What is the difference between linking two tables and then the PK is an FK in the other table, but the FK has not got the primary key option (so it does not have the gold key),
and having the PK in one table as a PK in another table?
Am I right to think that the second option is for a many-to-many relationship?
Thanks
FK means that any value in our table should be present in the foreign table.
Since the column in the foreign table should be declared as a PK or a UNIQUE key, this means it can be present only once in the foreign table.
PK means that any value in our table should be present only once.
Combined together, they mean that any value should be present only once both in our table and in foreign table.
This is a (0-1):1 relationship.
Related
I have a table person containing personal info, and I have another table person_contact to store contact information about that person (type shows if it's a phone record or email record, and record contains the actual phone number or email address).
I have designed the schema like this:
In person_contact I have declared pcont_id and person_id as PK while person_id is a FK referencing person.person_id. Do I need the PK pcont_id at all? When should I use a single PK in a one-to-many relationship and when is it better to use composite PK?
You don't need person_id as a part of your primary key in person_contact table. pcont_id should be the primary key if you have a one to many relationship between the two tables.
Do I need the PK pcont_id at all?
I suggest, it should be there and should be primary key of your table assuming you can have multiple contacts for one person.
If one person can have only one contact, in that case you don't need that table itself, you can store the data directly in the person table.
If you still want to store it separately then you don't need pcont_id column, your person_id column should be marked as primary key.
When should I use a single PK in a one-to-many relationship and when
is it better to use composite PK?
Composite primary key is used when you have a junction table/associative table to map a many-to-many relationship. In case of one-to-many relationship you don't need composite primary key with the foreign key column.
I am currently creating a database using SQL but I have found the need to use a foreign key to in 3 different fields in a single table.
I have CourseID1, CourseID2 and CourseID3 in students courses table. Each of those 3 fields need to be foreign and reference to the CourseID field in the course table which is a primary key.
Is this possible? how do I go about doing this?
Thank you
This is possible. You would do:
foreign key (CourseId1) references Courses(CourseId),
foreign key (CourseId2) references Courses(CourseId),
foreign key (CourseId3) references Courses(CourseId),
That said, you don't want to do this. Having multiple columns with numeric appendages usually means that you want an association/junction table. In this case, you want a table named something like StudentCourses which would have one row per student and per course that the student takes.
Can I have a foreign key with fewer number of columns of the primary/unique keys which it is referencing? (ORA-02270 error says no)
You can use fewer columns, but the columns you do use have to have a UNIQUE constraint on them.
And, of course, if you can put a UNIQUE constraint on just part of the primary key, then you have to ask yourself a question: "Why does my primary key have more columns than it needs in the first place?"
A FK isn't necessarily the PK of the other table. It can be, but it doesn't have to.
So you can use as FK as many columns as you need...
From oracle:
The foreign key in the child table will generally reference a primary key in the parent table.
Edit: the original question was about number of columns, not about using the incomplete PK as a FK.
The FK must reference something that is unique in the other table. So, you'll have to use any column that have a UNIQUE constraint, or the PK of the other table (since the PK must also be unique. For example, you select multiple columns from the table because each column, alone, is not unique, but the concatenation of multiple columns will be unique).
should I signal the foreign key in a database column name?
FKOrder vs. FK_Order vs. Order
The short answer is no - don't put "FK" in column names of foreign key columns. You can still signal the intent of the column though, here's how I do it:
Naming foreign key columns
It depends on your naming convention for the target of the FK. If you have Id, then I'd prepend the table name when creating FK columns.
Example 1:
For table User with PK Id and table Workitem with user ID FK, I'd call the column Workitem.UserId.
If there were more than one FK between the same tables, I'd make this clear in the column name:
Example 2:
For table User with PK Id and table Workitem with "assigned to user ID" and "created by user ID" FKs, I'd call the columns Workitem.CreatedByUserId and Workitem.AssignedToUserId.
