Using cursors in procedures and triggers

Chapter 20. Using Procedures, Triggers, and Batches
Using cursors in procedures and triggers

Cursors are used to retrieve rows one at a time from a query or stored procedure that has multiple rows in its result set. A cursor is a handle or an identifier for the query or procedure, and for a current position within the result set.

Cursor management overview

Managing a cursor is similar to managing a file in a programming language. The following steps are used to manage cursors:

Declare a cursor for a particular select statement or procedure using the DECLARE statement.
Open the cursor using the OPEN statement.
Use the FETCH statement to retrieve results one row at a time from the cursor.
Records are usually fetched until the
row not found warning
is returned, signaling the end of the result set.
Close the cursor using the CLOSE statement.

By default, cursors are automatically closed at the end of a transaction (on COMMIT or ROLLBACK statements). Cursors that are opened using the WITH HOLD clause will be kept open for subsequent transactions until they are explicitly closed.

Cursor positioning

A cursor can be positioned at one of three places:

On a row
Before the first row
After the last row

When a cursor is opened, it is positioned before the first row. The cursor position can be moved using the FETCH command (see "FETCH statement"). It can be positioned to an absolute position either from the start or from the end of the query results (using FETCH ABSOLUTE, FETCH FIRST, or FETCH LAST). It can also be moved relative to the current cursor position (using FETCH RELATIVE, FETCH PRIOR, or FETCH NEXT). The NEXT keyword is the default qualifier for the FETCH statement.

There are special positioned versions of the UPDATE and DELETE statements that can be used to update or delete the row at the current position of the cursor. If the cursor is positioned before the first row or after the last row, a

no current row of cursor

error will be returned.

Cursor positioning problems
Inserts and some updates to DYNAMIC SCROLL cursors can cause problems with cursor positioning. The database engine will not put inserted rows at a predictable position within a cursor unless there is an ORDER BY clause on the SELECT statement. In some cases, the inserted row will not appear at all until the cursor is closed and opened again.With SQL Anywhere, this occurs if a temporary table had to be created to open the cursor (see "Temporary tables used in query processing" for a description).The UPDATE statement may cause a row to move in the cursor. This will happen if the cursor has an ORDER BY that uses an existing index (a temporary table is not created). Using STATIC SCROLL cursors alleviates these problems but is more expensive.

Using cursors on SELECT statements in procedures

The following procedure uses a cursor on a SELECT statement. It illustrates several features of the stored procedure language. It is based on the same query used in the ListCustomerValue procedure described in "Returning result sets from procedures".

     CREATE PROCEDURE TopCustomerValue
         (    OUT TopCompany CHAR(36),
             OUT TopValue INT )
     BEGIN
         -- 1. Declare the "error not found" exception
         DECLARE err_notfound
             EXCEPTION FOR SQLSTATE '02000';
         -- 2.    Declare variables to hold
         --        each company name and its value
         DECLARE ThisName CHAR(36);
         DECLARE ThisValue INT;
         -- 3.    Declare the cursor ThisCompany
         --        for the query
         DECLARE ThisCompany CURSOR FOR
         SELECT company_name,
                 CAST( sum( sales_order_items.quantity *
                         product.unit_price ) AS INTEGER )
                 AS value
         FROM customer
             INNER JOIN sales_order
             INNER JOIN sales_order_items
             INNER JOIN product
         GROUP BY company_name;
         -- 4. Initialize the values of TopValue
         SET TopValue = 0;
         -- 5. Open the cursor
         OPEN ThisCompany;
         -- 6. Loop over the rows of the query
         CompanyLoop:
         LOOP
             FETCH NEXT ThisCompany
                 INTO ThisName, ThisValue;
             IF SQLSTATE = err_notfound THEN
                 LEAVE CompanyLoop;
             END IF;
             IF ThisValue > TopValue THEN
                 SET TopCompany = ThisName;
                 SET TopValue = ThisValue;
             END IF;
         END LOOP CompanyLoop;
         -- 7. Close the cursor
         CLOSE ThisCompany;
     END

Notes

The TopCustomerValue procedure has the following notable features:

The "error not found" exception is declared. This exception is used to detect when a loop over the results of a query has completed.

For more information about exceptions, see "Errors and warnings in procedures and triggers".

Two local variables ThisName and ThisValue are declared to hold the results from each row of the query.
The cursor ThisCompany is declared. The SELECT statement produces a list of company names and the total value of the orders placed by that company.
The value of TopValue is set to an initial value of 0, for later use in the loop.
The ThisCompany cursor is opened.
The LOOP statement loops over each row of the query, placing each company name in turn into the variables ThisName and ThisValue. If ThisValue is greater than the current top value, TopCompany and TopValue are reset to ThisName and ThisValue.
The cursor is closed at the end of the procedure.

The LOOP construct in the TopCompanyValue procedure is a standard form, exiting after the last row is processed. You can rewrite this procedure in a more compact form using a FOR loop. The FOR statement combines several aspects of the above procedure into a single statement.

     CREATE PROCEDURE TopCustomerValue2(
             OUT TopCompany CHAR(36),
             OUT TopValue INT )
     BEGIN
         -- Initialize the TopValue variable
         SET TopValue = 0;
         -- Do the For Loop
         CompanyLoop:
         FOR CompanyFor AS ThisCompany
             CURSOR FOR
             SELECT company_name AS ThisName ,
                 CAST( sum( sales_order_items.quantity *
                         product.unit_price ) AS INTEGER )
                 AS ThisValue
             FROM customer
                 INNER JOIN sales_order
                 INNER JOIN sales_order_items
                 INNER JOIN product
             GROUP BY ThisName
         DO
             IF ThisValue > TopValue THEN
                 SET TopCompany = ThisName;
                 SET TopValue = ThisValue;
                 END IF;
         END FOR CompanyLoop;
     END