MySQL Adding Foreign Key - mysql

Question

I have two tables:
Table 1: staff_incharge with following columns - First_name and staff_id (This is the Primary Key)
Table 2: student-info with following columns - First_name Last_name ID(Primary Key)
I have another column staff_id which I'm trying to make as a Foreign key, but it shows some error.
This is the query I'm using in MySQL Query Browser:
alter table `student_info`
add constraint foreign key (staff_id)
references staff_incharge(staff_id);
The error is:
You have an error in your SQL syntax; check the manual that
corresponds to your MySQL server version for the right syntax to use
near 'student_info add constraint foreign key (staff_id) references
staff_incharge' at line 1.
Can someone tell me what I'm doing wrong?
student_info table was created first and then staff_incharge was created next.
Output of the following query:
SHOW CREATE TABLE student_info;
CREATE TABLE `student_info` (
`First_name` varchar(15) NOT NULL DEFAULT '',
`Last_name` varchar(45) NOT NULL,
`ID` int(10) unsigned NOT NULL DEFAULT '0',
`staff_id` int(11) NOT NULL DEFAULT '0',
`City` varchar(15) NOT NULL DEFAULT '',
`Marks1` int(10) unsigned NOT NULL DEFAULT '0',
`Marks2` int(10) unsigned NOT NULL DEFAULT '0',
`Marks3` int(10) unsigned NOT NULL DEFAULT '0',
`Total_marks` int(10) unsigned NOT NULL DEFAULT '0',
`Branch` varchar(10) NOT NULL,
PRIMARY KEY (`ID`,`First_name`)
SHOW CREATE TABLE staff_incharge;
'staff_incharge', 'CREATE TABLE `staff_incharge` (
`First_name` varchar(20) NOT NULL,
`Staff_id` int(11) NOT NULL,
PRIMARY KEY (`Staff_id`)
)

Answer 1

Minimum code - without explicitly naming the foreign key constraint (MySQL will choose a semi-random name):
ALTER TABLE student_info
ADD FOREIGN KEY (staff_id)
REFERENCES staff_incharge (staff_id) ;
Naming the foreign key constraint:
ALTER TABLE student_info
ADD CONSTRAINT staff_incharge__student_info__FK -- name of your choice
FOREIGN KEY (staff_id)
REFERENCES staff_incharge (staff_id) ;

Answer 2

I got it! The problem was that I had given different default values for ID and staff_id.
The main thing is to ensure foreign key and referenced column should have same data types, length, attributes, default values, collation.
These links helped:
http://dev.mysql.com/doc/refman/5.5/en/innodb-foreign-key-constraints.html
http://forums.mysql.com/read.php?22,19755,499517