I am not able to establish foreign key constrain in mysql.
Mysql version: 5.0
Queries I have used are:
create table department (
deptid varchar(10),
primary key (deptid),
name varchar(20)
);
create table employee (
empid varchar(10),
primary key (empid),
name varchar(20),
age int,
deptid varchar(10),
foreign key (deptid) references department (deptid)
);
I am still able to add employee with department id which is not present in department table.
Thanks.
EDIT: I changed engine for my tables but still this constraint is not working. I changed engine by using following query: alter table department engine=InnoDB;
As mmarinero suggested that foreign key constraint doesn't work for MyISAM engine. I checked engine for my tables using following query:
show table status;
It showed MyISAM engine for my tables.
I dropped my tables and recreated them using following queries:
create table department (deptid varchar(10), primary key (deptid), name varchar(20)) engine=InnoDB;
create table employee (empid varchar(10), primary key (empid), name varchar(20), age int, deptid varchar(10), foreign key (deptid) references department (deptid)) engine=InnoDB;
Now my foreign constraint is working.
Thanks mmarinero.
Related
I have created two tables (emp and dept). Emp contains a foreign key (deptid). I am attempting to delete a row from the dept table and am receiving a foreign key constraint error. I then altered the emp table foreign key to add constraint and delete on cascade. The code is copied here
create table dept (
deptid int primary key,
deptname varchar(20),
locid int);
create table emp (
empid int primary key,
frname varchar(15),
lname varchar(20),
hiredate datetime,
deptid int,
foreign key (deptid) references dept(deptid));
alter table emp add constraint fk_deptid foreign key (deptid)
references dept(deptid) on delete cascade;
This is the error message that I receive:
Query Error: Error: ER_ROW_IS_REFERENCED_2: Cannot delete or update a parent row: a foreign key constraint fails (test.emp, CONSTRAINT emp_ibfk_1 FOREIGN KEY (deptid) REFERENCES dept (deptid))
The problem is that you are creating two foreign key constraints for the same column. Avoid doing this, it's confusing.
The first one is created while you create the table and is unnamed. Since you didn't provide a name for it, MySQL automatically names it for you as emp_ibfk_1. This one does not have CASCADE DELETE and prevents deletion of parent keys.
The second one is on a separate SQL statement and you name it fk_deptid. This one has CASCADE DELETE and allows deletion of parent keys.
While enforcing the foreign key constraints MySQL cannot delete the row since the first constraint does not allow it. That's the error you are getting.
This is a confusing situation and I would avoid doing something like this. I would suggest having a single FK constraint for the column; specifically I would remove the first one.
For example:
create table dept (
deptid int primary key,
deptname varchar(20),
locid int);
create table emp (
empid int primary key,
frname varchar(15),
lname varchar(20),
hiredate datetime,
deptid int
-- , foreign key (deptid) references dept(deptid) -- removed
);
alter table emp add constraint fk_deptid foreign key (deptid)
references dept(deptid) on delete cascade;
insert into dept (deptid, deptname, locid)
values (1, 'Math', 123);
insert into emp (empid, frname, lname, hiredate, deptid)
values (10, 'Peter', 'Fonda', null, 1);
delete from dept where deptid = 1; -- works!
CREATE TABLE Transcripts (sID VARCHAR(7), cNo VARCHAR(10),
semester VARCHAR(20), grade CHAR(1), PRIMARY KEY (sID)
);
CREATE TABLE Students (sID CHAR(7), sName VARCHAR(20),
bDate DATE, phone VARCHAR(12), major VARCHAR(30), avgGPA VARCHAR(4), PRIMARY KEY (sID),
FOREIGN KEY (sID)
REFERENCES Transcripts(sID)
);
CREATE TABLE Courses (cNo VARCHAR(10), cTitle VARCHAR(30),
creditHours VARCHAR(2), deptName VARCHAR(30), PRIMARY KEY (cNo),
FOREIGN KEY (cNo)
REFERENCES Transcripts(cNo)
);
So whenever I run this, the first foreign key works just fine, whenever I run the next table it gives me back this error "ERROR 1215 (HY000): Cannot add foreign key constraint" What did I do wrong?
