database self doubt

Question

what does this line means -  

Let r and s be two relations over the relation schemes R and S respectively and R has attribute A.

Comments

let R(A,B,C,D)  and S(L,M,N,O)  are two schema....and r,s are the instances of R&S respectively..i.e

r:->   A B C  D      s:-> L M N O

        1  5  7  8             1 4 5 7 8

        2  .........              2..........

        .............             ..............

Answer 1

Relation Schema means how a relation is defined. 

For example, a STUDENT schema will be something like this: $(rollno: integer, name: string, dob: date)$

or we can also remove the practicalities like the domain specification, and simply write:

$STUDENT (rollno, name, dob)$

A schema is usually denoted by uppercase letters.

A relation, on the other hand, is the actual table that we have in the database, it will have some values in it.

For example, a relation student, which can be defined using the STUDENT schema is written like, $student(STUDENT)$.

A relation is usually written in lowercase.

Sometimes, schema name is not mentioned, and is omitted. like $student (rollno, name, dob)$

In your example, the database has two relations(tables) r and s. The schema used for r is R, and for s is S. And the schema R will have an attribute A, along with other attributes. Just like $rollno$ is an attribute for schema $STUDENT$.

Comments

thanks as always