How do you understand traditional set operations in a database? Can't read it?

Traditional relational operations on mathematical sets and database-specific relational operations

The database itself is a two-dimensional table, which is equivalent to a mathematical set.

There are times when you need to perform operations on two tables, for example, to find the same part of two tables, and the mechanism for doing this is the traditional "intersection" of set operations.

There are times when you need the table itself to perform calculations, for example, just to show the value of a column in the table, this is a specialized operation of the relationship "projection".