An Introduction to database system----The operation of relational operations

This week is reviewing the "Introduction to Database System" This course, see this section of relational operations, for the addition of operations is not very understanding.

Through Baidu, I do not feel that it is easier to understand the explanation.

Here, I would like to share my understanding, if there is a mistake in the place, but also want to see this blog to give you guidance.

First of all, I have extracted the definition of the addition operation:

The result of setting the relationship R divided by the relationship S is the relationship T, then T contains all the properties and their values in R but not in S, and all combinations of the tuple of T and the tuple of s are in R.

Just see this definition, to tell the truth confused, perhaps I understand the ability is inferior.

So combine the examples to understand the definition:

First, give the relationship R and the relationship s.

1, the definition says, T contains all the properties in R but not in S, so R contains the attributes {B,c,d} contained in {a,b,c},s}, it is obvious that the relationship T should contain only

a property---A;

2, and its value, only consider the value of the A property in relation R {A1,A2,A3,A4}, the element of the relationship T attribute a should be a subset of {A1,A2,A3,A4};

3, and the tuple of T and S tuple all combinations are in R, it is obvious that s need to consider only attribute B and attribute C, so

So the values that meet the definition requirements are only A1.

Thus, the following results: