The following procedure uses a cursor on a SELECT statement. Based on the same query used in the ListCustomerValue procedure described in Returning result sets from procedures, it illustrates several features of the stored procedure language.
CREATE PROCEDURE TopCustomerValue( OUT TopCompany CHAR(36), OUT TopValue INT ) BEGIN -- 1. Declare the "row 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 CompanyName, CAST( sum( SalesOrderItems.Quantity * Products.UnitPrice ) AS INTEGER ) AS value FROM Customers INNER JOIN SalesOrders INNER JOIN SalesOrderItems INNER JOIN Products GROUP BY CompanyName; -- 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
The TopCustomerValue procedure has the following notable features:
The "row not found" exception is declared. This exception signals, later in the procedure, when a loop over the results of a query completes.
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 opens.
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 closes at the end of the procedure.
You can also write this procedure without a loop by adding an ORDER BY value DESC clause to the SELECT statement. Then, only the first row of the cursor needs to be fetched.
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 -- 1. Initialize the TopValue variable SET TopValue = 0; -- 2. Do the For Loop FOR CompanyFor AS ThisCompany CURSOR FOR SELECT CompanyName AS ThisName , CAST( sum( SalesOrderItems.Quantity * Products.UnitPrice ) AS INTEGER ) AS ThisValue FROM Customers INNER JOIN SalesOrders INNER JOIN SalesOrderItems INNER JOIN Products GROUP BY ThisName DO IF ThisValue > TopValue THEN SET TopCompany = ThisName; SET TopValue = ThisValue; END IF; END FOR; END;