retrieving data using SQL statements

SKASC
OracleAssignment
TOPIC: RETRIEVING DATA USING
SQL “SELECT”STATEMENT
- Arun.P(13bca005)

SQL “SELECT” STATEMENT
SELECT statement:-
Purpose:
Use a SELECT statement or subquery to retrieve data from one or more
tables, object tables, views, object views, or materialized views.
If part or all of the result of a SELECT statement is equivalent to an
existing materialized view, then Oracle Database may use the
materialized view in place of one or more tables specified in
the SELECTstatement. This substitution is called query rewrite. It takes
place only if cost optimization is enabled and
the QUERY_REWRITE_ENABLED parameter is set to TRUE. To
determine whether query write has occurred, use
the EXPLAIN PLAN statement.
Syntax
Query
[ORDER BY clause]
[result offset clause]
[fetch first clause]
[FOR UPDATE clause]
[WITH {RR|RS|CS|UR}]

A SELECT statement consists of a query with an
optional ORDERBY CLAUSE, an optional result offset clause an
optional fetch first clause an optional FOR UPDATE CLAUSE and
optionally isolation level. The SELECT statement is so named because
the typical first word of the query construct is SELECT. (Query includes
the VALUES expression and UNION, INTERSECT, and EXCEPT
expressions as well as SELECT expressions).
The SQL SELECT statement returns a result set of records from
one or more tables.[1][2]
A SELECT statement retrieves zero or more rows from one or
more database tables or database views. In most
applications, SELECT is the most commonly used Data
Manipulation Language (DML) command. As SQL is a declarative
programming language, SELECT queries specify a result set, but
do not specify how to calculate it. The database translates the
query into a "query plan" which may vary between executions,
database versions and database software. This functionality is
called the "query optimizer" as it is responsible for finding the best
possible execution plan for the query, within applicable
constraints.
The SELECT statement has many optional clauses:
 WHERE specifies which rows to retrieve.
 GROUP BY groups rows sharing a property so that
an aggregate function can be applied to each group.
 HAVING selects among the groups defined by the GROUP BY
clause.
 ORDER BY specifies an order in which to return the rows.

 AS provides an alias which can be used to temporarily rename
tables or columns.
The ORDERBY CLAUSE guarantees the ordering of the ResultSet. The
result offset clause and the fetch first clause can be used to fetch only a
subset of the otherwise selected rows, possibly with an offset into the
result set. The FOR UPDATE CLAUSE makes the result set's cursor
updatable. The SELECT statement supports the FOR FETCH ONLY
clause. The FOR FETCH ONLY clause is synonymous with the FOR
READ ONLY clause.
You can set the isolation level in a SELECT statement using the WITH
{RR|RS|CS|UR} syntax.
For queries that do not select a specific column from the tables involved
in the SELECT statement (for example, queries that use COUNT(*)),
the user must have at least one column-level SELECT privilege or table-
level SELECT privilege. See GRANT statement for more information.
Example
-- lists the names of the expression
-- SAL+BONUS+COMM as TOTAL_PAY and
-- orders by the new name TOTAL_PAY
SELECT FIRSTNME, SALARY+BONUS+COMM AS TOTAL_PAY
FROM EMPLOYEE
ORDER BY TOTAL_PAY

-- creating an updatable cursor with a FOR
UPDATE clause
-- to update the start date (PRSTDATE) and the
end date (PRENDATE)
-- columns in the PROJECT table
SELECT PROJNO, PRSTDATE, PRENDATE
FROM PROJECT
FOR UPDATE OF PRSTDATE, PRENDATE
-- set the isolation level to RR for this
statement only
SELECT *
FROM Flights
WHERE flight_id BETWEEN 'AA1111' AND 'AA1112'
WITH RR
A SELECT statement returns a ResultSet. A cursor is a pointer to a
specific row in ResultSet. In Java applications, all ResultSets have an
underlying associated SQL cursor, often referred to as the result set's
cursor. The cursor can be updatable, that is, you can update or delete
rows as you step through the ResultSet if the SELECT statement that
generated it and its underlying query meet cursor updatability
requirements, as detailed below. The FOR UPDATE clause can be used
to ensure a compilation check that the SELECT statement meets the
requiremments of a updatable cursors, or to limit the columns that can
be updated.
Requirements for updatable cursors and updatable ResultSets

Only simple, single-table SELECT cursors can be updatable. The SELECT
statement for updatable ResultSets has the same syntax as the SELECT
statement for updatable cursors. To generate updatable cursors:
 The SELECT statement must not include an ORDER BY clause.
 The underlying Query must be a SelectExpression.
 The SelectExpression in the underlying Query must not include:
o DISTINCT
o Aggregates
o GROUP BY clause
o HAVING clause
o ORDER BY clause
 The FROM clause in the underlying Query must not have:
o more than one table in its FROM clause
o anything other than one table name
o SelectExpressions
o subqueries
 If the underlying Query has a WHERE clause, the WHERE clause
must not have subqueries
.
Statement dependency system
The SELECT depends on all the tables and views named in the query
and the conglomerates (units of storage such as heaps and indexes)
chosen for access paths on those tables. CREATE INDEX does not
invalidate a prepared SELECT statement. A DROP INDEX statement
invalidates a prepared SELECT statement if the index is an access path
in the statement. If the SELECT includes views, it also depends on the
dictionary objects on which the view itself depends (see CREATE
VIEW statement).

Any prepared UPDATE WHERE CURRENT or DELETE WHERE
CURRENT statement against a cursor of a SELECT depends on the
SELECT. Removing a SELECT through
a java.sql.Statement.closerequest invalidates the UPDATE WHERE
CURRENT or DELETE WHERE CURRENT.
The SELECT depends on all aliases used in the query. Dropping an alias
invalidates a prepared SELECT statement if the statement uses the alias.
How to use expressions in SQL SELECT Statement?
Expressions combine many arithmetic operators, they can be used in
SELECT, WHERE and ORDER BY Clauses of the SQL SELECT
Statement.
Here we will explain how to use expressions in the SQL SELECT
Statement. About using expressions in WHERE and ORDER BY
clause, they will be explained in their respective sections.
The operators are evaluated in a specific order of precedence, when
more than one arithmetic operator is used in an expression. The order
of evaluation is: parentheses, division, multiplication, addition, and
subtraction. The evaluation is performed from the left to the right of
the expression.

SELECT Statement Example?
If we want to display the first and last name of an employee
combined together, the SQL Select Statement would be like
SELECT first_name + ' ' + last_name FROM
employee;
Output:
first_name + ' ' + last_name
---------------------------------
Rahul Sharma
Anjali Bhagwat
Stephen Fleming
Shekar Gowda
Priya Chandra
You can also provide aliases as below.
SELECT first_name + ' ' + last_name AS
emp_name FROM employee;
Output:
emp_name
-------------
Rahul Sharma

Anjali Bhagwat
Stephen Fleming
Shekar Gowda
Priya Chandra

retrieving data using SQL statements

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