> CREATE TABLE student(
-> student_id INT(2) NOT NULL AUTO_INCREMENT PRIMARY KEY,
-> first_name VARCHAR(10),
-> last_name VARCHAR(10)
-> );
> CREATE TABLE course(
-> course_id CHAR(5) NOT NULL PRIMARY KEY,
-> course_name VARCHAR(50),
-> student_id INT(2) NOT NULL,
-> CONSTRAINT student_student_id_fk
-> FOREIGN KEY (student_id)
-> REFERENCES student(student_id)
-> );
Thats how i created two tables namely student and course. Then i entered data in the student table. But when i enter some invalid data in the course table, it doesn't gives me any error.
For example:
VALUES('A1','SUB 1',34);
gets the entry in the course table even if there is no primary key '34' in the student table.
Also, i can delete records in the student table, even if there is referential integrity.
So, how can enforce the constraints?
The easiest way to do this is to change your storage engine to InnoDB which supports the constraints.
For old MyISAM tables, you will have to use triggers on BOTH sides to enforce an FK relationship
Some links for self-help
http://dev.mysql.com/tech-resources/articles/mysql-enforcing-foreign-keys.html
http://forge.mysql.com/wiki/Triggers
Create your tables like this
> CREATE TABLE student(
-> student_id INT(2) NOT NULL AUTO_INCREMENT PRIMARY KEY,
-> first_name VARCHAR(10),
-> last_name VARCHAR(10)
-> ) ENGINE = INNODB;
> CREATE TABLE course(
-> course_id CHAR(5) NOT NULL PRIMARY KEY,
-> course_name VARCHAR(50),
-> student_id INT(2) NOT NULL,
-> CONSTRAINT student_student_id_fk
-> FOREIGN KEY (student_id)
-> REFERENCES student(student_id)
-> ) ENGINE = INNODB;
For someone having the same problem, if you already created your table using MyISAM, you can change it to InnoDB using.
ALTER TABLE table_name ENGINE=InnoDB;
Related
I am trying to create three tables such as associate, manager and attendance. The attendance table should be having employee and manager details from the other two table which should enable marking the attendance. I created this SQL script. I'm not sure where I am making mistake.
CREATE TABLE associate (
id INT NOT NULL,
idmanager INT NOT NULL,
emp_id DATE NOT NULL,
emp_name VARCHAR(25) NOT NULL,
FOREIGN KEY (id) REFERENCES attendance (associate_id) ON DELETE CASCADE,
FOREIGN KEY (idmanager) REFERENCES attendance (manager_idmanager) ON DELETE CASCADE,
PRIMARY KEY (id)
) ENGINE=INNODB;
CREATE TABLE manager (
id INT NOT NULL,
mgr_usr_id VARCHAR(15) NOT NULL,
mgr_name VARCHAR(25) NOT null,
KEY (id),
KEY (mgr_usr_id),
FOREIGN KEY (id) REFERENCES associate (idmanager) ON DELETE CASCADE,
PRIMARY KEY (id)
) ENGINE=INNODB;
CREATE TABLE attendance (
sno INT NOT NULL,
manager_idmanager INT NOT NULL,
associate_id INT NOT NULL,
date_stamp DATETIME,
state BIT NOT NULL,
PRIMARY KEY (sno)
) ENGINE=INNODB;
Screenshot
It's an issue of ordering. For example, the first statement executed is
CREATE TABLE associate (
which references attendance. However, the attendance table has not yet been created. Switch the order so that any tables that reference other tables come last.
