SQL Tutorial - Duplicate Elimination using DISTINCT | Database Tutorial 5e
In this lecture of the databases course we learn how to use DISTINCT in SQL to eliminate duplicates from query results. However, the presence of unexpected duplicates in query results is often an indication of mistakes, e.g. missing join conditions. Therefore, before eliminating duplicates, it is important to understand why the duplicates occur. We will also learn how to determine whether a duplicate elimination (DISTINCT) is superfluous.
This is the databases online course given at the Vrije Universiteit Amsterdam by Jörg Endrullis. The course objective is to obtain a good understanding of relational database systems from the user perspective. This includes the ability to develop conceptual database models, as well as key concepts and skills in relational database theory and practice.
Among others, we treat in this course the following subjects:
(a) entity–relationship diagrams for the design of database schemata,
(b) the relational database model and integrity constraints,
(c) SQL (structured query language) for retrieving information from relational databases,
(d) database normalisation and functiona dependencies (reasoning about good database design),
(e) transactions and concurrency control.
The slides are based on work by Peter Boncz and Torsten Grust.
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