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.
Related
Is it possible, in MySQL, to create two unique indexes, together?
For example, there is a table that holds two IDs - customer ID and product ID.
I would like to create uniqueness for the pair only.
In simple words, combination of user and product can be present only once.
The same user with another products can be present as much as needed.
And the same product with another users can be present as much as needed.
But the pair itself to be unique. How?
This query actually has an OR relation. I am looking for an AND relation.
ALTER TABLE top ADD CONSTRAINT unique_pair UNIQUE (uid, pid)
This does what you describe:
ALTER TABLE top ADD CONSTRAINT unq_top_unique_pair UNIQUE (uid, pid);
As does:
CREATE UNIQUE INDEX unq_top_uid_pid ON TOP(uid, pid);
Both of these create a unique index, which is then used for enforcing the constraint.
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
Scenario:
User A and B executes at the same time select id from Product where id = ?, if the there are no results, both create a new product with given ID.
Problem:
This could lead to the creation of duplicate rows.
Question:
What are the possibles strategies to prevent that? I know that I can use compound/unique keys, to guarantee this, but are there any other strategies? Is there any SQL statement to lock query with same parameters?
You can use unique constraints
ALTER TABLE Persons ADD UNIQUE (P_Id)
or
ALTER TABLE Persons
ADD CONSTRAINT uc_PersonID UNIQUE (P_Id,LastName)
That way it would be impossible for a duplicate to be inserted.
Put a UNIQUE key on the field in question:
https://dev.mysql.com/doc/refman/5.0/en/constraint-primary-key.html
I have a table with name as listings and inside there I have a COLUMN namely as when some rows are deleted so the AUTO Incrmementation columns namely as "ID" goes into soemthing very bad values..Like missing values in between which I don't like and don't suit like a professional way..so therefore I want please if you people can guide me to how reset all ID columns values in rows on each INSERT or DELETE Query Exeution please..!
If you really want to find the lowest unused key value, don't use AUTO_INCREMENT at all, and manage your keys manually. However, this is NOT a recommended practice.
AS explained at Auto Increment after delete in MySQL
Primary autoincrement keys in database are used to uniquely identify a
given row and shouldn't be given any business meaning. So leave the
primary key as is and add another column called for example
courseOrder. Then when you delete a record from the database you may
want to send an additional UPDATE statement in order to decrement the
courseOrder column of all rows that have courseOrder greater than the
one you are currently deleting.
As a side note you should never modify the value of a primary key in a
relational database because there could be other tables that reference
it as a foreign key and modifying it might violate referential
constraints.
Well it is not recommended but you insisted , so use this to re order
By using something like:
ALTER TABLE table_name AUTO_INCREMENT = 1;
Two tables share a unique identifier 'id'. Both tables are meant to be joined by using 'id'.
Defining 'id' as an auto incrementing primary key in both tables may risk update inconsistencies.
Is there some general pattern to avoid such a situation or do I have to deal with updating table1 first and table2 by utilizing the last inserted id after (therefore not declaring id as auto inc in table2)?
First, if you use InnoDB table engine in MySQL you could use both transactions and foreign keys for data consistency.
Second, after the insert in the first table, you could get the last insert id (depending on the way you access the db) and use it as foreign key.
Eg
Table 1: Users: user_id, username
Table 2: User_Profiles: user_id, name, phone
In User_Profiles you don't need to define user_id as auto increment, but first insert a record in Users table and use the user_id for the User_Profiles record. If you do this in transaction, the Users record won't be seen outside of the transaction connection until it's completed, this way you guarantee that even if something bad happens after you insert the user, but before you have inserted the profile - there won't be messed up data.
You could also define that the user_id column in User_Profiles table is foreign key of Users table thus if someone deletes a record from the Users table, the database would automatically delete the one in User_Profiles. There are many other options - read more about that.
There is no problem with same column name 'id' in any number of tables.
Several persistence layer frameworks do it same way.
Just use aliases in your SQL to distinct your tables accordingly.
do I have to deal with updating table1 first and table2 by utilizing the last inserted id after (therefore not declaring id as auto inc in table2)?
Yes. And make id a foreign key so it can only exist in table2 if it already exists in table1.
Yes you do, and remember to wrap the operation in a transaction.