Keywords: SELECT statement, ORDER BY clause, needed to guarantee order of results

# SELECT statement

#### Syntax

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

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 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.

#### 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.
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.

#### 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:
  - DISTINCT
  - Aggregates
  - GROUP BY clause
  - HAVING clause
  - ORDER BY 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

Note: Cursors are read-only by default. To produce an updatable cursor besides meeting the requirements listed above, the concurrency mode for the ResultSet must be `ResultSet.CONCUR_UPDATABLE` or the SELECT statement must have FOR UPDATE in the FOR clause (see FOR UPDATE clause).

There is no SQL language statement to assign a name to a cursor. Instead, one can 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 the Java DB Developer's Guide.

#### 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.

Parent topic: Statements