I have the following tables category, Restaurant, and menu.
A restaurant can have more than one menu, and a restaurant can also have more than one category. Each menu obviously has some number of catagories, and each catagory can be a part of a number of menus.
Can I create a table that looks like this, and has 3 primary keys?
Category ID | Restaurant ID | Menu ID
1 (Appetizer) | 1 Chile's | 2 Dinner Menu
2 (Entree) | 1 Chile's | 2 Dinner Menu
3 (Dessert) | 1Chile's | 2 Dinner Menu
1 | 67 McDonalds | 3 Lunch Menu
1
Yes, you can make a primary key of the tuple (category_id, restaurant_id, menu_type_id). Better even, make a normal, integral primary key, and add another index on that tuple with uniqueness constraint. It's always good to have a fast, simple primary key, but you can certainly enforce uniqueness of the triple:
CREATE UNIQUE INDEX menu_index
ON menus (category_id, restaurant_id, menu_type_id)
Or you can define the index right in the table creation:
CREATE TABLE menus (menu_id INT AUTO_INCREMENT PRIMARY KEY,
category_id INT,
restaurant_id INT,
menu_type_id INT,
FOREIGN KEY (category_id) REFERENCES categories(category_id),
FOREIGN KEY (restaurant_id) REFERENCES restaurants(restaurant_id),
FOREIGN KEY (menu_type_id) REFERENCES menu_types(menu_type_id),
UNIQUE INDEX (category_id, restaurant_id, menu_type_id)
) ENGINE=InnoDB;
Related
I'm new to mysql and am trying to link two tables and am not sure how, i have a products table with a list of products in a restaurant, the column in this table are
ID(primary key)
name
price
The other table is called extras, this table contains extra things that you can add to your order but are optional, for when buying chicken you have an option of spicy and non spicy,
Some products have more than one extras, for instance a product can have the option of choosing three extras.
The extras table at the moment only has
ID(primary key)
name
not sure how to link the two or where to put foreign constraints.
UPDATE
Same extra may also belong to numerous products
A joining/linking table usually uses many-to-many relationships by joining the 2 parent tables/primary keys to allow many products to have many extras or no extras at all.
so for example:
eg:
Product IDs (primary keys) are: 1, 2, 3
Product names are: chicken wings, chicken breast, chicken fillet
Extras IDs (primary keys) are: 1, 2, 3
Extras names are: mild, medium, hot
Wings, breasts and fillet have the option of all three of the extras, so the product_extras table would end up looking something like this:
product_extras_id | product_id | extras_id
------------------------------------------
1 | 1 | 1
2 | 1 | 2
3 | 1 | 3
4 | 2 | 1
5 | 2 | 2
6 | 2 | 3
7 | 3 | 1
8 | 3 | 2
9 | 3 | 3
With products having many extras and extras applying to many products this is actually a many to many relationship.
create table products(ID int auto_increment Primary Key,
Name varchar(50),
Price decimal(6,2));
create table extras(ID int auto_increment Primary Key,
Name varchar(50));
create table product_extras(Product int Not Null,
Extra int Not Null,
FOREIGN KEY(Product) References products(ID) ON DELETE CASCADE,
FOREIGN KEY(Extra) References extras(ID) ON DELETE CASCADE);
Something akin to this should work for you, though you may want to change the datatypes based off preference and what data actually needs to go in there.
In your case one product may have many extras(1 to many) and many products may be having same extra thing(many to 1). Thus, this is a many to many relationship and for such relations we need 3 tables.
