i have two tables, employee details table(empno, emp_name , branch) and attendance table(date, Ot, attendence). I want to create a relation between two tables
Basically for this case you cannot. Your two tables must have something in common so that you can make one table related to the other one.
In this case, you probably need to add additional columns in the attendance table to indicate the attendence record belonging to which employee. For example, add "empno" column in the attendence table, and then set this "empno" as a foreign key of the employee details table by:
ALTER TABLE attendance ADD FOREIGN KEY (empno) REFERENCES employee_details(empno);
The SQL script may differ depending on which SQL database you're using.
Related
I have a CUSTOMERS table and a CONTACTS table the relation between them is one to many obviously.
also I have PROJECTS table and PROJECT_CUSTOMERS table with relation one to many and with relation one to one between CUSTOMERS and PROJECT_CUSTOMERS.
my problem is that I have a fifth table PROJECT_CONTACTS ....I can't figure which tables shall I refer to in this tables, currently I am refering to PROJECT_CUSTOMERS and CONTACTS table, is this correct or there is something better ?
Your title refers to "foreign keys" but your question just seems to be about what columns should go in what tables.
First, decide what situations can arise and what you want/need to say about them. (This will your tables, columns, candidate keys, foreign keys and constraints.)
Every table holds rows that make some predicate (statement template parameterized by column names) true. Your design seems to have:
CUSTOMERS(id, ...) -- ID identifies a customer and ...
CONTACTS(id, ...) -- ID identifies a contact and ...
PROJECTS(id, ...) -- ID identifies a project and ...
PROJECT_CUSTOMERS(pid, cust_id, ...) -- project PID has customer CUST_ID and ...
PROJECT_CONTACTS(pid, cont_id, cust_id)...)
-- project PID has contact CONT_ID and project pid has customer CUST_ID and ...
A foreign key has a table & column list referencing a table and column list that forms a candidate key. It says that lists of values in the first table appear as lists of values in the second table. Where that is so, declare a foreign key.
Its just a quick question :
I have user table and it has fields name, address and books_bought. books_bought is foreign key and its value is some PK from other table. Now after 1st insert, I will fill out all of this fields , but after second INSERT I want only to add a additional books_bought, so that am creating array of books_bought values?
You're doing it the wrong way around - this is a one-to-many relationship i.e. many books bought to one user. You need to have the foreign key on the many side of the relationship, so instead of having a foreign key to books_bought on the users table, add a foreign key to users on the books_bought table.
If you have a books table and a users table, then this is a many-to-many relationship and you will need a link table to go between them to hold the foreign keys.
You should not have more than one book in the books_bought cell because it will violate the atomicity constraint fo the database tables. You have to have a separate entry for each book_bought. This would cause a lot of redundant information as name, address would be repeated for each book bought by a specific person.
To solve this, you have to split the table into something like this:
R1(primary_key , name , address) and R2(foreign_key , books_bought)
Here foreign_key refers to primary_key of R1
I'm currently designing a database structure for our team's project. I have this very question in mind currently: Is it possible to have a foreign key act as a primary key on another table?
Here are some of the tables of our system's database design:
user_accounts
students
guidance_counselors
What I wanted to happen is that the user_accounts table should contain the IDs (supposedly the login credential to the system) and passwords of both the student users and guidance counselor users. In short, the primary keys of both the students and guidance_counselors table are also the foreign key from the user_accounts table. But I am not sure if it is allowed.
Another question is: a student_rec table also exists, which requires a student_number (which is the user_id in the user_accounts table) and a guidance_counsellor_id (which is also the user_id in the user_accounts) for each of its record. If both the IDs of a student and guidance counselor come from the user_accounts table, how would I design the student_rec table? And for future reference, how do I manually write it as an SQL code?
This has been bugging me and I can't find any specific or sure answer to my questions.
Of course. This is a common technique known as supertyping tables. As in your example, the idea is that one table contains a superset of entities and has common attributes describing a general entity, and other tables contain subsets of those entities with specific attributes. It's not unlike a simple class hierarchy in object-oriented design.
For your second question, one table can have two columns which are separately foreign keys to the same other table. When the database builds the query, it joins that other table twice. To illustrate in a SQL query (not sure about MySQL syntax, I haven't used it in a long time, so this is MS SQL syntax specifically), you would give that table two distinct aliases when selecting data. Something like this:
SELECT
student_accounts.name AS student_name,
counselor_accounts.name AS counselor_name
FROM
student_rec
INNER JOIN user_accounts AS student_accounts
ON student_rec.student_number = student_accounts.user_id
INNER JOIN user_accounts AS counselor_accounts
ON student_rec.guidance_counselor_id = counselor_accounts.user_id
This essentially takes the student_rec table and combines it with the user_accounts table twice, once on each column, and assigns two different aliases when combining them so as to tell them apart.
Yes, there should be no problem. Foreign keys and primary keys are orthogonal to each other, it's fine for a column or a set of columns to be both the primary key for that table (which requires them to be unique) and also to be associated with a primary key / unique constraint in another table.
in DB tables i've: Patient Table, PatientBasicInfomation Table, PatientImageFindings Table..
i've multiple Questions depend on this design..(note that i'm beginner in DB)
1) if i have for each Patient an ID.. so according to DB concepts both PatientBasicInfomation, PatientImageFindings should have this key (ID) as a foreign key?!
2) in the Patient Table i should reference to the PatientBasicInfomation, PatientImageFindings by using their private keys so they will be in Patient Table as a foreign keys?! am i thinking correct...
3) Now my big problem: i want to insert in PatientImageFindings Table a record but under condition ID + Date (where Date is an important Field in PatientImageFindings Table, i don't know if i should put it as a private key or not..), how could i do this insertion statement in my java class..(Insertion under conditions)
What you want to do is add a UNIQUE constraint across multiple columns.
This question provides an answer to do just that:
How do I specify unique constraint for multiple columns in MySQL?
alter table votes add unique index(user, email, address);
Unless you have another reason, you should enforce this at the database level and treat exceptions as they arise after attempted INSERTs.
If I have 2 accounts table: saving account and overdraft account, and then I have the table transaction in which column "from_account" can only have value equal to either saving account ID or overdraft account ID, how should I set the foreign key constraint? Is there any better schema design as the saving&overdraft account ID can collide.
Im using mysql btw :)
Thanx
A better schema design would be to have two distinct columns in the transaction table: One referencing the saving account table and the other referencing overdraft account table. Of course, these columns should be nullable and only one of them will point to an actual row in the other tables. But with this design you can enforce referential integrity constraints.
Another option would be to merge the two account tables into one, especially if they have many similar columns.
You can define an additional column ACCOUNT_TYPE and set a UNIQUE CONSTRAINT combining the FROM_ACCOUNT and the ACCOUNT_TYPE columns. You can also add a CHECK CONSTRAINT to the ACCOUNT_TYPE to limit its values to i.e 1,2.
Or you define one column for each referenced table plus a CHECK CONSTRAINT to ensure you never fill both at the same time.
And finally, consider merging both account tables, if there is no much difference between them.