3 Using SQL Datatypes in Application Development

Representing Character Data: Overview

You can use the following SQL datatypes to store alphanumeric data:

• CHAR and NCHAR datatypes store fixed-length character literals.

• VARCHAR2 and NVARCHAR2 datatypes store variable-length character literals.

• NCHAR and NVARCHAR2 datatypes store Unicode character data only.

• CLOB and NCLOB datatypes store single-byte and multibyte character strings of up to (4 gigabytes - 1) * (the value obtained from DBMS_LOB.GETCHUNKSIZE).

• The LONG datatype stores variable-length character strings containing up to two gigabytes, but with many restrictions. This datatype is provided only for backward compatibility with existing applications. In general, new applications should use CLOB and NCLOB datatypes to store large amounts of character data, and BLOB and BFILE to store large amounts of binary data.

Specifying Column Lengths as Bytes or Characters

You can specify the lengths of CHAR and VARCHAR2 columns as either bytes or characters. The lengths of NCHAR and NVARCHAR2 columns are always specified in characters, making them ideal for storing Unicode data, where a character might consist of multiple bytes.

Consider the following list of column length specifications:

• id VARCHAR2(32 BYTE)

  The id column contains only single-byte data, up to 32 bytes.

• name VARCHAR2(32 CHAR)

  The name column contains data in the database character set. If the database character set allows multibyte characters, then the 32 characters can be stored as more than 32 bytes.

• biography NVARCHAR2(2000)

  The biography column can represent 2000 characters in any Unicode-representable language. The encoding depends on the national character set, but the column can contain multibyte values even if the database character set is single-byte.

• comment VARCHAR2(2000)

  The representation of comment as 2000 bytes or characters depends on the initialization parameter NLS_LENGTH_SEMANTICS.

When using a multibyte database character encoding scheme, consider carefully the space required for tables with character columns. If the database character encoding scheme is single-byte, then the number of bytes and the number of characters in a column is the same. If it is multibyte, however, then there generally is no such correspondence. A character might consist of one or more bytes, depending upon the specific multibyte encoding scheme and whether shift-in/shift-out control codes are present. To avoid overflowing buffers, specify data as NCHAR or NVARCHAR2 if it might use a Unicode encoding that is different from the database character set.

Choosing Between the CHAR and VARCHAR2 Datatypes

When deciding which datatype to use for a column that will store alphanumeric data in a table, consider the following points of distinction:

• Space usage

  To store data more efficiently, use the VARCHAR2 datatype. The CHAR datatype blank-pads and stores trailing blanks up to a fixed column length for all column values, whereas the VARCHAR2 datatype does not add extra blanks.

• Comparison semantics

  Use the CHAR datatype when you require ANSI compatibility in comparison semantics (when trailing blanks are not important in string comparisons). Use the VARCHAR2 when trailing blanks are important in string comparisons.

• Future compatibility

  The CHAR and VARCHAR2 datatypes are fully supported. At this time, the VARCHAR datatype automatically corresponds to the VARCHAR2 datatype and is reserved for future use.

When an application interfaces with Oracle Database, there is a character set on the client and server side. Oracle Database uses the NLS_LANGUAGE parameter to automatically convert CHAR, VARCHAR2, and LONG data from the database character set to the character set defined for the user session, if these are different.

In the section "Datatype Comparison Rules," Oracle Database SQL Reference explains the comparison semantics that Oracle Database uses to compare character data. Because Oracle Database blank-pads values stored in CHAR columns but not in VARCHAR2 columns, a value stored in a VARCHAR2 column can take up less space than the same value in a CHAR column. For this reason, a full table scan on a large table containing VARCHAR2 columns may read fewer data blocks than a full table scan on a table containing the same data stored in CHAR columns. If your application often performs full table scans on large tables containing character data, then you may be able to improve performance by storing data in VARCHAR2 rather than in CHAR columns.

Performance is not the only factor to consider when deciding which datatype to use. Oracle Database uses different semantics to compare values of each datatype. You might choose one datatype over the other if your application is sensitive to the differences between these semantics. For example, if you want Oracle Database to ignore trailing blanks when comparing character values, then you must store these values in CHAR columns.

Using Character Literals in SQL Statements

Many SQL statements, functions, expressions, and conditions require you to specify character literal values. You can specify character literals with the following notations:

• Character literals with the 'text' notation, as in the literals 'users01.dbf' and 'Muthu''s computer'.