If your naming convention for PKs is more like UserId, then you'd factor that into the above examples so as not to end up with UserUserId.
Naming foreign key constraints
This is mine:
FK_childtablename_[differentiator]parenttablename
The differentiator is used when there is more than one FK between the same two tables (e.g. CreatedByUserId and AssignedToUserId). Often I use the child table's column name for this.
Example 1:
Given tables: Workitem and User
Where User has CreatedByUserId and AssignedToUserId
Foreign key names are FK_Workitem_User_CreatedByUser and FK_Workitem_AssignedToUser
I use double-underscores if tables/columns have underscores in the name:
Example 2:
Given tables: work_item and user
Where user has created_by_user_id and assigned_to_user_id
Foreign key names are FK_work_item__created_by_user and FK_work_item__assigned_to_user
Is usual to name the foreign key fields with an ID (IDORDER, IDPERSON, ...), if you have a table called PERSONS and another CITIES, if one person is in certain city, CITIES has an IDCITY field (K), PERSONS has a IDPERSON (K), and other field IDCITY (FK).
Hope this answers your question. I mean, a foreign key is only foreign when it's in other table, but not in theirs. But it's a good practice to name always the same to the same fields, even if they are in other tables, as a foreign key.
You shouldn't.
If a column becomes a foreign key later, you will have to change the column name, breaking all the scripts that are using it.
If there are multiple foreign keys, you don't know which column belongs to which key, so the only information you gain is that the column is a foreign key, but you already know it by looking at the keys.
Usually I name the foreign key column the same as the primary key, so I know immediately where the key maps.
I normally use the same name as the referenced column in the table holding the FK.
Only if this is potentially confusing (or a column with this name already exists, say id), would I be more explicit. In such a case, adding the entity type name before the rest - say ProductId.
My style is slightly different:
fk_table_column
eg: fk_user_id that is foreign key to User table on id column. I do not use any capital latter.
I have a table named 'Patient_detail'. There are 4 columns in this table. They are:
patient_id primary key(not null)
visit_serial_ID primary key(not null)
examination
plan
Now, how can I insert multiple records in 'examination' and 'plan' column against the single primary key 'patient_id' and 'visit_serial_ID' of the table 'Patient_detail'?
The datatypes of the fields are as follows:
patient_id: number(11)
visit_serial_ID: number(5)
examination: varchar2(50)
plan: varchar2(50)
You can't (that's the whole point of a primary key - you have one row per value). You need to split this into a couple of tables, one for visits and one for patients. The visit table would have a foreign key relationship to the patient table's primary key column.
EDIT
You need to think about what actual real-life things you are representing, and how they interrelate. For each separate real-life thing, you would usually have a separate table. This allows the one-to-one, many-to-one, many-to-many relationships that you are after. For instance, you are representing a patient who has a visit, during which there are a number of examinations, each of which has a corresponding plan (is this correct?). You should therefore have a patient table containing patient data, a visit table, containing visit data, and an examination table containing examination data, and maybe even a plan table containing plan data.
The visit table has a many-to-one relationship with the patient table (a patient can visit a number of time). To achieve this, it should have a foreign key to the patient_id column. Similarly, the examination table has a many-to-one relationship with the visit table, so, again, it should have a foreign key to the visit table's primary key.
There are further ways that the can be separated. As Sascha said, look up normalisation.
You can do this but your primary key would cease to be a primary key and you data would be denormalized. The best way to do this would be to split this table into two separate tables like this:
Patients
----------
PatientId
Visits
----------
VisitSerialId
Examination
Pland
PatientId
Notice that the Visits table has a foreign key realtionship to the Patients table. This will allow you to have the 1:M relationship you are looking for.
You can't. That's what a primary key is - something that is unique to every row. You would need a separate table with its own primary key, and with a foreign key relationship to this table, in order to store multiple entries. This would then be a straightforward one-to-many relationship.
As darasd said you can't. What you're looking for is call normalization.