I have these relationships:
User(uid:integer,uname:varchar), key is uid
Recipe(rid:integer,content:text), key is rid
Rating(rid:integer, uid:integer, rating:integer) , key is (uid,rid).
I built the table in the following way:
CREATE TABLE User(
uid INTEGER PRIMARY KEY ,
uname VARCHAR NOT NULL
);
CREATE TABLE Recipes(
rid INTEGER PRIMARY KEY,
content VARCHAR NOT NULL
);
Now for the Rating table: I want it to be impossible to insert a uid\rid that does not exist in User\Recipe.
My question is: which of the following is the correct way to do it? Or please suggest the correct way if none of them are correct. Moreover, I would really appreciate if someone could explain to me what is the difference between the two.
First:
CREATE TABLE Rating(
rid INTEGER,
uid INTEGER,
rating INTEGER CHECK (0<=rating and rating<=5) NOT NULL,
PRIMARY KEY(rid,uid),
FOREIGN KEY (rid) REFERENCES Recipes,
FOREIGN KEY (uid) REFERENCES User
);
Second:
CREATE TABLE Rating(
rid INTEGER REFERENCES Recipes,
uid INTEGER REFERENCES User,
rating INTEGER CHECK (0<=rating and rating<=5) NOT NULL,
PRIMARY KEY(rid,uid)
);
EDIT:
I think User is problematic as a name for a table so ignore the name.
Technically both versions are the same in Postgres. The docs for CREATE TABLE say so quite clearly:
There are two ways to define constraints: table constraints and column constraints. A column constraint is defined as part of a column definition. A table constraint definition is not tied to a particular column, and it can encompass more than one column. Every column constraint can also be written as a table constraint; a column constraint is only a notational convenience for use when the constraint only affects one column.
So when you have to reference a compound key a table constraint is the only way to go.
But for every other case I prefer the shortest and most concise form where I don't need to give names to stuff I'm not really interested in. So my version would be like this:
CREATE TABLE usr(
uid SERIAL PRIMARY KEY ,
uname TEXT NOT NULL
);
CREATE TABLE recipes(
rid SERIAL PRIMARY KEY,
content TEXT NOT NULL
);
CREATE TABLE rating(
rid INTEGER REFERENCES recipes,
uid INTEGER REFERENCES usr,
rating INTEGER NOT NULL CHECK (rating between 0 and 5),
PRIMARY KEY(rid,uid)
);
This is a SQL Server based solution, but the concept applies to most any RDBMS.
Like so:
CREATE TABLE Rating (
rid int NOT NULL,
uid int NOT NULL,
CONSTRAINT PK_Rating PRIMARY KEY (rid, uid)
);
ALTER TABLE Rating ADD CONSTRAINT FK_Rating_Recipies FOREIGN KEY(rid)
REFERENCES Recipies (rid);
ALTER TABLE Rating ADD CONSTRAINT FK_Rating_User FOREIGN KEY(uid)
REFERENCES User (uid);
This ensures that the values inside of Rating are only valid values inside of both the Users table and the Recipes table. Please note, in the Rating table I didn't include the other fields you had, just add those.
Assume in the users table you have 3 users: Joe, Bob and Bill respective ID's 1,2,3. And in the recipes table you had cookies, chicken pot pie, and pumpkin pie respective ID's are 1,2,3. Then inserting into Rating table will only allow for these values, the minute you enter 4 for a RID or a UID SQL throws an error and does not commit the transaction.
Try it yourself, its a good learning experience.
In Postgresql a correct way to implement these tables are:
CREATE SEQUENCE uid_seq;
CREATE SEQUENCE rid_seq;
CREATE TABLE User(
uid INTEGER PRIMARY KEY DEFAULT nextval('uid_seq'),
uname VARCHAR NOT NULL
);
CREATE TABLE Recipes(
rid INTEGER PRIMARY KEY DEFAULT nextval('rid_seq'),
content VARCHAR NOT NULL
);
CREATE TABLE Rating(
rid INTEGER NOT NULL REFERENCES Recipes(rid),
uid INTEGER NOT NULL REFERENCES User(uid),
rating INTEGER CHECK (0<=rating and rating<=5) NOT NULL,
PRIMARY KEY(rid,uid)
);
There is no real difference between the two options that you have written.
