I created a table like this:
CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
PRIMARY KEY (P_Id)
)
but I can't understand the purpose of the primary key
PRIMARY KEY (P_Id)
in that table.
From MSDN:
A table typically has a column or combination of columns that contain
values that uniquely identify each row in the table. This column, or
columns, is called the primary key (PK) of the table and enforces the
entity integrity of the table. You can create a primary key by
defining a PRIMARY KEY constraint when you create or modify a table.
Related
I have this table:
CREATE TABLE cars_info.cars
(
id SERIAL,
owner_id INTEGER,
brand VARCHAR(50) NOT NULL,
model VARCHAR(50) NOT NULL,
color VARCHAR(50) NOT NULL,
register_number VARCHAR(50) NOT NULL,
created DATE NOT NULL,
PRIMARY KEY(id, brand, model, color, register_number, created),
CONSTRAINT fk_owner_id
FOREIGN KEY(owner_id)
REFERENCES persons_info.persons(id)
);
But when I tried create another table like this:
CREATE TABLE cars_info.violations
(
id SERIAL PRIMARY KEY,
car_id INTEGER NOT NULL,
message VARCHAR(100) NOT NULL,
active BOOLEAN NOT NULL,
CONSTRAINT fk_car_id
FOREIGN KEY(car_id)
REFERENCES cars_info.cars(id)
);
I get an error about that
Target external table "cars" does not have a unique constraint corresponding to the given keys
How can I fix that? I'm a beginner in SQL and don't know how to go about googling that
Your primary key definition for cars
PRIMARY KEY(id, brand, model, color, register_number, created)
makes no sense: The id column, being serial, is itself unique and it alone should be the primary key.
Delete your primary key definition and change the id column definition to:
id serial not null primary key
Unrelated, but best practice is to name table in the singular; name your tables car and violation rather than cars and violations
I'm new to SQL and there are a lot of things going on that I still don't seem to quite understand. I have the following table
CREATE TABLE Person
(
First_Name varchar(20) NOT NULL,
Name varchar(20) NOT NULL,
Address varchar(50) NOT NULL,
PRIMARY KEY (First_Name, Name, Address)
);
I know want to create another table that has the primary key from the table Person as foreign key and also as primary key:
CREATE TABLE Purchase
(
No_Installments int,
Rate int,
Person varchar(50) NOT NULL PRIMARY KEY,
CONSTRAINT PFK
FOREIGN KEY (Person) REFERENCES Person (First_Name, Name, Address)
);
For some reason this doesn't work and I get an error every time. I've already looked up the other threads here on stackoverflow, but they don't really seem to help me. What am I doing wrong?
If you have a compound PK made up from three columns, then any child table that wants to establish a foreign key relationship must ALSO have all those 3 columns and use all 3 columns to establish the FK relationship.
FK-PK relationship is an all or nothing proposal - you cannot reference only parts of a primary key - either you reference all columns - or you don't reference.
CREATE TABLE Purchase
(
No_Installments int,
Rate int,
Person varchar(50) NOT NULL PRIMARY KEY,
First_Name varchar(20) NOT NULL,
Name varchar(20) NOT NULL,
Address varchar(50) NOT NULL,
CONSTRAINT PFK
FOREIGN KEY (First_Name, Name, Address)
REFERENCES Person (First_Name, Name, Address)
);
Have an integer primary key, using identity, auto_increment, serial or whatever for your database:
CREATE TABLE Person (
PersonId int identity PRIMARY KEY
First_Name varchar(20) NOT NULL,
Name varchar(20) NOT NULL,
Address varchar(50) NOT NULL,
CONSTRAINT unq_person_3 UNIQUE (First_Name, Name, Address)
);
Then use the identity column for the reference:
CREATE TABLE Purchase (
PurchaseId int identity PRIMARY KEY,
No_Installments int,
Rate int,
PersonId int,
CONSTRAINT PFK
FOREIGN KEY (PersonId) REFERENCES Person (PersonId)
);
Notes:
You really don't want to have to deal with a composite primary key. Have you thought about what the joins will look like?
You don't want a primary key where the values are subject to change. What happens when someone changes his/her name? When someone moves?
Person should not be the primary key in Purchases. Are you only allowing someone to make one purchase?
As noted initially, how you generate such a column varies by database; identity happens to be the way that SQL Server does this.
You probably want to assign a unique ID to each person, not relying on their name to be unique or for an address to be required. That ID will be your Primary key and foreign key. Your purchase table should also have its own id for its primary key -- otherwise because primary keys must be unique, each person can only have one purchase.
