Natural join - sql, PL-SQL Programming

Natural Join - SQL

In the absence of NATURAL JOIN Example has to be replaced by something rather more longwinded, as shown in Example.

Example: Joining IS_CALLED and IS_ENROLLED_ON in original SQL

SELECT IC.StudentId, Name, CourseId

FROM IS_CALLED AS IC, IS_ENROLLED_ON AS IE

WHERE IC.StudentId = IE.StudentId

866_Joining in SQL.png

Explanation

  • The FROM clause now has two elements. When there are two elements, t1 and t2, the result is equivalent to t1 CROSS JOIN t2, which is SQL's counterpart of t1 TIMES t2 in Tutorial D. However, TIMES requires its operands to have disjoint headings, whereas CROSS JOIN is defined for all pairs of SQL tables. When t1 and t2 each have a column named c, the result has two columns named c. In general, when t1 has m columns named c and t2 has n, t1 CROSS JOIN t2 has m+n columns named c.
  • Following the FROM clause is a WHERE clause, denoting an invocation of the operator WHERE. The operands are the table resulting from the FROM clause and the condition following the word WHERE. SQL's WHERE operator is equivalent to Tutorial D's operator of the same name when its table operand represents a relation.
  • The result of the FROM clause has two columns of the same name, StudentId. The condition specified in the WHERE clause uses range variables, IC and IE, to distinguish between these two columns. The distinction is possible here, thanks to the fact that the same column name isn't used more than once in either of the two operand tables (as we shall see later, that is a condition that does not always apply, even though the same column name cannot be used more than once in a base table).
  • The range variables are defined in the FROM clause alongside the table expressions to which they apply. The key word AS separating the table expression from the range variable name is optional. If the table expression consists of just a table name, unaccompanied by a range variable, then that table name serves also as a range variable name.
  • A range variable is so-called because it is considered to "range over" each element in turn of a collection, the collection in the example at hand being the rows of a table. Note carefully that although the expression IE.StudentId is a column reference, it is not a column name. It references a particular column named StudentId. The prefix "IE." is required because without it the column reference would be ambiguous.
Posted Date: 1/18/2013 4:46:02 AM | Location : United States







Related Discussions:- Natural join - sql, Assignment Help, Ask Question on Natural join - sql, Get Answer, Expert's Help, Natural join - sql Discussions

Write discussion on Natural join - sql
Your posts are moderated
Related Questions
Tautology - Equivalences Rules: If there Tautologies are not all the time as much easy to note as the one above so than we can use these truth tables to be definite that a sta


Example of COALESCE operator Example: Give the total of marks for each exam (simplified solution) SELECT CourseId, COALESCE ((SELECT SUM (Mark) FROM EXAM_MARK AS EM

Lightweight system to provide and take info from workers in the field and office, have basic design outlined already just require build and implementation Desired Skills CSS,

Assignments in pl/sql The Variables and constants are initialized every time a block or subprogram is entered. By default, the variables are initialized to NULL. Therefore, unle

Question: a) Given the following relation: Location(loc_id, bldg_code, room, capacity) The underlined field is a primary key. (i) Write a PL/SQL program using the impl

Oracle 11 G new features associated with this release:- Enhanced ILM  - Information Lifecycle Management (ILM) has been around for the almost 10 years, but Oracle has made

Example of DELETE - SQL As with UPDATE, a FOR PORTION OF clause can be specified if the target table has a defined period name, as illustrated in Example. Example: Deleting

PRIMARY KEY: PRIMARY KEY  indicates that the table is subject to a key constraint, in this case declaring that no two rows in the table assigned to ENROLMENT can ever have the

Rollback Behavior When a FORALL statement fails, the database changes are rolled back to an implicit savepoint marked before each of the SQL statement execution. The Changes t