Chapter 14. Data Retrieval

One of the hallmarks of the PL/SQL language is its tight integration with the Oracle database, both for changing data in database tables and for extracting information from those tables. This chapter explores the many features available in PL/SQL to query data from the database and make that data available within PL/SQL programs.

When you execute a SQL statement from PL/SQL, the Oracle RDBMS assigns a private work area for that statement and also manages the data specified by the SQL statement in the System Global Area (SGA). The private work area contains information about the SQL statement and the set of data returned or affected by that statement.

PL/SQL provides a number of different ways to name this work area and manipulate the information within it; all of these ways involve defining and working with cursors. They include:

Implicit cursors

A simple and direct SELECT . . . INTO retrieves a single row of data into local program variables. It's the easiest (and often the most efficient) path to your data, but can often lead to coding the same or similar SELECTs in multiple places in your code.

Explicit cursors

You can declare the query explicitly in your declaration section (local block or package). In this way, you can open and fetch from the cursor in one or more programs, with a granularity of control not available with implicit cursors.

Cursor variables

Offering an additional level of flexibility, cursor variables (declared from a REF CURSOR type) allow you to pass a pointer to a query's underlying result set from one program to another. Any program with access to that variable can open, fetch from, or close the cursor.

Cursor expressions

New to Oracle9i, the CURSOR expression transforms a SELECT statement into a REF CURSOR result set and can be used in conjunction with table functions to improve the performance of applications.

Dynamic SQL queries

Oracle allows you to construct and execute queries dynamically at runtime using either native dynamic SQL (NDS) (new to Oracle8i and covered in Chapter 15) or DBMS_SQL Details on this built-in package are available in Oracle Built-in Packages (O'Reilly).

This chapter explores implicit cursors, explicit cursors, cursor variables, and cursor expressions in detail.

14.1 Cursor Basics

In its simplest form, you can think of a cursor as apointer into a table in the database. For example, the following cursor declaration associates the entire employee table with the cursor named employee_cur:

CURSOR employee_cur IS SELECT * FROM employee;

Once I have declared the cursor, I can open it:

OPEN employee_cur;

Then I can fetch rows from it:

FETCH employee_cur INTO employee_rec;

Finally, I can close the cursor:

CLOSE employee_cur;

In this case, each record fetched from this cursor represents an entire record in the employee table. You can, however, associate any valid SELECT statement with a cursor. In the next example I have a join of three tables in my cursor declaration:

DECLARE

CURSOR joke_feedback_cur

IS

SELECT J.name, R.laugh_volume, C.name

FROM joke J, response R, comedian C

WHERE J.joke_id = R.joke_id

AND J.joker_id = C.joker_id;

BEGIN

...

END;

Here, the cursor does not act as a pointer into any actual table in the database. Instead, the cursor is a pointer into the virtual table or implicit view represented by the SELECT statement (SELECT is called a virtual table because the data it produces has the same structure as a table—rows and columns—but it exists only for the duration of the execution of the SQL statement). If the triple join returns 20 rows, each containing three columns, then the cursor functions as a pointer into those 20 rows.