1. Geo-Databases: lecture 3 Simple 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

2. Simple queries in SQL
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3. Motivation
The user wants to describe which information is to be extracted from a table. Example queries:
What are the names and cities of all students?
(select certain columns of a table)
Which students live in Berlin?
(select certain rows of a table)
How many students live in Potsdam?
(combine information of a table)
These queries refer to a single table
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4. Navigating vs. Set-oriented
The first, pre-relational DBMS offered simple navigating access. Database operations were dataset-oriented, that means in every operation a single dataset is processed.
Example:
FIND FIRST Student # find first entry in the student data sets
While not end-of-file do
begin
GET Student; S_Name; # copy name of the current set into ‘S_Name‘
… # operate on ‘S_Name‘
FIND NEXT Student; # go to next entry
end;
In contrast, SQL databases operate (tuple-)set-oriented. An SQL operation is declarative and normally processes sets of datasets.
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5. Projection
The cutback of a table in vertical direction is called projection.
Using this operator may produce duplicates!
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6. Selection (1)
The cutback of a table in horizontal direction is called selection.
The special character “ * “ (wildcard) can be used as a synonym for all columns.
The WHERE clause of a SELECT command is used to specify the selection condition.
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7. Selection (2)
The selection condition consists of atomic conditions linked by the (logical) Boolean operators AND, OR, NOT.
Example: We want to know all students from Bonn or Berlin.
Comparison operators in atomic conditions: =, <>, <, <=, >, >=
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8. Selection (3)
In case of character strings SQL allows for “fuzzy“ queries.
(comparison operator LIKE)
Question:
Which students live in a city that starts with “B“?
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9. Aggregate functions
Aggregate functions perform operations on tuple sets.
Question:
How many students are from Cologne?
Other important aggregate functions are:
SUM : compute sum Attention in case of NULL values!
AVG : compute average value
MIN / MAX : find minimum / maximum value
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10. Grouping
The group by function is suited to enhance the applicability of the aggregate functions.
Question:
How many students live in each city?
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11. Grouping (2)
The GROUP BY operator can be used to group the tuples of a table on the basis of the occuring values wrt. selected columns.
Example:
How many students from the same city do the same course?
SELECT Ort, Fach, COUNT(Fach) AS Anz
FROM Studenten
GROUP BY Ort, Fach
In case a GROUP BY command is used, the SELECT-list is subject to the following restrictions:
It may only comprise attributes that are contained in the GROUP BY-list, and
It may refer to other attributes solely via aggregate functions.
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12. Elimination of Duplicates
The Theory of the relational query languages does not allow for duplicates (relation as a mathematical set).
In SQL:
No automatic duplicate elimination
Forced duplicate elimination with the key word DISTINCT
Example: Overview of all cities that students live in.
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13. Sorting
In the theory of the relational query languages, the tuples of a relation do not have a specified order.
In SQL:
Sorting of the data sets during output can be forced using the ORDER BY clause.
Example: List of all students, sorted by residence and name
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14. References
Jim Melton, Alan R. Simon, SQL 1999: Understanding Relational Language Components, Morgan Kaufmann Publishers, 2001
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