CREATE TABLE products (
id INT PRIMARY KEY AUTO_INCREMENT,
name VARCHAR(50),
price DECIMAL(10.2)
);
CREATE TABLE extras (
id INT PRIMARY KEY AUTO_INCREMENT,
name VARCHAR(50)
);
CREATE TABLE products_extras (
id INT PRIMARY KEY AUTO_INCREMENT,
pro_id INT,
FOREIGN KEY(pro_id) REFERENCES products(id),
ext_id INT,
FOREIGN KEY(ext_id) REFERENCES extras(id)
);
I have two mysql tables. The first one is created using the following code:
create table project(
project_id int not null auto_increment,
project_name varchar(30)
primary key(project_id));
The second table:
create table matches(
match_id int not null auto_increment,
match_name varchar(30),
project_id int(4) foreign key (project_id) references projects(project_id));
Both these commands works fine. I want to add the project_name column from the first table to the second table. I tried using
alter table projects drop primary key, add primary key(project_id, project_name);
and then
alter table matches add column project_name varchar(30), add foreign key (project_name) references projects(project_name);
But got the following error:
ERROR 1005 (HY000): Can't create table 'matches.#sql-55e_311' (errno: 150)
How do i include both the columns from the first table into the second table.
The current structure of my second table is as follows:
+------------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+----------------+
| match_id | int(11) | NO | PRI | NULL | auto_increment |
| match_name | varchar(30) | YES | | NULL | |
| project_id | int(4) | NO | MUL | NULL | |
+------------+-------------+------+-----+---------+----------------+
I want to add the project_name as the fourth column in my second table.
To use a compound Primary Key as Foreign Key, you'll have to add the
same number of columns (that compose the PK) with same datatypes to
the child table and then use the combination of these columns in the
FOREIGN KEY definition.
see related post here https://stackoverflow.com/a/10566463/4904726
so try this way
alter table matches add foreign key (project_id, project_name) references projects(project_id, project_name);
Do you understand what a FK constraint says? It says a list of values for some columns in a table must appear as a list of values for some columns forming a (declared) PK or UNIQUE NOT NULL in the referenced table. So what you are writing doesn't make sense. If you wanted a PK (project_id, project_name) then that should also be the form of your FK.
But (project_id, project_name) is not two 1-column PKs, it is one 2-column PK, and it is probably not what you want since presumably in projects it isn't just pairs that are unique it is each column. Presumably you want two 1-column PKs and two one-column FKs, one referencing each PK.
If projects project_id was NOT NULL you could write:
alter table projects add primary key(project_name);
alter table matches add column project_name varchar(30),
add foreign key (project_name) references projects(project_name);
But if project_name can be NULL in projects then you cannot make it a PK and you cannot sensibly have a FK to it. You can make it UNIQUE. Unfortunately MySQL lets you write such a FK declaration to a non-NULL UNIQUE column while it also tells you not to do it:
The handling of foreign key references to non-unique keys or keys that contain NULL values is not well defined for operations such as UPDATE or DELETE CASCADE. You are advised to use foreign keys that reference only keys that are both UNIQUE (or PRIMARY) and NOT NULL.
So if you want projects project_name to be nullable then should declare it UNIQUE but you should enforce the logical matches project_name match with a trigger.
I'm normalizing at the moment in 2NF and I'm quite confused, Here is what i have so far in 0 NF Colour is a multi valued attribute of Product
Product = Product_ID (PK)
Colour{Colour ID, Colour_Name}
Then in 1NF colour gets its own table so its
Colour=
+Colour_ID (PK)
+Product_ID (FK)
+Colour_Name
in 2NF its then changed to 2 Separate Tables
Colour =
+Colour_ID (PK)
+Product_ID(FK)
and a new table
Product_Colour =
+Colour_ID
+Colour_Name
I'm i've finished out the ERD and doing the mySQL but are they (Colour_ID) both primary keys respectively or is one a foreign key, but if one is then a foreign key do i need a primary key for that table?
Thanks,
You definitely need a colour table:
colour_id INT PRIMARY KEY
colour_name VARCHAR(255) # or other appropriate data type
Then, if you know that each product will always have exactly two colours, your product table might look like this:
product_id INT PRIMARY KEY,
color1 INT NOT NULL FOREIGN KEY (colour.colour_id),
color2 INT NOT NULL FOREIGN KEY (colour.colour_id)
You could drop the second NOT NULL in case products will always have one or two colours.
