Databases: relational algebra projection

[atrus_ovis]

Homework Statement

Say you have two tables, S(A,B) , R(A,B), where A is the key for both.
Lets say there is a tuple x0,y1 in S, and a tuple x0,y2 in R
You use then this projection
π(S$$\cup$$R)

My question is,do the rules of primary key constraints apply to a projection?
Will the result include
...
x0,y1
x0,y2
...

or is there going to be a problem, because of non unique primary keys in column one?

The Attempt at a Solution

N/A

 

[KataKoniK]

Hello,

The rules of primary key constraints do apply to a projection. The result of the projection will include the tuples x0,y1 and x0,y2, as they are both unique in the union of S and R. However, if there are any duplicate values in column A, the projection will only include one instance of that value. This is because the primary key constraint ensures that each value in the key column is unique, and a projection will not include duplicate values.

In terms of potential problems, it is important to note that a projection can only be performed on attributes that are included in the key. So if there are any non-unique values in the key column, they will not be included in the projection. Additionally, if the key column is not included in the projection, then the result will not be unique and may cause issues with further operations.

I hope this helps clarify any concerns you may have. Let me know if you have any further questions.