This is the foreign key that produces the error:
FOREIGN KEY (cNo)
REFERENCES Transcripts(cNo)
But the Transcripts(cNo) is not part of any KEY in that table.
A foreign key must reference column(s) of a UNIQUE or PRIMARY KEY of the parent table.
See MySQL Creating tables with Foreign Keys giving errno: 150 for a good checklist required for a foreign key.
Index the foreign key column before adding the foreign key.
CREATE TABLE Courses (cNo VARCHAR(10), cTitle VARCHAR(30),
creditHours VARCHAR(2), deptName VARCHAR(30), PRIMARY KEY (cNo),
INDEX (cNo),
FOREIGN KEY (cNo)
REFERENCES Transcripts(cNo)
);
to define cNo as a foreign key on Courses table, it must be primary key
or a unique key of other table.
CREATE TABLE Transcripts(
sID VARCHAR(7),
cNo VARCHAR(10) NOT NULL UNIQUE,
semester VARCHAR(20),
grade CHAR(1),
PRIMARY KEY (sID)
);
http://sqlfiddle.com/#!9/fddf8
changing Transcripts, as I've written above wil solve your problem.
Isn't the logic backwards? Shouldn't `Transcripts have two FKs referencing the other two tables? And, as already pointed out, declare the other two tables first.
The current tables I have are as follows:
CREATE TABLE course(
CourseNum INT(11),
CourseName VARCHAR(30),
NumOfUnit INT(11),
PRIMARY KEY(CourseNum)
);
CREATE TABLE timeandloc(
CourseNum INT(11),
Quarter VARCHAR(20),
DayTime VARCHAR(40),
RoomNum INT,
PRIMARY KEY(CourseNum, Quarter, DayTime),
FOREIGN KEY(CourseNum) REFERENCES course (CourseNum)
);
I was able to add those fine using a query, but when I try to add this table:
CREATE TABLE student(
StudentName VARCHAR(30),
CourseNum INT(11),
Quarter VARCHAR(20),
PRIMARY KEY(StudentName, CourseNum, Quarter),
FOREIGN KEY(CourseNum) REFERENCES course(CourseNum),
FOREIGN KEY(Quarter) REFERENCES timeandloc(Quarter)
);
I get
Error code: 1215. Cannot add foreign key constraint.
It seems to be this line that's the culprit:
FOREIGN KEY(Quarter) REFERENCES timeandloc(Quarter)
When I try to add the table without that line, everything works fine without a hitch.
I'm very new to MySQL and databases in general so I'm not sure what's wrong. Any help would be great. Thanks.
create a separate index on Quarter field in timeandloc table and then create problematic create table query.
alter table timeandloc add index idx_Quarter(Quarter);
CREATE TABLE student(
StudentName VARCHAR(30),
CourseNum INT(11),
Quarter VARCHAR(20),
PRIMARY KEY(StudentName, CourseNum, Quarter),
FOREIGN KEY(CourseNum) REFERENCES course(CourseNum),
FOREIGN KEY(Quarter) REFERENCES timeandloc(Quarter)
);
Update:
For performance point of view joining field in both tables (parent/child) should be indexed. When we create foreign key then mysql itself create an index on the field we create foreign key but could not allow without index on referenced column in referenced table. As index works from left to right, so in your case index for Quarter column will not be used from primary key, so need to create a separate index on it.
I have got 4 tables here:
CREATE TABLE paper (
id VARCHAR(20) PRIMARY KEY,
name VARCHAR(30)
) ENGINE =INNODB;
CREATE TABLE subscriber (
id VARCHAR(10) PRIMARY KEY,
name VARCHAR(20),
address VARCHAR(30),
suburb VARCHAR(20),
state VARCHAR(3),
postcode VARCHAR(4))
round_id INTEGER NOT NULL,
FOREIGN KEY (round_id) REFERENCES round (id)
ON UPDATE CASCADE ON DELETE RESTRICT
) ENGINE = INNODB;
CREATE TABLE current_order (
paper_id VARCHAR(20),
subscriber_id VARCHAR(10),
PRIMARY KEY (paper_id, subscriber_id),
FOREIGN KEY (paper_id) REFERENCES paper (id)
ON UPDATE CASCADE ON DELETE RESTRICT,
FOREIGN KEY(subscriber_id) REFERENCES subscriber (id)
ON UPDATE CASCADE ON DELETE RESTRICT)
ENGINE = INNODB;
CREATE TABLE receipt (
id INTEGER PRIMARY KEY AUTO_INCREMENT,
receipt_date DATE,
paid_till_date DATE,
paper_id VARCHAR(20),
subscriber_id VARCHAR(10))
ENGINE = INNODB;
Now table paper has a primary key ID, it is referenced by paper_id in table current_order.
