Say I have a junction table to resolve a many to many relationship I have between two tables. My junction table also has its own 'Method' column to describe the relationship.
Normally, I would make a composite primary key of [a], [b] and [method] (the method needs to be part of what makes a row unique), but my problem is that [method] field can be NULL. Therefore I cannot add it to the primary key.
So what I've done is create a unique index:
ALTER TABLE A_B ADD UNIQUE INDEX `Unique` (`a`, `b`, `method`);
The table has no primary key. Is this an okay thing to do or do I need to do something differently?
Using a primary key is not the only way to ensure records uniqueness. There is a unique constraint you can implement to accomplish what is needed.
http://www.w3schools.com/sql/sql_unique.asp
Related
I am using mysql to create a database.
I have one base table named GP, with its Primary Key a composite Primary Key(SAT_ID, DATE).
I want to create multiple tables with the same Primary Key (SAT_ID,DATE), but would like to avoid data redundancy.
Is there a way to create a Primary Key for the children tables (for example ID INT NOT NULL AUTO_INCREMENT) that references the composite Primary Key (SAT_ID,DATE), so that I can avoid having the same composite Primary Key (SAT_ID,DATE) in every other table ?
I know the question can seem silly but there is something I don't understand about composite keys and data redundancy.
Thanks
#pepper's solution (suggested in the comments) works just fine:
You could modify your GP table to have an autoincrement ID as PK and
an unique index on (SAT_ID, DATE), then you can use ID as foreign
key in your other tables
Today I faced something, Someone defined foreign keys as primary keys in Pivot Table! Why is that? Does it effect on performance ? What are the benefits and use cases ? Should we define foreign keys as primary keys in pivot table in order to make them unique ??
Example of my question:
Why should define our many-to-many relationship foreign keys as primary keys ?
This is the proper way to build a junction table, aka bridge table, in a N-M relationship.
Rationale:
each order_id (resp user_id) should refer to a record in master table orders (resp users), so you do want a foreign key on each of these columns to enforce data integrity
each (order_id, user_id) tuple should occur not more than once in the bridge table: this can be enforced by using this columns tuple as a composite primary key, or by setting a unique constraint on it.
Of course, it is also possible to define an indenpendent primary key for the junction table (like an auto-incrementend pk), but that is mostly a matter of taste.
As far as I'm aware, you can only assign primary keys and unique columns to foreign keys... yet I have a table that has a primary key between two columns:
alter table NAME add constraint PK primary key(VALUE1, VALUE2)
I'm trying to make Value1 a foreign key in another table, but it's not recognizing it as a primary key or unique - obviously because the primary key is shared between two values... So what do I do from here? I'm pretty new to SQL syntax...
You are correct that you can only assign primary keys and unique columns to foreign keys. I am not much aware of the business requirement here but ideally, you should be having a third table which has the VALUE1 as a primary key. If not you should create one.
The main idea is that you can't link a foreign key to a value that can hold duplicates on the referenced table. So if your main table has a compound key (more than 1 column), linking the foreign key to one (or many but not all) of it's columns would be linking the table to more than one row (since that column might have duplicates by itself).
If you really need to establish the link between the two then you have a problem, either:
Your primary key isn't really 2 or more columns. You can read about normalizing your database (in standard normal forms) to solve this.
Your relationship between the tables isn't 1 to N (it's N to M). You can't add a foreign key, you will have to create a 3rd table with both primary keys to link them.
I'm using MySQL 5.7 with WB6.3 for my study project to design the ER diagram.
When I try to create many-to-many relationship. By default MySQL will mark the primary keys of both tables as primary key in the lookup table as well.
Is it really need to have primary keys from both table as primary key in the lookup table as well ?
Please see the table car_item in my diagram below I've removed primary key from both red(idcar,iditem). Please tell me if PK is needed for them
There should be a unique key for (car_idcar, item_list_iditem_list), but it doesn't necessarily have to be the primary key. This way, you ensure that you don't create duplicate relationships between the same rows in the two tables.
