MySQL Error Number 1005 Can’t create table ‘.mydb#sql-328_45.frm’ (errno: 150)

If you get this error while trying to create a foreign key, it can
be pretty frustrating. The error about not being able to create a .frm
file seems like it would be some kind of OS file permission error or
something but this is not the case. This error has been reported as a
bug on the MySQL developer list for ages, but it is actually just a
misleading error message.

In every case this is due to something about the relationship that
MySQL doesn’t like. Unfortunately it doesn’t specify what the exact
issue is. Here is a running list of causes that people have reported
for the dreaded errno 150. I’ve tried to put them in order based on the
frequency that I hear about a particular cause.

You may want to start by running the MySQL command “SHOW INNODB
STATUS” immediately after receiving the error. This command displays
log info and error details. (Thanks Jonathan for the tip)

Known Causes:

1. The two key fields type and/or size doesn’t match exactly. For
   example, if one is INT(10) the key field needs to be INT(10) as well
   and not INT(11) or TINYINT. You may want to confirm the field size
   using SHOW CREATE TABLE because Query Browser will sometimes visually
   show just INTEGER for both INT(10) and INT(11). You should also check
   that one is not SIGNED and the other is UNSIGNED. They both need to be
   exactly the same.
2. One of the key field that you are trying to reference does not have
   an index and/or is not a primary key. If one of the fields in the
   relationship is not a primary key, you must create an index for that
   field.
3. The foreign key name is a duplicate of an already existing key.
   Check that the name of your foreign key is unique within your database.
   Just add a few random characters to the end of your key name to test
   for this.
4. One or both of your tables is a MyISAM table. In order to use
   foreign keys, the tables must both be InnoDB. (Actually, if both tables
   are MyISAM then you won’t get an error message – it just won’t create
   the key.) In Query Browser, you can specify the table type.
5. You have specified a cascade ON DELETE SET NULL, but the relevant
   key field is set to NOT NULL.  You can fix this by either changing your
   cascade or setting the field to allow NULL values. (Thanks to Sammy and
   J Jammin)
6. Make sure that the Charset and Collate options are the same both at
   the table level as well as individual field level for the key columns.
7. You have a default value (ie default=0) on your foreign key column (Thanks to Omar for the tip)
8. One of the fields in the relationship is part of a combination
   (composite) key and does not have it’s own individual index. Even
   though the field has an index as part of the composite key, you must
   create a separate index for only that key field in order to use it in a
   constraint. (Thanks to Alex for this tip)
9. You have a syntax error in your ALTER statement or you have
   mistyped one of the field names in the relationship
10. The name of your foreign key exceeds the max length of 64 chars. 

The MySQL documentation includes a page explaining requirements for foreign keys.
Though they don’t specifically indicate it, these are all potential
causes of errno 150. If you still haven’t solved your problem you may
want to check there for deeper technical explanations.If you run into
this error and find that it’s caused by something else, please leave a
comment and I’ll add it to the list.

To Find detailed error message in Mysql

If you get for example:

ERROR 1005 at line 12: Can’t create table ‘./database/users.frm’ (errno: 150)

You can find out the real reason by executing bin/perror 150 in the MySQL directory.

This will give you a nice error message:

MySQL error: 150 = Foreign key constraint is incorrectly formed

Collected from here directly.