SELECT statement

A SELECT statement consists of a query with an optional ORDER BY clause and an optional FOR UPDATE clause. 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 ORDER BY clause guarantees the ordering of the ResultSet. The FOR UPDATE clause makes the result an updatable cursor. The SELECT statement supports the FOR FETCH ONLY clause. The FOR FETCH ONLY clause is synonymous with the FOR READ ONLY clause.

Remember: In order to get an updatable ResultSet, you must include a FOR UPDATE clause with the SELECT clause.

Syntax

Query
[ORDER BY clause]
[FOR UPDATE clause]
WITH {RR|RS|CS|UR}

You can set the isolation level in a SELECT statement using the WITH {RR|RS|CS|UR} syntax.

-- 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 are cursors. A cursor is 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. You use a FOR UPDATE clause when you want to generate an updatable cursor.

Note: The ORDER BY clause allows you to order the results of the SELECT. Without the ORDER BY clause, the results are returned in random order.

If a SELECT statement meets the requirements listed below, cursors are updatable only if you specify FOR UPDATE in the FOR clause (see FOR UPDATE clause).

Requirements for updatable cursors and updatable ResultSets

Only simple, single-table SELECT cursors and FORWARD_ONLY ResultSets 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:
- DISTINCT
- Aggregates
- GROUP BY clause
- HAVING clause
The FROM clause in the underlying Query must not have:
- more than one table in its FROM clause
- anything other than one table name
- SelectExpressions
- subqueries

There is no SQL language statement to assign a name to a cursor. Instead, you use the JDBC API to assign names to cursors or retrieve system-generated names. For more information, see "Naming or Accessing the Name of a Cursor" in Chapter 5 of the Derby Developer's Guide.

Cursors are read-only by default. For a cursor to be updatable, you must specify FOR UPDATE in the FOR clause (see FOR UPDATE clause).

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.close request 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.