Projection in SQL - correct version

Student StudentId is enrolled on some course.

SELECT DISTINCT StudentId FROM IS_ENROLLED_ON

In more complicated examples it is sometimes quite difficult to tell whether omission of DISTINCT would give rise to duplicate rows. It might therefore seem good advice to always write DISTINCT. Indeed, I would certainly advocate such practice to students having to write SQL expressions in solutions to questions in exam papers, for example, but if it were followed blindly in commercial systems, then many queries would run very much slower than need be because typical SQL implementations have little or nothing in the way of built-in intelligence to recognize cases where duplicate rows cannot possibly arise. Although such intelligence is quite feasible within acceptable limits (and was used in Business System 12, for example), the inclusion of DISTINCT allows SQL implementations to place the responsibility for duplicate elimination on the user.

Posted Date: 1/18/2013 5:10:13 AM | Location : United States

Ask an Expert

Related Discussions:- Projection in sql - correct version, Assignment Help, Ask Question on Projection in sql - correct version, Get Answer, Expert's Help, Projection in sql - correct version Discussions

Write discussion on Projection in sql - correct version
Your posts are moderated

Write your message here..

Related Questions

Keyword and parameter description in pl sql, Keyword and Parameter Descript... Keyword and Parameter Description: label_name: This is an undeclared identifier which optionally labels the PL/SQL block. When used, label_name should be enclosed by the do

Explicitly specifying the join condition - sql, Explicitly specifying the j... Explicitly specifying the join condition - SQL SELECT * FROM IS_CALLED JOIN IS_ENROLLED_ON ON ( IS_CALLED.StudentId = IS_ENROLLED_ON.StudentId ) Now, the key word JO

Testing triggers, Demonstrate your knowledge of PL/SQL programming by writi... Demonstrate your knowledge of PL/SQL programming by writing and thoroughly testing triggers and stored procedures associated with an e-commerce application that provides security l

Example of add constraint in sql, Example of ADD CONSTRAINT in SQL Exa... Example of ADD CONSTRAINT in SQL Example: Alternative formulation for MAX_ENROLMENTS ALTER TABLE IS_ENROLLED_ON ADD CONSTRAINT MAX_ENROLMENTS CHECK ((SELECT COUNT (*)

Mysql developer/programmer, Project Description: I want somebody who can... Project Description: I want somebody who can help me with an idea that I have been working on for a few months now. The person will require extensive knowledge of warcraft 3 PvP

Existential quantification - sql, Existential Quantification - SQL Ex... Existential Quantification - SQL Existential quantification-stating that something is true of at least one object under consideration-can be expressed by OR(r,c), meaning tha

Authorize and fetch data from instagram, Authorize and fetch data from Inst... Authorize and fetch data from Instagram Project Description: Incorporate Instagram feed on mobile site platform: c#, ms sql, jquerymobile, jquery Web admin Author

Example of table literal - sql, Example of Table Literal - SQL Exampl... Example of Table Literal - SQL Example: A Table Literal (correct version) VALUES ('S1', 'C1', 'Anne'), ('S1', 'C2', 'Anne'), ('S2', 'C1', 'Boris'), ('S3', 'C3'

Multiset types - sql, Multiset types - SQL An SQL multiset is what in ... Multiset types - SQL An SQL multiset is what in mathematics is also known as a bag-something like a set except that the same element can appear more than once. The body of an

Effects of null for union - sql, Effects of NULL for union - SQL The ... Effects of NULL for union - SQL The treatment of NULL in invocations of EXCEPT is as for UNION. This is different from its treatment in those of NOT IN and quantified compari