What are projection and selection?

Question

What is the difference between projection and selection? Is it:

• Projection --> for selecting the columns of table; and
• Selection ---> to select the ro

Answer 1

Projections and Selections are two unary operations in Relational Algebra and has practical applications in RDBMS (relational database management systems).

In practical sense, yes Projection means selecting specific columns (attributes) from a table and Selection means filtering rows (tuples). Also, for a conventional table, Projection and Selection can be termed as vertical and horizontal slicing or filtering.

Wikipedia provides more formal definitions of these with examples and they can be good for further reading on relational algebra:

• Projection: https://en.wikipedia.org/wiki/Projection_(relational_algebra)
• Selection: https://en.wikipedia.org/wiki/Selection_(relational_algebra)
• Relational Algebra: https://en.wikipedia.org/wiki/Relational_algebra