Duplicates and Set Operations in SQL

This document discusses the use of duplicates and set operations in SQL. It also covers aggregate functions, such as average, minimum, maximum, sum, and count.

Questions

• How does SQL handle duplicates?
• What are the set operations in SQL?
• What are aggregate functions?

Answers

• SQL can define how many copies of tuples appear in the result of a query. The default is to eliminate duplicates, but the `union all`, `intersect all`, and `except all` operators can be used to retain all duplicates.
• The set operations in SQL are union, intersect, and except. These operations correspond to the relational algebra operations `∪`, `∩`, and `−`.
• Aggregate functions operate on the multiset of values of a column of a relation, and return a value. The most common aggregate functions are average, minimum, maximum, sum, and count.