A simple (i.e. single-column) foreign key may be declared in-line with the column declaration or not. It's merely a question of style. A third way should be to omit foreign key declarations from the CREATE TABLE entirely and later add them using ALTER TABLE statements; done in a transaction (presumable along with all the other tables, constraints, etc) the table would never exist without its required constraints. Choose whichever you think is easiest fora human coder to read and understand i.e. is easiest to maintain.
EDIT: I overlooked the REFERENCES clause in the second version when I wrote my original answer. The two versions are identical in terms of referential integrity, there are just two ways of syntax to do this.
Related
Warning: possible duplicate
Hi,
I have the following table
CREATE TABLE names (
id uuid DEFAULT uuid_generate_v4(),
name VARCHAR(10) NOT NULL UNIQUE,
PRIMARY KEY (id)
);
Now, uuid basically says, use a unique VARCHAR(10) as an index. The question that I have is: can't I just use name as an id, since it's unique anyway and takes up the same amount of space or would this have negative impact on foreign keys?
I'm not new to SQL, I've just never came across this conflict and now I'm wondering if I've been using some concepts wrong the whole time.
I'm trying to create a procedure that inserts data into a table of registers but i don't want to repeat the second parameter, this is the table
CREATE TABLE Inscription
(
idClass INT references tb_class,
idStudent INT references tb_student,
)
The idea is that a student (idStudent) can register in various classes but not in the same class (idClass), I tried to add a unique constraint in the idStudent column but that only allows a student to register in one single class.
I always suggest that all tables have a numeric primary key. In addition, your foreign key references are not correct. And what you want to do is add a unique constraint.
The exact syntax depends on the database. The following is for SQL Server:
CREATE TABLE Inscriptions (
idInscription int identity(1, 1) primary key
idClass int references tb_classes(idClass),
idStudent int references tb_students(idStudnt)
unique (idClass, idStudent)
);
Notice that I name the tables as the plural of the entity, but the id using the singular.
The Inscriptions table probably wants other columns as well, such as the date/time of the inscription, the method, and other related information.
You are looking to create a constraint on your table that includes both columns idClass and idStudent.
Once that constraint is created, an attempt to insert duplicate class/student will result in an error being raised.
As your table does not seem to include a primary key, you would better make that constraint your primary key.
NB : you did not tell which RDBMS you are using hence cannot give you the exact syntax to use...
Your unique key needs to encompass both idClass and idStudent, so any particular combination cannot repeat itself.
I am trying to create my very first table in postgres, but when I execute this SQL:
create table public.automated_group_msg (
automated_group_msg_idx integer NOT NULL DEFAULT nextval ('automated_group_msg_idx'::regclass),
group_idx integer NOT NULL,
template_idx integer NOT NULL,
CONSTRAINT automated_group_msg_pkey PRIMARY KEY (automated_group_msg_idx),
CONSTRAINT automated_group_msg_group_idx_fkey FOREIGN KEY (group_idx)
REFERENCES public.groups (group_idx) MATCH SIMPLE
ON UPDATE CASCADE ON DELETE CASCADE,
CONSTRAINT automated_msg_template_idx_fkey FOREIGN KEY (template_idx)
REFERENCES public.template (template_idx) MATCH SIMPLE
ON UPDATE CASCADE ON DELETE CASCADE
)
WITH (
OIDS = FALSE
);
I get the following error:
ERROR: relation "automated_group_msg_idx" does not exist
Your error is (likely) because the sequence you're trying to use doesn't exist yet.
But you can create a sequence on the fly using this syntax:
create table public.automated_group_msg (
id serial primary key,
... -- other columns
)
Not directly related to your question, but naming columns with the table name in the name of the column is generally speaking an anti-pattern, especially for primary keys for which id is the industry standard. It also allows for app code refactoring using abstract classes whose id column is always id. It's crystal clear what automated_group_msg.id means and also crystal clear that automated_group_msg.automated_group_msg_id is a train wreck and contains redundant information. Attribute column names like customer.birth_date should also not be over-decorated as customer.customer_birth_date for the same reasons.