Alternatively, don't put the FOREIGN KEY constraints in the CREATE statements, but them at the end of your script with ALTER TABLE statements. Consider:
CREATE TABLE associate (
id INT NOT NULL,
idmanager INT NOT NULL,
emp_id DATE NOT NULL,
emp_name VARCHAR(25) NOT NULL,
PRIMARY KEY (id)
) ENGINE=INNODB;
CREATE TABLE attendance (
sno INT NOT NULL,
manager_idmanager INT NOT NULL,
associate_id INT NOT NULL,
date_stamp DATETIME,
state BIT NOT NULL,
PRIMARY KEY (sno)
) ENGINE=INNODB;
ALTER TABLE associate ADD FOREIGN KEY (id) REFERENCES associate(id) ON DELETE CASCADE;
Edit
The above is just syntax. To model the requested problem consider orthogonality of information. You might also see/hear "normalization." The basic concept is this: have only one copy of your information. The schema should have a single point of authority for all data. For example, if a user has a birthdate, make sure you don't have an ancillary column that also stores their birthday; it's superfluous information and can lead to data errors.
In this case, what is the relationship? What must come first for the other to exist? Can an attendance be had without a manager? How about a manager without attendance? The former makes no sense. In this case then, I would actually use a third table, to form a hierarchy.
Then, consider that maybe roles change in a company. It would not behoove the DB architect to hard code roles as tables. Consider:
CREATE TABLE employee (
id INTEGER NOT NULL AUTO_INCREMENT,
name VARCHAR(25) NOT NULL,
PRIMARY KEY (id)
) ENGINE=INNODB;
CREATE TABLE role (
id INTEGER NOT NULL AUTO_INCREMENT,
name VARCHAR(30) NOT NULL,
description VARCHAR(254) NOT NULL,
PRIMARY KEY( id ),
UNIQUE( name )
) ENGINE=INNODB;
INSERT INTO role (name, description) VALUES
('associate', 'An associate is a ...'),
('manager', 'A manager follows ...');
CREATE TABLE employee_role (
employee_id INTEGER NOT NULL,
role_id INTEGER NOT NULL,
PRIMARY KEY (employee_id, role_id),
FOREIGN KEY (idemployee_id) REFERENCES employee_id (id) ON DELETE CASCADE,
FOREIGN KEY (role_id) REFERENCES role (id) ON DELETE CASCADE
) ENGINE=INNODB;
CREATE TABLE attendance (
sno INTEGER NOT NULL,
employee_id INTEGER NOT NULL,
date_stamp DATETIME,
state BIT NOT NULL,
PRIMARY KEY (sno),
FOREIGN KEY (idemployee_id) REFERENCES employee_id (id) ON DELETE CASCADE
) ENGINE=INNODB;
From this schema, the attendance needs only one foreign key because everyone is an employee. Employee's can have multiple roles, and they can change. Further, role definitions can change without needing to resort to costly DDL statements (data definition layer changes, like ALTER TABLE), and can be modified with simple DML (data manipulation layer changes, like UPDATE TABLE). The former involves rewriting all entries in the tables, and changing schemas, while the latter involves changing individual entries.
what is wrong?
mysql> create table price(
-> p_code char(1) not null,
-> p_description varchar(20),
-> p_rentfee decimal(2,2) not null,
-> p_dylatefee decimal(2,2));
Query OK, 0 rows affected (0.18 sec)
mysql> create table movie(
-> mv_no char(4) not null,
-> mv_name varchar(50) not null,
-> mv_year char(4) not null,
-> mv_cost decimal(2,2) not null,
-> mv_genre varchar(15) not null,
-> p_code char(1) not null,
-> foreign key (p_code) references price(p_code));
ERROR 1215 (HY000): Cannot add foreign key constraint
mysql>
price.p_code is not the primary key for price. Try:
create table price(
p_code char(1) not null PRIMARY KEY,
p_description varchar(20),
p_rentfee decimal(2,2) not null,
p_dylatefee decimal(2,2));
In general, foreign keys must reference a primary/unique key, a whole primary/unique key, and nothing but a primary/unique key.
In some RDBMS, for example SQL Server, you can reference a column with a unique index (not key) (see can we have a foreign key which is not a primary key in any other table?), but this is non-standard behavior.
Engine should be the same e.g. InnoDB
Datatype should be the same, and with same length. e.g. VARCHAR(20)
Collation Columns charset should be the same. e.g. utf8
Watchout: Even if your tables have same Collation, columns still could have different one.
