Relationship To Other Query Languages
Conjunctive queries also correspond to select-project-join queries in relational algebra (i.e., relational algebra queries that do not use the operations union or difference) and to select-from-where queries in SQL in which the where-condition uses exclusively conjunctions of atomic equality conditions, i.e. conditions constructed from column names and constants using no comparison operators other than "=", combined using "and". Notably, this excludes the use of aggregation and subqueries. For example, the above query can be written as an SQL query of the conjunctive query fragment as
select l.student, l.address from attends a1, gender g1, attends a2, gender g2, lives l where a1.student=g1.student and a2.student=g2.student and l.student=g1.student and a1.course=a2.course and g1.gender='male' and g2.gender='female';
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