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Most exceptions programmers face in NET development are synchronous exceptions Synchronous exceptions are caused by the operations that a thread executes and by methods that are called in a thread As an example, the IL operation castclass, which is emitted for safe_cast operations, can throw a System::InvalidCastException, and a call to System::Console::WriteLine can throw a System::FormatException Synchronous exceptions are typically mentioned in a function s documentation In contrast to that, asynchronous exceptions can be thrown at any instruction Exceptions that can be thrown asynchronously include the following: System::StackOverflowException System::OutOfMemoryException System::ExecutionEngineException For many applications, the best way to react to these exceptions is to shut down the process and restart the application A process shutdown typically cleans up the native resources, so there is often no need to treat these exceptions specially In fact, the default behavior of the CLR is to shut down the process after such an exception.

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The MERGE statement is a powerful means of transforming data because it provides the functionality of checking the data to see if an update is indeed required for a given row. Suppose you re loading data from a data source into your table. You want to insert customer data only if the customer is a new customer. If the customer s data is already present in your table, you don t want to reload the data, but you may want to update the customer s information based on the new data you just received. The MERGE statement is actually an UPDATE ELSE-INSERT operation performed by a single SQL statement. You could do the same thing without using the MERGE statement by performing a twopass operation. In the first pass, you update all rows that have matching customer IDs in the table. In the second pass, you insert all rows that don t have a matching customer ID in your table. The following listings show the traditional two-pass update/insert method using separate UPDATE and INSERT statements. First, the update: SQL> UPDATE catalog c SET (catalog_name, catalog_desc, catalog_category, catalog_price) = SELECT (catalog_name, catalog_desc, catalog_category, catalog_price)

This chapter s first math method is the use of expr. expr is a utility that evaluates various types of expressions, including ones containing string, logical, and mathematical functions. The following are examples of how to use expr to carry out mathematical operations: Addition: answer=`expr $c + $d` Subtraction: answer=`expr $c - $d` Multiplication: answer=`expr $c \* $d` Division: answer=`expr $c / $d` Remainder: answer=`expr $c % $d` There are no explicit trigonometric or exponentiation functions in expr.

FROM catalog_data d WHERE c.catalog_id=d.catalog_id; Second, the insert: SQL> INSERT INTO catalog c SELECT * FROM catalog_data d WHERE c.catalog_id NOT IN (select catalog_id from catalog_data); You could do the preceding work using a lengthy PL/SQL code piece. The PL/SQL procedures must match each input row against the table to see if it already exists. Based on the results of the checks, code that either inserts or updates rows is executed. Whether you use SQL or PL/SQL, you can t avoid the inefficient multiple processing of the same data to complete your update/insert processing. Both methods are fairly tedious and take a long time. The MERGE statement, sometimes referred to as the upsert statement (because it does both an update and an insert using a single SQL statement), is a much more efficient way of performing traditionally multiple-pass operations. It s almost like using if-then-else logic. Listing 13-8 shows an update and insert process using the MERGE statement. The MERGE statement in Listing 13-8 indicates that if customer_id exists, then update; otherwise, insert into the table. Listing 13-8. Using the MERGE Statement to Perform an Update/Insert SQL> MERGE INTO target t USING source s ON (t.product_id=s.product_id) WHEN MATCHED THEN UPDATE SET t.price=s.price, t.discount=s.discount WHEN NOT MATCHED THEN INSERT (product_id, product_code, product_desc, product_price, product_discount) VALUES (s.product_id, s.product_code, s.product_desc, s.product_price, s.product_discount); The WHEN MATCHED THEN UPDATE SET clause determines if an UPDATE or an INSERT operation will take place. The previous statement will update a row in the table target if that row already exists. If there is no such row, Oracle will insert a new row in the table. In addition to a straightforward insert/delete operation, you may perform conditional updates/inserts and optionally delete some rows, as shown in the following sections.