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The DBA_TAB_COLUMNS view is another useful data dictionary view that provides a lot of information about table columns. Listing 5-14 shows a simple query using this view. Listing 5-14. Using the DBA_TAB_COLUMNS Data Dictionary View SQL> SELECT column_name, data_type, nullable FROM dba_tab_columns WHERE owner='HR' AND table_name = 'EMPLOYEES'; COLUMN_NAME DATA_TYPE NULLABLE ------------------------------------------------EMPLOYEE_ID NUMBER N FIRST_NAME VARCHAR2 Y LAST_NAME VARCHAR2 N EMAIL VARCHAR2 N PHONE_NUMBER VARCHAR2 Y HIRE_DATE DATE N JOB_ID VARCHAR2 N SALARY NUMBER Y 8 rows selected. SQL> Of course, you could have obtained this type of information easily by using the DESCRIBE command. Listing 5-15 shows how to use this command. Listing 5-15. Using the DESCRIBE Command SQL> DESCRIBE new_employees Name Null Type --------------------------- -------- -------------EMPLOYEE_ID NOT NULL NUMBER(6) FIRST_NAME NOT NULL VARCHAR2(20) LAST_NAME NOT NULL VARCHAR2(25) HIRE_DATE NOT NULL DATE JOB_ID NOT NULL VARCHAR2(10) SALARY NUMBER(8,2)

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bash and expr; ksh uses scientific notation for large numbers). This is the maximum 64-bit integer. If you need numbers that are larger than this, you must use the bc or dc calculator.

The performance gain from using a multitable insert is directly proportional to the complexity of the data and the number of target tables. Oracle claims that you can achieve a processing speed gain of 400 percent or more.

When you load all rows of a table, you can use either an unconditional all row insert or a conditional all row insert. In the following example, the source table is called sales_activity, whose data is loaded at the same time into two destination tables: sales and cost. The unconditional insert uses the keywords INSERT ALL, meaning that all the source rows (sales_activity) are loaded into the sales and cost tables. SQL> INSERT ALL INTO target1 VALUES (product_id, customer_id, sysdate, product_quantity) INTO target2 VALUES (product_id,sysdate,product_price,product_discount) SELECT s.product_id, s.customer_id, sysdate, s.product_quantity, s.product_price, s.product_discount FROM source s; After the INSERT ALL keywords, there are two INTO statements, each denoting an insert into a separate table. Notice that the SELECT statement contains all the necessary columns required by both INTO statements for inserting into the two tables. The conditional insert of all rows from the source table is similar to the unconditional version, except that the keyword WHEN indicates the conditions under which the inserts will be made. The following example shows how to perform a conditional all-row insert: SQL> INSERT ALL WHEN product_id IN(SELECT product_id FROM primary) THEN INTO target1 VALUES (product_id, customer_id, sysdate, product_quantity) WHEN product_id IN (SELECT product_id FROM secondary) THEN INTO target2 VALUES (product_id, sysdate, product_price, product_discount) SELECT s.product_id, s.customer_id, sysdate, s.product_quantity, s.product_price, s.product_discount FROM source s; This example still inserts all the rows from sales_data, because it uses the key phrase INSERT ALL.

When an asynchronous exception is thrown after the call to the native XYZConnect function but before the returned handle is assigned to the Handle property, a resource is correctly allocated, but the handle is not stored in the wrapper object. Therefore, the finalizer of the wrapper class cannot use this handle for cleanup. A similar problem can occur in the destructor: ~XYZHandle() { if (hxyz) { ::XYZDisconnect(hxyz); hxyz = 0; } GC::KeepAlive(this); } When an asynchronous exception is thrown after the call to XYZDisconnect but before 0 is assigned to hxyz, a further Dispose call could cause double cleanup. If you use the thread-safe variant based on InterlockedExchangePointer, you will likely end up with a resource leak instead of double cleanup. To avoid asynchronous exceptions in these critical phases, the CLR version 2.0 provides a set of very special features. Each of these features targets different kinds of asynchronous exceptions, which are discussed in the following sections.

Sometimes, you re interested in loading only some rows from a table, either based on a condition or unconditionally. You can do this in a multitable insert by using the keywords INSERT FIRST. Listing 13-11 shows how only some of the source table s rows are loaded into each target table, based on a separate condition for each table.