Unique - Foreign key should refer to field that is unique (usually primary key) in the referenced table.
p_code should be a primary key in your price table:
create table price(
-> p_code char(1) not null,
-> p_description varchar(20),
-> p_rentfee decimal(2,2) not null,
-> p_dylatefee decimal(2,2),
-> PRIMARY KEY ( p_code ));
set p_code to be a key ,either set it to be a unique key or primary key.
The referenced column price.p_code must be unique (primary or unique key need to be created).
Both tables must be InnoDb tables, use ENGINE = INNODB in CREATE TABLE statement.
The data type for the child column must match the parent column exactly. For example, since price.p_code is an char(1), movie.p_code also needs to be an char(1) and price.p_code need be a Primary Key or need create a Index.
I want to create a table relationship with MYSQL PHPMYADMIN.
I have this Create table:
CREATE TABLE students(code_students int(8)not null AUTO_INCREMENT,
name_students varchar(25),
age_students int(3),
degree_program varchar(25),
code_advisor int(8)not null,
primary key(code_students, code_advisor)
);
and i want to make a create table named advise relationship between code_students, code_advisor.
Ok this is my tryout.
CREATE TABLE advise (
code_students int(8),
code_advisor int(8),
primary key(code_students, code_advisor),
foreign key(code_students)references students(code_students),
foreign key(code_advisor)references students(code_advisor)
);
mySQL says :
A FOREIGN KEY constraint that references a non-UNIQUE key is not standard SQL. It is an InnoDB extension to standard SQL
Try adding the UNIQUE keyword to your first table:
CREATE TABLE students(
code_students int(8)not null unique AUTO_INCREMENT,
name_students varchar(25),
age_students int(3),
degree_program varchar(25),
code_advisor int(8)not null unique,
primary key(code_students, code_advisor)
);
Check out this sqlFiddle and see how it works, then try removing the UNIQUE keywords and see that you get the same error that you mentioned. ( Hit the Build Schema button )
http://sqlfiddle.com/#!2/46b69
I'm making a database, with several tables, and so far I could make the tables, and update it with some demo data, but when a tried to make a new table for connecting two of my tables I just get this can't create table error whatever I did. Tried to rename the entities to an entirely different name maybe that's the problem but it wasn't.
CREATE DATABASE IF NOT EXISTS ETR;
CREATE TABLE Hallgato (
OktAzonosito INT(6) PRIMARY KEY,
EHAazonosito VARCHAR(11),
TeljesNev VARCHAR(50),
Szemelyazonosito INT(6),
AnyaNyelv VARCHAR(20),
VegzettsegSzint VARCHAR(25),
AnyjaNeve VARCHAR(35),
SzuletesiHely VARCHAR(30),
SzuletesiEv DATE,
Allampolgarsag VARCHAR(30),
Neme VARCHAR(5),
Adoazonosito INT(6),
TAJszam INT(6),
BankszamlaSzam INT(9)
) ENGINE = InnoDB;
CREATE TABLE OktAdat (
OktAzonosito INT(6),
NyelvVizsgaNyelve VARCHAR(15),
NyVegzesDatuma DATE,
NyIntezmeny VARCHAR(35),
EgyebVegzetteseg VARCHAR(15),
EgyVegzesDatuma DATE,
ID INT(5) PRIMARY KEY NOT NULL auto_increment,
FOREIGN KEY (OktAzonosito) REFERENCES Hallgato(OktAzonosito) ON DELETE CASCADE
) ENGINE = InnoDB;
CREATE TABLE Oktato (
TEHAazonosito VARCHAR(11) PRIMARY KEY,
Nev VARCHAR(50),
SzuletesiEv DATE,
Szakterulet VARCHAR(25),
Telefonszam INT(9),
Email VARCHAR(50)
) ENGINE = InnoDB;
CREATE TABLE Kurzus (
KurzusKod VARCHAR(8) PRIMARY KEY,
KurzusNev VARCHAR(30),
Idotartam INT(3),
EloadasHelye VARCHAR(25),
Tipus CHAR(3),
Vizsgatipus VARCHAR(7),
KreditErtek INT(2),
OktatoKod VARCHAR(11)
) ENGINE = InnoDB;
CREATE TABLE Terem (
Sorszam INT(3) PRIMARY KEY NOT NULL,
Epuletszam INT(1),
Kapacitas INT(3),
IRszam INT(4),
Utca CHAR(25),
Hazszam INT(2)
) ENGINE = InnoDB;
CREATE TABLE DiakKurz (
kd_id INT (5) PRIMARY KEY NOT NULL auto_increment,
DKKod VARCHAR(8),
EHA VARCHAR(11),
FOREIGN KEY (EHA) REFERENCES Hallgato(EHAazonosito) ON DELETE CASCADE,
FOREIGN KEY (DKKod) REFERENCES Kurzus(KurzusKod) ON DELETE CASCADE
) ENGINE = InnoDB;
The problem is with creating the DiakKurz table.
