Relational algebra, as you may already know, is a procedural query language. It consists of a set of operations that take one or two relations as input and produce a new relation as their result. It forms the backbone of any relational database management system (DBMS), as it provides the foundational operations with which more complex queries can be built.
The fundamental operations of relational algebra are as follows:
Projection: This operation retrieves certain columns from the table and discards the other columns. For instance, if you just want to retrieve student names from the 'student' table, you can project the 'name' column.
Selection: As the name suggests, this operation selects tuples (rows) that satisfy a given predicate. For example, you can select students who were born after 1991.
Cartesian Product: This operation combines information from two relations. Combining 'student' and 'enrollment' tables would give us information about which student is enrolled in which course.
The code given executes these operations in Python. This might not be as straightforward as SQL's SELECT, FROM, WHERE approach. But understanding these operations will enable you to work with any relational database more effectively.
It's important to note that real-world databases can be massive, and operating on such large data sets would require more efficient algorithms and data structures, typically provided by DBMS. For now, understanding these operations on a theoretical level is crucial to grasp how relational databases work under the hood.
xxxxxxxxxxif __name__ == "__main__": # Our example tables student = [{'id':1, 'name':'John', 'birth_year':1991}, {'id':2, 'name':'Mike', 'birth_year':1995}, {'id':3, 'name':'Anna', 'birth_year':1993}] enrollment = [{'student_id':1, 'course_id':1}, {'student_id':2, 'course_id':2}, {'student_id':3, 'course_id':3}] #The Projection Operation projection = [x['name'] for x in student] print('Projection result: ', projection) #The Selection Operation selection = [x for x in student if x['birth_year']>1991] print('Selection result: ', selection) #The Cartesian Product Operation cartesian_product = [(x, y) for x in student for y in enrollment] print('Cartesian Product result: ', cartesian_product)