What is the difference between Select and Project Operations
PROJECT eliminates columns while SELECT eliminates rows.
relational-algebra
What is a Projection?
Projection is one of the basic operations of Relational Algebra. It takes a relation and a (possibly empty) list of attributes of that relation as input. It outputs a relation containing only the specified list of attributes with duplicate tuples removed. In other words the output must also be a relation. Example, if the relation … Read more
How can I find MAX with relational algebra?
Assuming you have a relation, A, with a single attribute, ‘a’ (reducing a more complex relation to this is a simple task in relational algebra, I’m sure you got this far), so now you want to find the maximum value in A. One way to do it is to find the cross product of A … Read more
Difference between a theta join, equijoin and natural join
A theta join allows for arbitrary comparison relationships (such as ≥). An equijoin is a theta join using the equality operator. A natural join is an equijoin on attributes that have the same name in each relationship. Additionally, a natural join removes the duplicate columns involved in the equality comparison so only 1 of each … Read more
What are projection and selection?
Exactly. Projection means choosing which columns (or expressions) the query shall return. Selection means which rows are to be returned. if the query is select a, b, c from foobar where x=3; then “a, b, c” is the projection part, “where x=3” the selection part.