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

How pl/sql resolves the calls? , How Calls Are Resolved? The figure sho... How Calls Are Resolved? The figure shows that how the PL/SQL compiler resolves the subprogram calls. When the compiler encounters the procedure or function call, it tries to di

Special cases of projection, Special cases of projection This section ... Special cases of projection This section describes the identity projection, r {ALL BUT}, and the projection on no attributes, r { }, which yields TABLE_DUM when r is empty, ot

Like operator-comparison operators, LIKE Operator You use the LIKE opera... LIKE Operator You use the LIKE operator to compare the character value to a pattern. The Case is significant. LIKE returns the Boolean value TRUE when the character patterns mat

What is a record, What Is a Record  ? A record is a group of related... What Is a Record  ? A record is a group of related data items that stored in the fields, each with its own name and datatype. Assume that you have different data about an em

What is a collection, What Is a Collection The collection is an ordered... What Is a Collection The collection is an ordered group of elements, all of similar type (for e.g. the grades for a class of students). Each element has a unique subscript whic

Object types - syntax, Object Types An object type is a user-defined co... Object Types An object type is a user-defined complex datatype which encapsulates the data structure along with the functions and procedures required to manipulate the data. Th

Closing a cursor variable, Closing a Cursor Variable The CLOSE stateme... Closing a Cursor Variable The CLOSE statement disables the cursor variable. After that, the related result set is undefined. The syntax for the same is as shown below: CLOS

Cursor variables, Cursor Variables Similar to a cursor, cursor variable... Cursor Variables Similar to a cursor, cursor variable points to the current row in the result set of a multi-row query. But, dissimilar a cursor, a cursor variable can be opene

Using extend - collection method, Using EXTEND To enlarge the size of ... Using EXTEND To enlarge the size of a collection, use EXTEND. This process has 3 forms. The EXTEND appends one null element to a collection. And the EXTEND(n) appends n null e

Sql is a database language, SQL Is a Database Language: The commands g... SQL Is a Database Language: The commands given to a DBMS by an application are written in the database language of the DBMS. The term data sublanguage is sometimes used instea