime) REFERENCES playlists(uid,sid,plname,plmdate,plmtime)

Your last table Create should be:

CREATE TABLE u_share_pl(
uid INT NOT NULL,
from_uid INT NOT NULL,
sid INT NOT NULL,
plname VARCHAR(20) NOT NULL,
plmdate DATE NOT NULL,
plmtime TIME NOT NULL,
PRIMARY KEY(uid, from_uid, plname, plmdate, plmtime),
FOREIGN KEY(uid) REFERENCES users(uid),
FOREIGN KEY(from_uid,sid,plname,plmdate,plmtime) REFERENCES playlists(uid,sid,plname,plmdate,plmtime)
);