The more flexible approach would be to use a third table, allowing you to add an arbitrary number of colours to each product:
product table:
product_id INT PRIMARY KEY
product_colors table:
product_id INT FOREIGN KEY (product.product_id)
colour_id INT FOREIGN KEY (colour.colour_id)
PRIMARY KEY(product_id, colour_id)
So, in this case, both id fields are foreign keys on their own while together forming the primary key so it's impossible to add the same colour to the same product twice.
(pseudo SQL syntax used)
I visualize this:
Product = Product_ID (PK), Colour_ID (FK)
Colour = Colour_ID (PK), Colour_Name
________________________ _________________________
| Product_ID | Colour_ID | | Colour_ID | Colour_Name |
-------------------------- ---------------------------
|____________|___________| |___________|_____________|
So I understand how to create foreign keys and I know what is the purpose of the FK. But I have a problem in understanding How to use them. I asked a question regarding Foreign keys HERE(Click link)
Here is what I made:
CREATE TABLE user(
id INT(11) NOT NULL AUTO_INCREMENT,
username VARCHAR(50) NOT NULL,
password VARCHAR(20) NOT NULL,
PRIMARY KEY (id)
);
CREATE TABLE items(
i_id INT(11) NOT NULL AUTO_INCREMENT,
name TINYTEXT NOT NULL,
price DECIMAL(8,2) NOT NULL,
PRIMARY KEY (i_id)
);
CREATE TABLE user_purchase(
i_id INT(11) NOT NULL,
name TINYTEXT NOT NULL,
id INT(11) NOT NULL,
FOREIGN KEY (i_id) REFERENCES items(i_id),
FOREIGN KEY (name) REFERENCES items(name),
FOREIGN KEY (id) REFERENCES user(id)
);
Now my question is how do I make the most out of this using PHP? From the link above, people have suggested that it's good to use only one foreign key in the user_purchase table, but what if I want several columns? Why don't we use several foreign keys for different columns of the same table?
I am using mysql and php. I would appreciate it if you could show some examples of how you use PHP with the tables which have foreign keys to get get information using MYSQL commands. I really need a thorough explanation.
I also need to understand the terms Normalization and Denormalization. I would appreciate if you could give some links which explain these terms in great detail with examples or if you have any suggestion for some great books for beginners in database design, implementation, etc, I would really appreciate.
Thanks a lot.
Foreign key columns/constraints disambiguation
So I understand how to create foreign keys and I know what is the
purpose of the FK. But I have a problem in understanding How to use
them.
Assuming you are referring to the foreign key constraints, the short answer would be you just don't use them.
And here comes the long one:
We are accustomed to refer to columns being foreign keys to other tables. Especially during the normalization process, phrases like "user_purchase.i_id is a foreign key to the items table" would be very common. While that's a perfectly valid way to describe the relationship, it can get a little fuzzy when we reach the implementation phase.
Suppose you have created your tables without the FOREIGN KEY clauses:
CREATE TABLE user(
id INT(11) NOT NULL AUTO_INCREMENT,
username VARCHAR(50) NOT NULL,
password VARCHAR(20) NOT NULL,
PRIMARY KEY (id)
);
CREATE TABLE items(
i_id INT(11) NOT NULL AUTO_INCREMENT,
name TINYTEXT NOT NULL,
price DECIMAL(8,2) NOT NULL,
PRIMARY KEY (i_id)
);
CREATE TABLE user_purchase(
i_id INT(11) NOT NULL,
name TINYTEXT NOT NULL,
id INT(11) NOT NULL,
);
Notice that, relation-wise, the foreign key columns are still implemented. There's a column that references the user table (id) and another one that references the items table (i_id) -- let's put the name column aside for a moment. Consider the following data:
user user_purchase items
| id username | | id i_id | | i_id name price |
| 23 john | | 55 10 | | 10 chocolate bar 3.42 |
| 55 mary | | 70 10 | | 33 mobile phone 82.11 |
| 70 fred | | 70 33 | | 54 toothpaste 8.67 |
| 55 10 | | 26 toy car 6.00 |
| 70 26 |
The relation is there. It is implemented by means of the user_purchase table, which holds information as to who bought what. If we were to query the database for a relevant report, we would do:
select * from user_purchase p
join user u on (p.id=u.id)
join items i on (p.i_id=i.i_id)
And that's how we use the relation and the foreign key columns involved.