• National character literals with the N'text' or n'text' notation, where N or n specifies the literal using the national character set. For example, N'résumé' is a National character literal.

  Oracle Database translates N-quoted text into the national character set by way of the database character set. If client-side characters do not have corresponding encoding in the database character set, then Oracle Database converts them into question marks. To avoid the potential loss of data during the text literal conversion, set the environment variable $ORA_NCHAR_LITERAL_REPLACE to TRUE. This setting transparently replaces the N'text' internally and preserves the text literal for SQL processing.

The UNISTR function provides support for Unicode character literals by enabling you to specify the Unicode encoding value of characters in the string, as in UNISTR('\1234'). This technique is useful, for example, when inserting data into NCHAR columns. Because every character has a corresponding Unicode encoding, the client application can safely send character data to the server without data loss.

q'(name LIKE '%DBMS_%%')'
q'<'Data,' he said, 'Make it so.'>'
q'"name like '['"'
nq'ïŸ1234ï'

What Are the Numeric Datatypes?

The following SQL datatypes store numeric data:

• NUMBER

• BINARY_FLOAT

• BINARY_DOUBLE

Use the NUMBER datatype to store real numbers in a fixed-point or floating-point format. Numbers using this datatype are guaranteed to be portable among different Oracle Database platforms, and offer up to 38 decimal digits of precision. You can store positive and negative numbers of magnitude 1 x 10^-130 through 9.99 x10¹²⁵, as well as zero, in a NUMBER column.

The BINARY_FLOAT and BINARY_DOUBLE datatypes store floating-point data in the 32-bit IEEE 754 format and the double precision 64-bit IEEE 754 format respectively. Compared to the Oracle NUMBER datatype, arithmetic operations on floating-point data are usually faster for BINARY_FLOAT and BINARY_DOUBLE. Also, high-precision values require less space when stored as BINARY_FLOAT and BINARY_DOUBLE.

In client interfaces supported by Oracle Database, the native instruction set supplied by the hardware vendor performs arithmetic operations on BINARY_FLOAT and BINARY_DOUBLE datatypes. The term native floating-point datatypes refers to datatypes including BINARY_FLOAT and BINARY_DOUBLE and to all implementations of these types in supported client interfaces.

The floating-point number system is a common way of representing and manipulating numeric values in computer systems. A floating-point number is characterized by the following components:

• Binary-valued sign

• Signed exponent

• Significand

• Base

A floating-point value is the signed product of its significand and the base raised to the power of its exponent, as shown in the formula in Example 3-1.

(-1)sign . significand . base exponent

For example, the number 4.31 can be represented in the following expression:

(-1)0 . 431 . 10 -2

The components of the preceding expression are as follows:

• 0 is the binary-valued sign

• 431 is the significant

• 10 is the base

• -2 is the exponent

Using Floating-Point Number Formats

A floating-point number format specifies how components of a floating-point number are represented. The choice of representation determines the range and precision of the values the format can represent. By definition, the range is the interval bounded by the smallest and the largest values the format can represent and the precision is the number of digits in the significand.

Formats for floating-point values support neither infinite precision nor infinite range. There are a finite number of bits to represent a number and only a finite number of values that a format can represent. A floating-point number that uses more precision than available with a given format is rounded.

A floating-point number can be represented in a binary system (one that uses base 2), as in the IEEE 754 standard, or in a decimal system (one that uses base 10), such as Oracle NUMBER. The base affects many properties of the format, including how a numeric value is rounded.

For a decimal floating-point number format like Oracle NUMBER, rounding is done to the nearest decimal place (for example. 1000, 10, or 0.01). The IEEE 754 formats use a binary format for floating-point values and round numbers to the nearest binary place (for example: 1024, 512, or 1/64).

The native floating-point datatypes supported by the database round to the nearest binary place, so they are not satisfactory for applications that require decimal rounding. Use the Oracle NUMBER datatype for applications in which decimal rounding is required on floating-point data.

(-1)s 2E (b0 b1 b2 ... bp-1)

Using Comparison Operators for Native Floating-Point Datatypes

Oracle Database defines the following comparison operators for operations involving floating-point datatypes:

• Equal to

• Not equal to

• Greater than

• Greater than or equal to

• Less than

• Less than or equal to

• Unordered

Note the following special cases:

• Comparisons ignore the sign of zero (-0 is equal to, not less than, +0).