CREATE TABLE Person (
id serial NOT NULL,
First_Name varchar(20) NOT NULL,
Name varchar(20) NOT NULL,
Address varchar(50) NOT NULL,
PRIMARY KEY (id));
CREATE TABLE Purchase (
id serial NOT NULL,
No_Installments int,
Rate int,
Person int NOT NULL,
FOREIGN KEY (Person) REFERENCES Person (id),
PRIMARY KEY (id));
I have a composite primary key that I would like to insert into another table.
create table courses_instructors
(
courseID int foreign key references Course(C_ID) not null,
instructorID int foreign key references Instructor(I_ID) not null,
primary key (courseID, instructorID), --coourseID and instructorID combined is the composite PK
courseTerm varchar(50) not null,
courseNumber int not null,
courseLocation varchar(50),
courseTime varchar(50),
courseMaxOccupancy int,
courseSeatAvailable int
)
create table courses_students
(
studentID int foreign key references student(S_ID) not null,
courseID int, -- foreign key -- I want this value to the be value that represents the composite PK from the courses_instructors
primary key(studentID, courseID), -- these 2 fields combined would make the composite PK, but with the courseID value I will be able to identify who is the instructor for a course and the other details from the course_instructor table
courseOutcome varchar(50)
)
All the course come from a course table which only contains the course name and the disciple along with a descrption. The course table has a primary key that identifies each course uniquely.
To refer composit primary key, Courses_Students table should be having both the columns CourseID and InstructorID.
And then
ALTER TABLE Courses_Students
ADD CONSTRAINT FK_Courses_Students
FOREIGN KEY(CourseID, InstructorID) REFERENCES Courses_Instructors(CourseID, InstructorID)
Or the table definitions should look like,
create table courses_instructors
(
courseID int foreign key references Course(C_ID) not null,
instructorID int foreign key references Instructor(I_ID) not null,
primary key (courseID, instructorID), --coourseID and instructorID combined is the composite PK
courseTerm varchar(50) not null,
courseNumber int not null,
courseLocation varchar(50),
courseTime varchar(50),
courseMaxOccupancy int,
courseSeatAvailable int
)
create table courses_students
(
studentID int foreign key references student(S_ID) not null,
courseID int,
instructorId int,
FOREIGN KEY(CourseID, InstructorID) REFERENCES Courses_Instructors(CourseID, InstructorID),
primary key(studentID, courseID, InstructorId),
courseOutcome varchar(50)
)
You can either refer two columns as already mentioned or add a surrogate key such as an identity column or GUID to the primary table and refer with to it - It usually performs better.
The course_instructors table is an intersection table implementing an m-m relationship between, as may be easily guessed, course entities and instructor entities. Almost invariably, I don't add a surrogate key to such a table for the simple reason that such a key would never be used. A typical user has a reference to one entity or the other and wishes to see all the other entities it relates to. Or sometimes the user has references to both entities and wished to get the details of their relationship.
This rule is not without exceptions, however, and your use case is just such an example. The table not just expresses a relationship between two entities but becomes an entity unto itself: a class offering. A student will select a class from a published schedule for the class and day/time desired. This will be identified by a class code number of some sort.
This code is what will be used to register for the desired class. In cases such as these, it makes sense to create a surrogate key for the intersection table -- which then becomes the class code printed in the catalog. Thus you would use this class code to refer to the relationship that defines the class offering and not use the composite key.
It looks like you already have such a composite key: the course_number field. You don't have it defined as unique but doesn't it uniquely identity the combination of course, instructor, location and time that makes up each class offering?
I have created PERSON table in Oracle by this SQL syntax:
Create table person
(
p_id int not null,
personName char(5) not null );
Then I am trying to create ORDERS table with the following syntax:
CREATE TABLE Orders
(
O_Id int NOT NULL,
OrderNo int NOT NULL,
P_Id int,
PRIMARY KEY (O_Id),
CONSTRAINT fk_PerOrders FOREIGN KEY (P_Id)
REFERENCES person(p_id) );
But I am getting the following error .
No matching unique or primary key for this column-list.
What is the problem ? How can I solve this ?
Add primary key to person table:
CREATE TABLE person(
p_id int not null,
personName char(5) not null,
PRIMARY KEY (p_ID)
);
SqlFiddleDemo
Foreign keys enforce a one-to-many relationship. That is, however many records there are in the dependent table they can only reference a single record in the parent table. This means the referenced column(s) in the parent table must be constrained by a PRIMARY or UNIQUE key.
The error message is telling you that there is no such constraint on person(p_id). And lo! if we compare the two DDL statements you have posted we can see that you have created a primary key for ORDERS but not for PERSON.
The solution is simple: constrain P_ID by adding a primary key to PERSON. You can either drop and re-create the table, or you can use an alter table statement to add a primary key.
You should add primary key to person table.
try this:
ALTER TABLE Person
ADD CONSTRAINT p_id PRIMARY KEY (p_id);
I have two tables Users & Stores
Users table primary key is combination of address & phone number (users enroll through a web page and I don't want the same user to enroll twice), the userId column is serial but not a primary key
In Stores table the column of ownerID is it's userID from the users table - but since it's not primary key the reference can not be set (although it's serial)
how can I achieve this result?
Referenced column need not to be a primary key.
A foreign key can reference columns that either are a primary key OR a unique constraint.
This can be done in this way:
CREATE TABLE Users(
address varchar(100),
phone_number varchar(20),
userid serial,
constraint pk primary key (address, phone_number ),
constraint userid_unq unique (userid)
);
create table Stores(
storeid int primary key,
ownerID integer,
constraint b_fk foreign key (ownerID)
references Users(userid)
);
You should make userid primary key, so you could reference it easily in foreign keys. To eliminate duplication of phone and address, you can define a unique constraint or unique index for those columns.