Now, what if we do:
insert into user_purchase (id,i_id) values (23,99)
Apparently, this is an invalid entry. Although there is a user with id=23, there's no item with i_id=99. The RDBMS would allow that to happen, because it doesn't know any better. Yet.
That's where foreign key constraints come into play. By specifying FOREIGN KEY (i_id) REFERENCES items(i_id) in the user_purchase table definition, we essentially give the RDBMS a rule to follow: entries with i_id values that are not contained in the items.i_id column are not acceptable. In other words, while a foreign key column implements the reference, a foreign key constraint enforces the referential integrity.
Note, however, that the above select wouldn't change, just because you defined a FK constraint. Thus, you don't use FK constraints, the RDBMS does, in order to protect your data.
Redundancies
...what if I want several columns? Why don't we use several foreign
keys for different columns of the same table?
Ask yourself: Why would you want that? If the two foreign keys are to serve the same purpose, the redundancy will eventually get you in trouble. Consider the following data:
user_purchase items
| id i_id name | | i_id name price |
| 55 10 chocolate bar | | 10 chocolate bar 3.42 |
| 70 10 chocolate bar | | 33 mobile phone 82.11 |
| 70 33 mobile phone | | 54 toothpaste 8.67 |
| 55 10 toothpaste | | 26 toy car 6.00 |
| 70 26 toy car |
What's wrong with this picture? Did user 55 buy two chocolate bars, or a chocolate bar and a toothpaste? This kind of ambiguity can lead to a lot of effort to keep data in-sync, which would be unnecessary if we just kept one of the foreign keys. In fact, why not drop the name column altogether, since it is implied by the relation.
Of course, we could resolve this by implementing a composite foreign key, by setting PRIMARY KEY(i_id,name) for the items table (or defining an extra UNIQUE(i_id,name) index, it doesn't realy matter) and then setting a FOREIGN KEY(i_id,name) REFERENCES items(i_id,name). This way, only (i_id,name) couples that exist in the items table would be valid for user_purchases. Apart from the fact that you would still have one foreign key, this approach is totally unnecessary, provided that the i_id column is already enough to identify an item (can't say the same for the name column...).
However, there's no rule against using multiple foreign keys to a table. In fact, there are circumstances that demand such an approach. Consider a person(id,name) table and a parent(person,father,mother) one, with the following data:
person parent
| id name | | person father mother |
| 14 John | | 21 14 59 |
| 43 Jane | | 14 76 43 |
| 21 Mike |
| 76 Frank |
| 59 Mary |
Obviously, all three columns of the parent table are foreign keys to person. Not for the same relation, though, but for three different ones: Since a person's parents are persons too, the two corresponding columns must reference the same table person does. Note, however, that the three fields not only can but also have to refer different persons in the same parent row, since noone is his own parent and noone's father is his mother as well.
Foreign keys are used in joins. For instance, if you want to know the usernames that purchased a particular item, you would write:
select u.username
from items i
join user_purchase up on i.i_id = up.i_id
join user u on u.id = up.id
where i.name = "Some product name"
They may also be used by the database engine itself. It can detect if you create a row in user_purchase whose id or i_id column doesn't match anything in the referenced column in the other table.
You should not replicate the name column in the user_purchase table. The name is just an attribute of the item, it's not specific to any particular purchase. If you need to get the name of the item that was purchased, join with the items table.
Instead of reading so many links, just try to implement this in any simple project. I'm just explaining how we gonna use the above tables.
