Conjunctive query

In database theory, conjunctive queries are a restricted form of first-order queries, which are said to be particularly easy to grasp for novice users. A large part of queries issued on relational databases can be written as conjunctive queries, and large parts of other first-order queries can be written as conjunctive queries.

Conjunctive queries are of the general form $(x_{1},\ldots ,x_{k}).\exists x_{k+1},\ldots x_{m}.A_{1}\wedge \ldots \wedge A_{r}$, with the variables $x_{1},\ldots ,x_{k}$ being called distinguished variables, and the variables $x_{1},\ldots ,x_{m}$ being called undistinguished variables.

Conjunctive queries can express a large part of queries, which are frequently issued on relational data bases. To give an example, imagine a relational data base for storing information about students, their address, the courses they visit and their gender. Finding all male students and their addresses who attend a course that is also attended by a female student is expressed by the following conjunctive query:

$\$hallo\$$

(student, address) . \exists(student2, course) . & &\\

 attends(student, course) & \wedge & gender(student, 'male') \wedge attends(student2, course) \wedge \\
 gender(student2, 'female') & \wedge & lives(student, address)