Effects of null for unique specification - sql, PL-SQL Programming

Effects of NULL for UNIQUE Specification

When a UNIQUE specification u for base table t includes a column c that is not subject to a NOT NULL constraint, the appearance of several rows having NULL in place of a value for c and equal values for the other columns specified in u is permitted. It is only when each column of the specified "key" has a value that those column values may not appear in the same combination in more than one row of t.

Posted Date: 1/18/2013 11:32:32 PM | Location : United States







Related Discussions:- Effects of null for unique specification - sql, Assignment Help, Ask Question on Effects of null for unique specification - sql, Get Answer, Expert's Help, Effects of null for unique specification - sql Discussions

Write discussion on Effects of null for unique specification - sql
Your posts are moderated
Related Questions
Overview of control structures According to the structure theorem, any computer program can be written by using the basic control structures as shown in figure below. They can b

Keyword & Parameter Description: boolean_expression: This is an expression which yields the Boolean value that is TRUE, FALSE, & NULL. character_expression: This

If two relations R and S are joined, then the non matching tuples of both R and S are ignored in __________________.

Due to an increase in overhead costs, the buying price of all items needs to be increased. Management wants to see a report before deciding how much each product will go up. Add to

Example of WHEN or THEN Constraints A concrete example showing how SQL supports WHEN/THEN constraints CREATE TABLE SAL_HISTORY (EmpNo CHAR (6), Salary INTEGER NOT NULL,

Parameter and Keyword Description: label_name: This is an undeclared identifier which optionally labels a loop. When used, the label_name should be enclosed by double ang

Manipulating Collections Within PL/SQL, the collections add procedural power and flexibility. The biggest benefit is that your program can compute subscripts to process the spec

Example of Check Constraints Example: Workaround for when subqueries not permitted in CHECK constraints CREATE FUNCTION NO_MORE_THAN_20000_ENROLMENTS ( ) RETURNS BOOLEAN

Define basic operators of relational algebra with an example each

Passing Cursor Parameters You use the OPEN statement to pass the parameters to a cursor. Unless you want to accept the default values, each proper parameter in the cursor decl