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This is where init comes in. There is an action value among the /etc/inittab entries called respawn that is quite useful for certain tasks. When an entry is added to the inittab and the action defined for that entry is respawn, init makes sure that a process corresponding to that entry is always running. If the process dies or is killed, init will automatically restart or respawn it. The process restart is nearly instantaneous, and the amount of time during which no instance of the desired process is running is negligible. The following are a few example lines from a system inittab file:

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Data in an Oracle database is organized in rows and columns inside tables. The individual columns are defined with properties that limit the values and format of the column contents. Let s review the most important Oracle built-in data types before we look at Oracle SQL statements.

The CHAR data type is used for fixed-length character literals: SEX CHAR(1) The VARCHAR2 data type is used to represent variable-length character literals: CITY VARCHAR2 (20)

The CLOB data type is used to hold large character strings as long as 4GB. BLOB and BFILE data types are used to store large amounts of binary data.

There are two important SQL data types used to store numeric data: The NUMBER data type is used to store real numbers, either in a fixed-point or floatingpoint format. The BINARY FLOAT and BINARY DOUBLE data types store data in a floating-point format.

There are a couple of special data types that let you handle date and time values: The DATE data type stores the date and time (such as year, month, day, hours, minutes, and seconds). The TIMESTAMP data type stores time values that are precise to fractional seconds.

1:2345:respawn:/sbin/mingetty 2:2345:respawn:/sbin/mingetty 3:2345:respawn:/sbin/mingetty 4:2345:respawn:/sbin/mingetty 5:2345:respawn:/sbin/mingetty 6:2345:respawn:/sbin/mingetty tty1 tty2 tty3 tty4 tty5 tty6

A few compiler switches are incompatible with managed compilation models. Other compiler switches must be set when /clr[:*] is used. These incompatibilities may also influence the way you migrate a project.

Oracle offers several conversion functions that let you convert data from one format to another. The most common of these functions are the TO_CHAR, TO_NUMBER, TO_DATE, and TO_TIMESTAMP functions. The TO_CHAR function converts a floating number to a string, and the TO_NUMBER function converts a floating number or a string to a number. The TO_DATE function converts character data to a DATE data type. Here are some examples: SQL> SELECT TO_CHAR(TO_DATE('20-JUL-05', 'DD-MON-RR') ,'YYYY') "Year" FROM DUAL; Year --------------------------------------------------------------------------2005 SQL> SQL> SELECT TO_CHAR(SYSDATE, 'DD-MON-YYYY') FROM DUAL;

In 5 you saw how Oracle SQL statements include DDL, DML, and other types of statements. Let s begin with a review of the basic SQL statements.

The SELECT statement is the most common SQL statement (it is also called a projection). A SELECT statement retrieves all or some of the data in a table, based on the criteria that you specify. The most basic SELECT statement is one that retrieves all the data in the table: SQL> SELECT * FROM employees; To retrieve only certain columns from a table, you specify the column names after the SELECT keyword, as shown in the following example: SQL> SELECT first_name, last_name, hiredate FROM employees; If you want only the first ten rows of a table, you can use the following statement: SQL> SELECT * FROM employees WHERE rownum <11; If you want just a count of all the rows in the table, you can use the following statement: SQL> SELECT COUNT(*) FROM employees; If a table has duplicate data, you can use the DISTINCT clause to eliminate the duplicate values, as shown here: SQL> SELECT DISTINCT username FROM V$SESSION; The optional WHERE clause in a SELECT statement uses conditions to help you specify that only certain rows be returned. Table A-1 lists some of the common conditions you can use in a WHERE clause. Table A-1. Common Conditions Used in WHERE Clauses

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