Positive Relational Algebra
A constant relation over a set of attributes U is a positive relational algebra query with associated set of attributes U.
If R(U) is a relation schema in τ, the relation symbol R is a positive relational algebra query with associated set of attributes U.
If Q and Q′ are positive relational algebra queries with sets of attributes U and U′, respectively, the following are positive relational algebra queries:
The selection σA=B (Q) or σA=c (Q), with set of attributes U, where A and B are attributes in U, and c is a constant value
The projection π X...