• In Oracle Database, NaN is equal to itself. NaN is greater than everything except itself. That is, NaN == NaN and NaN > x, unless x is NaN.

  See Also:

  "Behavior of Special Values for Native Floating-Point Datatypes" for more information on comparison results, ordering, and other behaviors of special values

Performing Arithmetic Operations with Native Floating-Point Datatypes

Oracle Database defines operators for the following arithmetic operations:

• Multiplication

• Division

• Addition

• Subtraction

• Remainder

• Square root

You can define the mode used to round the result of the operation. Exceptions can be raised when operations are performed. Exceptions can also be disabled.

Formerly, Java required floating-point arithmetic to be exactly reproducible. IEEE 754 does not require such behavior. The standard allows for the result of operations, including arithmetic, to be delivered to a destination that uses a range greater than that used by the operands to the operation.

You can compute the result of a double-precision multiplication at an extended double-precision destination. When this is done, the result must be rounded as if the destination were single-precision or double-precision. The range of the result, that is, the number of bits used for the exponent, can use the range supported by the wider (extended double-precision) destination. This occurrence may result in a double-rounding error in which the least significant bit of the result is incorrect.

This state of affairs can only occur for double-precision multiplication and division on hardware that implements the IA-32 and IA-64 instruction set architecture. Thus, with the exception of this case, arithmetic for these datatypes is reproducible across platforms. When the result of a computation is NaN, all platforms produce a value for which IS NAN is true. However, all platforms do not have to use the same bit pattern.

Using Conversion Functions with Native Floating-Point Datatypes

Oracle Database defines functions that convert between floating-point and other formats, including string formats that use decimal precision (precision may be lost during the conversion). For example, you can use the following functions:

• TO_BINARY_DOUBLE, which converts float to double, decimal (string) to double, and float or double to integer-valued double

• TO_BINARY_FLOAT, which converts double to float, decimal (string) to float, and float or double to integer-valued float

• TO_CHAR, which converts float or double to decimal (string)

• TO_NUMBER, which converts a float, double, or string to a number

Oracle Database can raise exceptions during conversion. The IEEE 754 specification defines the following exceptions:

• Invalid

• Inexact

• Divide by zero

• Underflow

• Overflow

Oracle Database does not raise these exceptions for native floating-point datatypes. Generally, situations that would raise an exception produce the values described in Table 3-5.

Client Interfaces for Native Floating-Point Datatypes

Oracle Database has implemented support for native floating-point datatypes in the following client interfaces:

• SQL

• PL/SQL

• OCI and OCCI

• Pro*C/C++

• JDBC

Representing Datetime Data: Overview

Oracle Database supports the following datetime datatypes:

• DATE

• TIMESTAMP

• TIMESTAMP WITH TIME ZONE

• TIMESTAMP WITH LOCAL TIME ZONE

Manipulating the Date Format

For input and output of dates, the standard Oracle Database default date format is DD-MON-RR. The RR datetime format element enables you store 20th century dates in the 21st century by specifying only the last two digits of the year.

As explained in Oracle Database SQL Reference, the century of the return value varies according to the specified two-digit year and the last two digits of the current year. For example, the following format refers to the year 2004 in a query issued between 1950 and 2049, but to the year 2005 in a query issued between 2050 and 2099:

'13-NOV-04'

SELECT TO_CHAR(TO_DATE('27-OCT-98', 'DD-MON-RR') ,'YYYY') "Year"FROM DUAL;

SELECT TO_CHAR(SYSDATE, 'DD-MON-YYYY BC')
FROM DUAL;

TO_CHAR(SYSDAT
--------------
24-JAN-2004 AD

Manipulating the Time Format

Time is stored in the following 24-hour format:

HH24:MI:SS

By default, the time in a DATE column is 12:00:00 A.M. (midnight) if no time portion is entered or if the DATE is truncated.

In a time-only entry, the date portion defaults to the first day of the current month. To enter the time portion of a date, use the TO_DATE function with a format mask indicating the time portion, as shown in Example 3-3.

-- create test table
CREATE TABLE birthdays 
( Bname VARCHAR2(20),
  Bday  DATE 
);

-- insert a row
INSERT INTO birthdays (bname, bday) 
VALUES
( 'ANNIE',
  TO_DATE('13-NOV-92 10:56 A.M.','DD-MON-YY HH:MI A.M.') 
);

Performing Date Arithmetic

Oracle Database provides a number of features to help with date arithmetic, so that you do not need to perform your own calculations on the number of seconds in a day, the number of days in each month, and so on. Some useful features include the following:

• ADD_MONTHS function, which returns the date plus the specified number of months.