Suppose you 3 users in user table and 5 items in items table.
user table
id | username | password
1 abc 123
2 def 456
3 qwe 987
items table
i_id | name | price
1 item 1 6
2 item 2 8
3 item 3 11
4 item 4 3
5 item 5 14
your user_purchase table look like this
CREATE TABLE user_purchase(
i_id INT(11) NOT NULL,
id INT(11) NOT NULL,
FOREIGN KEY (i_id) REFERENCES items(i_id),
FOREIGN KEY (id) REFERENCES user(id)
);
There is no need of item name again in this table. So I have removed.
i_id | id
1 1
1 2
2 2
3 3
In the above table we will get, user 1 has purchased item 1, user 2 has purchased item 1,item 2 and user 3 has purchased item 3.
This is how normalization works. You can use MySQL JOIN for getting user name and item details
SELECT B.user_name,C.name AS item_name,C.price
FROM user_purchase A
JOIN user B ON A.id = B.id
JOIN items C ON A.i_id = C.i_id
Here is foreign key use to join
A.id = B.id
A.i_id = C.i_id
You treet tables with foreign keys in php the same way, as if they had no foreign keys. Foreign keys are defined in the database and have (almost) nothing to do with php. The only thing you have to do in php is reacting to potential errors that can be returned by sql queries, which brake the foreign key constraint (typically DELETE queries).
And for your database schema, you should drop column "name" from "table user_purchase". It is redundat.
Just looking at the normalization/de-normalization point:
Normalization is a process of trying to remove redundancy in your database - in your example the name field in user_purchase is redundant - I can find out the name of the item by looking it up in the items table using i_id.
So if we were to look at normalizing user_purchase we'd probably remove the name field and use a JOIN to retrieve that when we needed it. This would, of course, also mean we don't need the second FOREIGN KEY reference to items.
De-normalization is basically going the opposite way - adding redundancy - usually done for performance reasons.
However, in your example you might also consider de-normalization for business reasons too. For example you might decide it is important to store the product name as it was when the user actually purchased it (rather than what it's called now) - just in case you need to be able to re-print an invoice for example. However even in this case you wouldn't want the FOREIGN KEY back to items (as it would "break" if the product was re-named).
Say I have
ID | PRODUCT
1 | Apples
1 | Oranges
1 | Bananas
2 | Walnuts
2 | Almonds
3 | Steak
3 | Chicken
Is this possible to have this type of setup in MySQL? I created a test table, and made an ID column with primary index and auto incrementing. When I try to insert a couple rows all having the same ID, mysql returns an error.
Is this possible to do in mysql?
How did the duplicate records (ID) INSERTED when you set ID as Primary Key? Basically, it will not. Primary Keys are UNIQUE. If you want records to be like that, make another column which served as your primary key
CREATE TABLE sampleTable
(
ID INT NOT NULL AUTO_INCREMENT,
GROUP_ID INT NOT NULL,
PRODUCT VARCHAR(25),
CONSTRAINT pk_name PRIMARY KEY (ID),
CONSTRAINT uq_name UNIQUE (GROUP_ID, PRODUCT)
)
a UNIQUE constraint was added so to avoid duplicated rows.
That is possible, but it is not normalized as you have a repeating primary key. Primary keys must be unique, which means 1 can only occur once. If you have custom ID's for each product, then either use a compound primary key (id, product) or a surrogate key. A surrogate key would be an auto incrementing column that uniquely identifies the row. Your table would then look like this:
CREATE TABLE fruits (
auto_id int AUTO_INCREMENT PRIMARY KEY,
id int,
product varchar(15))
You mustn't create a primary key for the ID column, as this implies that it's unique.
Just take a normal index for your example.
On the other hand, in most cases, it makes sense to have a primary key, so I'd recommend adding a 3rd row (GENRE?) with a normal INDEX on it and leaving the primary key as it is.
When inserting data, just insert the GENRE and the PRODUCT, the ID will be automatically filled.