You just need to create the sequence before creating the table
CREATE SEQUENCE automated_group_msg_idx;
The relation "astronomers discover stars" is m:n, as an astronomer can discover many stars, and a star can as well be discovered from many astronomers.
In any case however, there will be a single date of discovery for every star. If there are many astronomers working on it, they should do it simultaneously, otherwise, only the first one gets the credits of the discovery.
So, in PostgreSQL, we have:
CREATE TABLE astronomer (
astronomer_id serial PRIMARY KEY,
astronomer_name text NOT NULL UNIQUE
);
CREATE TABLE star (
star_id serial PRIMARY KEY,
star_name text NOT NULL UNIQUE,
discovery_date date
);
CREATE TABLE discovered (
star_id integer NOT NULL REFERENCES star,
astronomer_id integer NOT NULL REFERENCES astronomer,
CONSTRAINT pk_discovered PRIMARY KEY (star_id, astronomer_id)
);
Now the trick question: I want to enforce that whenever there is a discovery date for a star, there will be at least one entry in the discovered table, and vice versa.
I could create a unique key (star_id, discovery_date) in table star, and then use that combination as a foreign key in table discovered instead of the star_id. That would solve half of the problem, but still leave it possible to have a discovered_date without astronomers for it.
I can't see any simple solution other than using a trigger to check, or only allowing data enter via a stored procedure that will at the same time insert a discovery_date and entries into the table discovered.
Any ideas?
Thank you!
I would just move the discovery_date column to the discovered table
CREATE TABLE astronomer (
astronomer_id serial PRIMARY KEY,
astronomer_name text NOT NULL UNIQUE
);
CREATE TABLE star (
star_id serial PRIMARY KEY,
star_name text NOT NULL UNIQUE
);
CREATE TABLE discovered (
star_id integer NOT NULL REFERENCES star,
astronomer_id integer NOT NULL REFERENCES astronomer,
discovery_date date not null,
CONSTRAINT pk_discovered PRIMARY KEY (star_id, astronomer_id)
);
Now you have the problem of multiple dates for the same star but as you say in the question only the first one(s) will get the credit.
Hoping someone can shed some light on this: Do lookup tables need their own ID?
For example, say I have:
Table users: user_id, username
Table categories: category_id, category_name
Table users_categories: user_id, category_id
Would each row in "users_categories" need an additional ID field? What would the primary key of said table be? Thanks.
You have a choice. The primary key can be either:
A new, otherwise meaningless INTEGER column.
A key made up of both user_id and category_id.
I prefer the first solution but I think you'll find a majority of programmers here prefer the second.
You could create a composite key that uses the both keys
Normally if there is no suitable key to be found in a table you want to create a either a composite key, made up of 2 or more fields,
ex:
Code below found here
CREATE TABLE topic_replies (
topic_id int unsigned not null,
id int unsigned not null auto_increment,
user_id int unsigned not null,
message text not null,
PRIMARY KEY(topic_id, id));
therefor in your case you could add code that does the following:
ALTER TABLE users_categories ADD PRIMARY KEY (user_id, category_id);
therefor once you want to reference a certain field all you would need is to pass the two PKs from your other table, however to link them they need to each be coded as a foreign key.
ALTER TABLE users_categories ADD CONSTRAINT fk_1 FOREIGN KEY (category_id) REFERENCES categories (category_id);
but if you want to create a new primary key in your users_categories table that is an option. Just know that its not always neccessary.
If your users_categories table has a unique primary key over (user_id, category_id), then - no, not necessarily.
Only if you
want to refer to single rows of that table from someplace else easily
have more than one equal user_id, category_id combination
you could benefit from a separate ID field.
Every table needs a primary key and unique ID in SQL no matter what. Just make it users_categories_id, you technically never have to use it but it has to be there.