Geo-Databases: lecture 4 Complex Queries in SQL Prof. Dr. Thomas H. Kolbe Institute for Geodesy and Geoinformation Science Technische Universität Berlin Credits: This material is mostly an english translation of the course module no. 8 (‘Geo-Datenbanksysteme‘) of the open e-content platform www.geoinformation.net.

Complex queries in SQL 04/05/2019

Motivation In many cases interrelated information is spread across multiple tables, e.g. in order to avoid redundancy. Problem: How can we associate the professors with their lectures? 04/05/2019

Combination of tables The assignment of professors with their lectures follows a general principle: 1. Create all pairs of tuples of Professoren and Vorlesungen! 2. Filter the valid combinations from this tuple set. 04/05/2019

Cross Join All pairs of tuples are generated using the following query. The result is the so-called cross join (also called cartesian product) of the two tables. Schema of the resulting table: Concatenation of attributes SELECT * FROM Professoren CROSS JOIN Vorlesungen 04/05/2019

General Join Operator The general join allows to constrain the considered tuples using an arbitrary selection condition. The selection condition specifies which tuple pairs are valid. Wanted: Which professors teach which subjects? SELECT * FROM Professoren JOIN Vorlesungen ON Professoren.PersNr = Vorlesungen.PersNr Result: 04/05/2019

Natural Join Tests all columns having the same name from both tables on identity Schema of the resulting table: Like cross join, but attributes with the same name are only given once Example: Show the lecturer for each course! SELECT * FROM Professoren NATURAL JOIN Vorlesungen 04/05/2019

Problem of previous Join Operators In some situations we may be interested in joining two tables, where tuples, that have no corresponding partner should also be included in the result set. Example: Create a table with all professors and their lectures! Problem: The last tuple has no corresponding partner. Therefore it would not appear in the result set using the previously introduced join operators. Previously presented join operators do not fit the problem! 04/05/2019

Outer Join Left outer join: tuples of the left table, that have no corresponding partner are retained. For these tuples the attributes of the right table are set to NULL. Similarly the right outer join retaines all the tuples from the right table. The full outer join combines the functionality of both left and right outer join. Example: SELECT * FROM Professoren LEFT OUTER JOIN Vorlesungen ON Professoren.PersNr=Vorlesungen.PersNr 04/05/2019

Overview of Join Operators type from SQL 92 alternative syntax Cartesian product SELECT * FROM Professoren CROSS JOIN Vorlesungen SELECT * FROM Professoren, Vorlesungen Natural join SELECT * FROM Professoren NATURAL JOIN Vorlesungen SELECT attributelist (see below) FROM Professoren, Vorlesungen WHERE Professoren.PersNr= Vorlesungen.PersNr General SELECT * FROM Professoren JOIN Vorlesungen ON Professoren.PersNr= Vorlesungen.PersNr SELECT * FROM Professoren, Vorlesungen WHERE Professoren.PersNr= Vorlesungen.PersNr Right outer join SELECT * FROM Professoren RIGHT OUTER JOIN Vorlesungen No equivalence attributelist: all attributes from Professoren, non-redundant attributes from Vorlesung 04/05/2019

Nested Queries A nested query (or subquery) “SELECT…FROM…WHERE…“ is allowed: in the from clause (as a substitute for table names) as operand of the operators UNION, INTERSECT, EXCEPT, IN, EXISTS In case the result of the subquery is a scalar (that means a table comprising one row and usually one column) also: in a comparison operation as an attribute within the select clause Example: Formulation of a professors-lectures join with subquery: set operation subquery (=nested SQL-query) 04/05/2019

Existential Quantifier Existential quantifiers offer another way to perform complex queries operates on a table T returns the logical value TRUE if T is not empty Example: All professors that do not have lectures. negation existential quantifiers Remark: A FOR ALL -operator is not provided in SQL! 04/05/2019

Set Operations (1) Alternatively, the previous query can be formulated using set operations. The operators IN and NOT IN are used to test for membership within a set. Wanted: All professors that don‘t have lectures. all professors professors with lectures 04/05/2019

Set Operations (2) SQL provides the classical set operations: Remarks: R UNION S for set union R INTERSECT S for the intersection and R EXCEPT S for the set difference of two tables R and S Remarks: Schema compatibility of R and S is required The set operations eliminate duplicates! If duplicates should be retained: UNION ALL,… union operator all staff members all employees all professors 04/05/2019

References Jim Melton, Alan R. Simon, SQL 1999: Understanding Relational Language Components, Morgan Kaufmann Publishers, 2001 04/05/2019