• SYSDATE function, which returns the current date and time set for the operating system on which the database resides.

• SYSTIMESTAMP function, which returns the system date, including fractional seconds and time zone, of the system on which the database resides.

• TRUNC function, which when applied to a DATE value, trims off the time portion so that it represents the very beginning of the day (the stroke of midnight). By truncating two DATE values and comparing them, you can determine whether they refer to the same day. You can also use TRUNC along with a GROUP BY clause to produce daily totals.

• Arithmetic operators such as + and -. For example, SYSDATE-7 refers to 7 days before the current system date.

• INTERVAL datatypes, which enable you to represent constants when performing date arithmetic rather than performing your own calculations. For example, you can add or subtract INTERVAL constants from DATE values or subtract two DATE values and compare the result to an INTERVAL.

• Comparison operators such as >, <, =, and BETWEEN.

Converting Between Datetime Types

Oracle Database provides several useful functions that enable you to convert to a from datetime datatypes. Some useful functions include:

• EXTRACT, which extracts and returns the value of a specified datetime field from a datetime or interval value expression

• NUMTODSINTERVAL, which converts a NUMBER or expression that can be implicitly converted to a NUMBER value to an INTERVAL DAY TO SECOND literal

• NUMTOYMINTERVAL, which converts a NUMBER or expression that can be implicitly converted to a NUMBER value to an INTERVAL YEAR TO MONTH literal

• TO_DATE, which converts character data to a DATE datatype

• TO_CHAR, which converts DATE data to character data

• TO_DSINTERVAL, which converts a character string to an INTERVAL DAY TO SECOND value

• TO_TIMESTAMP, which converts character data to a value of TIMESTAMP datatype

• TO_TIMESTAMP_TZ, which converts character data to a value of TIMESTAMP WITH TIME ZONE datatype

• TO_YMINTERVAL, which converts a character string to an INTERVAL YEAR TO MONTH type

  See Also:

  Oracle Database SQL Reference for details about each function

Importing and Exporting Datetime Types

TIMESTAMP WITH TIME ZONE and TIMESTAMP WITH LOCAL TIME ZONE values are always stored in normalized format, so that you can export, import, and compare them without worrying about time zone offsets. DATE and TIMESTAMP values do not store an associated time zone, and you must adjust them to account for any time zone differences between source and target databases.

Representing Geographic Data

To represent Geographic Information System (GIS) or spatial data in the database, you can use Oracle Spatial features, including the type MDSYS.SDO_GEOMETRY. You can store the data in the database by using either an object-relational or a relational model. You can use a set of PL/SQL packages to query and manipulate the data.

Representing Multimedia Data

Oracle interMedia enables Oracle Database to store, manage, and retrieve images, audio, video, or other heterogeneous media data in an integrated fashion with other enterprise information. Oracle interMedia extends Oracle Database reliability, availability, and data management to multimedia content in traditional, Internet, electronic commerce, and media-rich applications.

Whether you store such multimedia data inside the database as BLOB or BFILE values, or store it externally on a Web server or other kind of server, you can use interMedia to access the data using either an object-relational or a relational model, and manipulate and query the data using a set of object types.

Oracle interMedia provides the ORDAudio, ORDDoc, ORDImage, ORDImageSignature, ORDVideo, and SI_StillImage object types and methods for the following purposes:

• Extracting metadata and attributes from multimedia dataRetrieving and managing multimedia data from Oracle interMedia, Web servers, file systems, and other serversPerforming manipulation operations on image data

Representing Large Amounts of Data

Oracle Database provides several datatypes for representing large amounts of data. These datatypes are grouped under the general category of Large Objects (LOBs). Table 3-6 describes the different LOBs.

An instance of type BLOB, CLOB, or NCLOB can exist as either a persistent LOB instance or a temporary LOB instance. Persistent and temporary instances differ as follows:

• A temporary LOB instance is declared in the scope of your application.

• A persistent LOB instance is created and stored in the database.

With the exception of declaring, freeing, creating, and committing, operations on persistent and temporary LOB instances are performed the same way.