Table subscriber has a primary key ID, it is referenced by subscriber_id in table current_order.
Tabe current_order has a composite primary key (paper_id,subscriber_id).
So these three table has been linked together through the foreign key relationship.
If I want to have the last table receipt linked with these three table, how do I do that? My idea was to
1:set a compound foreign key (paper_id,subscriber_id) referencing to the compound primary key (paper_id,subscriber_id) in table current_order.
2:set two single foreign key (paper_id) and (subscriber_id) referencing to (paper_id) and )subscriber_id) in table current_order separately.
Neither method worked and it came up with error 1452:cannot add or update child row.
So I am really desperate to know what is the proper way to set relationship between table receipt and table current_order?
here is the E-R Digram:
E-R DIGRAM
There is two links between table receipt and table current_order and I am required to set relationship according to that.
i'm not a mysql expert but i would try this:
CREATE TABLE receipt (
id INTEGER PRIMARY KEY AUTO_INCREMENT,
receipt_date DATE,
paid_till_date DATE,
paper_id VARCHAR(20),
subscriber_id VARCHAR(10),
FOREIGN KEY (paper_id,subscriber_id) REFERENCES current_order (paper_id,subscriber_id)
ON UPDATE CASCADE ON DELETE RESTRICT)
ENGINE = INNODB;
does it works?
I am new to databases and I keep getting an error when I try to create these tables. Where is a mistake?
I have found loads of questions similar to mine, but they didn't help me to resolve this problem.
CREATE TABLE IF NOT EXISTS course
(
cou_id VARCHAR(3),
course_name VARCHAR(25),
CONSTRAINT pk_course PRIMARY KEY (cou_id, course_name)
);
CREATE TABLE IF NOT EXISTS students_marks
(
stu_id INT,
student_name VARCHAR(25),
course_name VARCHAR(25),
first_mark NUMERIC(2,0),
second_mark NUMERIC(2,0),
third_mark NUMERIC(2,0),
CONSTRAINT pk_studentsmarks PRIMARY KEY (stu_id),
CONSTRAINT fk_studentsmarks_course FOREIGN KEY (course_name) REFERENCES course (course_name)
);
This is from INNODB STATUS.
LATEST FOREIGN KEY ERROR
------------------------
130603 20:17:22 Error in foreign key constraint of table testdb/students_marks:
FOREIGN KEY (course_name) REFERENCES course (course_name)
):
Cannot find an index in the referenced table where the
referenced columns appear as the first columns, or column types
in the table and the referenced table do not match for constraint.
Note that the internal storage type of ENUM and SET changed in
tables created with >= InnoDB-4.1.12, and such columns in old tables
cannot be referenced by such columns in new tables.
See http://dev.mysql.com/doc/refman/5.5/en/innodb-foreign-key-constraints.html
for correct foreign key definition.
Your primary key is set to cou_id and course_name in your course table. To add a foreign key constraint in your students_marks table, you'd have to reference both fields (and include both fields in your table definition):
CREATE TABLE IF NOT EXISTS course
(
cou_id VARCHAR(3),
course_name VARCHAR(25),
CONSTRAINT pk_course PRIMARY KEY (cou_id, course_name)
);
CREATE TABLE IF NOT EXISTS students_marks
(
stu_id INT,
student_name VARCHAR(25),
cou_id VARCHAR(3),
course_name VARCHAR(25),
first_mark NUMERIC(2,0),
second_mark NUMERIC(2,0),
third_mark NUMERIC(2,0),
CONSTRAINT pk_studentsmarks PRIMARY KEY (stu_id),
CONSTRAINT fk_studentsmarks_course FOREIGN KEY (cou_id,course_name) REFERENCES course(cou_id,course_name)
);
SQL Fiddle Demo
You are referencing only one field of a composed primary key!