Projection and existential quantification - sql, PL-SQL Programming

Projection and Existential Quantification - SQL

Intuitively it might seem that projection in SQL is simply a matter of specifying the required columns in the SELECT clause, as in Example.

Example: Projection (incorrect)

Student StudentId is enrolled on some course.

SELECT StudentId FROM IS_ENROLLED_ON

Unfortunately, though, if some student is currently enrolled on more than one course (as indeed student S1, Anne, is in our example database), then the row giving that student's id appears twice in the result, which because of that duplicate appearance does not represent a relation. To avoid multiple appearances of the same row SQL requires you to write the word DISTINCT. (The key word ALL can be given instead of DISTINCT, clarifying that duplicate rows are not to be eliminated. As ALL is the default option).

Posted Date: 1/18/2013 5:09:17 AM | Location : United States







Related Discussions:- Projection and existential quantification - sql, Assignment Help, Ask Question on Projection and existential quantification - sql, Get Answer, Expert's Help, Projection and existential quantification - sql Discussions

Write discussion on Projection and existential quantification - sql
Your posts are moderated
Related Questions
I need SQL to infopath data connection Project Description: Want data retrieval connection from SQL to SharePoint infopath Skills required are Sharepoint, SQL

Effect of Anonymous Columns Now, recall that a VALUES expression denotes a table with undefined column names. If an initial value is to be specified when a base table is creat

Use the RETURNING Clause Frequently, the application requires information about the row affected by a SQL operation, for illustration, to produce a report or take a subsequent

Using INNER JOIN INNER JOIN is used to retrieve the data from all tables listed based on a condition of equality listed after keyword ON. If the condition is not meet, rows ar

Data Types and Representations This explains the concept possible representation, abbreviated possrep, and explains how these can be used in conjunction with constraints to de

Substitution and Instantiation - SQL It shows how NULL might appear in substitution for a parameter of a predicate and how it might thus participate in instantiation of that p

Rephrase Conditional Control Statements When computing a logical expression, the PL/SQL uses short-circuit evaluation. That is, the PL/SQL stops evaluating the expression as s

Package Specification The package specifications contain the public declarations. The scopes of these declarations are local to your database representation and global to the

Character Types The Character types allow you to store alphanumeric data, represent words and text, and manipulate the character strings. CHAR You use the CHAR dataty

Complete the following steps to create a procedure to calculate the tax on an order. The BB_TAX table contains the states that require taxes to be submitted for Internet sales. If