You'll have to set an INDEX for column EHAazonosito in table Hallgato
CREATE TABLE Hallgato (
OktAzonosito INT(6) PRIMARY KEY,
EHAazonosito VARCHAR(11), INDEX (EHAazonosito),
TeljesNev VARCHAR(50),
Szemelyazonosito INT(6),
AnyaNyelv VARCHAR(20),
VegzettsegSzint VARCHAR(25),
AnyjaNeve VARCHAR(35),
SzuletesiHely VARCHAR(30),
SzuletesiEv DATE,
Allampolgarsag VARCHAR(30),
Neme VARCHAR(5),
Adoazonosito INT(6),
TAJszam INT(6),
BankszamlaSzam INT(9)
) ENGINE = InnoDB;
This
FOREIGN KEY (EHA) REFERENCES Hallgato(EHAazonosito) ON DELETE CASCADE,
doesn't work because EHAazonosito is not the primary key of the Hallgato table. A foreign key can only reference a primary (or unique) key.
Btw: are you awary that the 6 in int(6) is not about limiting the values in the column?
It doesn't do anything. int(6) is the same as int. The 6 is only there to give client applications a hint(!) with how many digits the column should be displayed. It does not enforce anything at all.
Have you tried looking up the error?
If MySQL reports an error number 1005 from a CREATE TABLE statement,
and the error message refers to error 150, table creation failed
because a foreign key constraint was not correctly formed. Similarly,
if an ALTER TABLE fails and it refers to error 150, that means a
foreign key definition would be incorrectly formed for the altered
table. To display a detailed explanation of the most recent InnoDB
foreign key error in the server, issue SHOW ENGINE INNODB STATUS.
http://dev.mysql.com/doc/refman/5.5/en/innodb-foreign-key-constraints.html
it happened also to me after a dump of the whole schema, what helped was to wrap whole dump within disabled foreign key checks
SET foreign_key_checks=0;
...dump
SET foreign_key_checks=1;
I have two tables called lawyer & client.And they are the users of my site .so i have another table called users to store their username,password.But i have to add a foreign key which references clientID(from client table),lawyerId(from lawyer table) to User table.
how can i do this?
This sounds backwards; as lawyers & clients are users, their IDs should be referencing the ID in the users table, not the other way around (not every user is a lawyer and client; in fact, some could presumably be neither.) Or am I misreading what you wrote?
You cannot have multiple foreign key references for a single primary key column. Here is a simple example of how to re-design your schema to facilitate your requirements.
mysql> CREATE TABLE Users(
-> userID VARCHAR(20) NOT NULL PRIMARY KEY,
-> password VARCHAR(20) NOT NULL
-> )
-> ENGINE=INNODB;
mysql> CREATE TABLE Client(
-> userID VARCHAR(20) NOT NULL PRIMARY KEY,
-> FOREIGN KEY (userID) REFERENCES Users (userID)
-> )
-> ENGINE=INNODB;
mysql> CREATE TABLE Lawyer(
-> userID VARCHAR(20) NOT NULL PRIMARY KEY,
-> FOREIGN KEY (userID) REFERENCES Users (userID)
-> )
-> ENGINE=INNODB;