Conjunctive queries tutorial

Conjunctive queries are NP-complete with respect to combined complexity, while the data complexity of conjunctive queries is very low, in the parallel complexity class AC0, which is contained in LOGSPACE and thus in polynomial time.

When writing CQs in Datalog form, we can also choose to use equality:

Conjunctive Queries are the simplest form of queries that can be expressed over a database, but as we will see they have many interesting properties and a deep theory behind them.
There are many ways to define a Conjunctive Query.
We will start from a logical perspective, using Datalog notation.

For the study of the computational complexity of evaluating conjunctive queries, two problems have to be distinguished.
The first is the problem of evaluating a conjunctive query on a relational database where both the query and the database are considered part of the input.