A table can only have one primary key, and the car_item table already has primary key id_car_item, so the foreign keys for the relation can't also be a primary key. But there can be an arbitrary number of unique keys.
Some purists might say that if there are two unique keys (a primary key is also a unique key), one of them is redundant. In your case, the id_car_item column may not really be necessary, as it's not common to refer to relations in other tables, the relation table is just used to join the other two tables. But this isn't always the case. For instance, a user table might have a unique username column (since you don't allow multiple users to have the same name), but also a userid primary key that's used as the foreign key in other tables (this allows renaming users without having to update all the foreign keys). Some database designers like to have an auto-increment column in every table, as it's useful as a reference in user interface applications.
It is common practice to have a composite primary key in a table that implements many-to-many. This key should consist of primary keys from both tables on each side of the many-to-many relationship.
Such approach guarantees data consistency and eliminates duplication.
In your case you should define a composite primary key for table car_item that would consist of two fields { car_idcar, item_list_iditem_list }
The column id_car_item is not needed and can be dropped, it was probably added automatically, as many RDBMS add surrogate PK by default.
Does a Join table (association table) have a primary key ? many to many relationship. I've seen some Join tables with a primary key and some without can someone please explain when you would have a primary key in a join table and why?
Thank you in advance;-)
In a pure 'join' or junction table all the fields will be part of the primary key. For example let's consider the following tables:
CREATE TABLE USERS
(ID_USER NUMBER PRIMARY KEY,
FIRST_NAME VARCHAR2(32),
LAST_NAME VARCHAR2(32));
CREATE TABLE ATTRIBUTES
(ID_ATTRIBUTE NUMBER PRIMARY KEY,
ATTRIBUTE_NAME VARCHAR2(64));
A junction table between these to allow many users to have many attributes would be
CREATE TABLE USER_ATTRIBUTES
(ID_USER NUMBER REFERENCES USERS(ID_USER),
ID_ATTRIBUTE NUMBER REFERENCES ATTRIBUTES(ID_ATTRIBUTE),
PRIMARY KEY(ID_USER, ID_ATTRIBUTE));
Sometimes you'll find the need to add a non-primary column to a junction table but I find this is relatively rare.
Share and enjoy.
All tables should have a primary key. :-)
You can either use a compound foreign key, or a blind integer key.
You would use the compound foreign key when there are no other elements in your association table.
You could use the blind integer key when the association table has elements of its own. The compound foreign key would be defined with two additional indexes.
It depends on the records you are associating. You can create a composite primary key on the id's of the associated records as long as you don't need multiple records per association.
However, its far more important that you make sure both these columns are indexed and have referential integrity defined.
Relationship tables frequently have three candidate keys, one of which need not be enforced with a constraint, and the choice of which key (if any) should be 'primary' is arbitrary.
Consider this example from Joe Celko:
CREATE TABLE Couples
(boy_name INTEGER NOT NULL UNIQUE -- nested key
REFERENCES Boys (boy_name),
girl_name INTEGER NOT NULL UNIQUE -- nested key,
REFERENCES Girls(girl_name),
PRIMARY KEY(boy_name, girl_name)); -- compound key
The "Couples" table lets you insert
these rows from the original set:
('Joe Celko', 'Brooke Shields')
('Alec Baldwin', 'Kim Bassinger')
Think about this table for a minute.
The PRIMARY KEY is now redundant. If
each boy appears only once in the
table and each girl appears only once
in the table, then each (boy_name,
girl_name) pair can appear only once.
From a theoretical viewpoint, I could
drop the compound key and make either
boy_name or girl_name the new primary
key, or I could just leave them as
candidate keys.
SQL products and theory do not always
match. Many products make the
assumption that the PRIMARY KEY is in
some way special in the data model and
will be the way to access the table
most of the time.
...HOWEVER I suspect you question is implying something more like, "Assuming I'm the kind of person who shuns natural keys in favour of artificial identifiers, should I add an artificial identifier to a table that is composed entirely of artificial identifiers referenced from other tables?"