Representing Searchable Text

Rather than writing low-level code to do full-text searches, you can use Oracle Text. It stores the search data in a special kind of index, and lets you query the data with operators and PL/SQL packages. This technology enables you to create your own search engine using data from tables, files, or URLs, and combine the search logic with relational queries. You can also search XML data this way with the XPath notation.

Representing XML

If you have information stored as files in XML format, or if you want to take an object type and store it as XML, then you can use the XMLType built-in type.

XMLType columns store their data as CLOBs. You can take an existing CLOB, VARCHAR2, or any object type, and call the XMLType constructor to turn it into an XML object.

When an XML object is inside the database, you can use queries to traverse it (using the XML XPath notation) and extract all or part of its data.

You can also produce XML output from existing relational data and split XML documents across relational tables and columns. You can use the following packages to transfer XML data into and out of relational tables:

• DBMS_XMLQUERY, which provides database-to-XMLType functionality

• DBMS_XMLGEN, which converts the results of a SQL query to a canonical XML format

• DBMS_XMLSAVE, which provides XML to database-type functionality

You can use the following SQL functions to process XML:

• EXTRACT, which applies a VARCHAR2 XPath string and returns an XMLType instance containing an XML fragment

• SYS_XMLAGG, which aggregates all of the XML documents or fragments represented by an expression and produces a single XML document

• SYS_XMLGEN, which takes an expression that evaluates to a particular row and column of the database, and returns an instance of type XMLType containing an XML document

• UPDATEXML, which takes as arguments an XMLType instance and an XPath-value pair and returns an XMLType instance with the updated value

• XMLAGG, which takes a collection of XML fragments and returns an aggregated XML document

• XMLCOLATTVAL, which creates an XML fragment and then expands the resulting XML so that each XML fragment has the name column with the attribute name

• XMLCONCAT, which takes as input a series of XMLType instances, concatenates the series of elements for each row, and returns the concatenated series

• XMLELEMENT, which takes an element name for identifier, an optional collection of attributes for the element, and arguments that make up the content of the element

• XMLFOREST, which converts each of its argument parameters to XML, and then returns an XML fragment that is the concatenation of these converted arguments

• XMLSEQUENCE, which either takes as input an XMLType instance and returns a varray of the top-level nodes in the XMLType, or takes as input a REFCURSOR instance, with an optional instance of the XMLFormat object, and returns as an XMLSequence type an XML document for each row of the cursor

  XMLTRANSFORM, which takes as arguments an XMLType instance and an XSL style sheet, applies the style sheet to the instance, and returns an XMLType

Representing Dynamically Typed Data

Some languages allow datatypes to change at runtime or let a program check the type of a variable. For example, C has the union keyword and the void * pointer, while Java has the typeof operator and wrapper types such as Number. Oracle Database includes features that enable you to create variables and columns that can hold data of any type and test such data values to determine their underlying representation. Using these features, a single table column can represent a numeric value in one row, a string value in another row, and an object in another row.

You can use the built-in type SYS.ANYDATA to represent values of any scalar or object type. This type is an object type with methods to bring in a scalar value of any type, and turn the value back into a scalar or object. In the same way, you can use the built-in type SYS.ANYDATASET to represent values of any collection type.

To manipulate and check type information, you can use SYS.ANYTYPE in combination with the DBMS_TYPES package. The program in Example 3-4 represents data of different underlying types in a table, then interprets the underlying type of each row and processes each value appropriately.

-- This example defines and executes a PL/SQL procedure that 
-- uses methods built into SYS.ANYDATA to access information about 
-- data stored in a SYS.ANYDATA table column.
DROP TYPE Employee_type FORCE;
DROP TABLE mytab;
CREATE OR REPLACE TYPE Employee_type AS OBJECT ( empno NUMBER, 
  ename VARCHAR2(10) ); 
/ 
CREATE TABLE mytab ( id NUMBER, data SYS.ANYDATA ); 
INSERT INTO mytab VALUES (1, SYS.ANYDATA.ConvertNumber(5)); 
INSERT INTO mytab VALUES (2, 
                          SYS.ANYDATA.ConvertObject(Employee_type(5555, 'john'))); 
COMMIT; 
CREATE OR REPLACE PROCEDURE p
IS 
  CURSOR cur IS SELECT id, data FROM mytab; 
  v_id           mytab.id%TYPE; 
  v_data         mytab.data%TYPE; 
  v_type         SYS.ANYTYPE; 
  v_typecode     PLS_INTEGER; 
  v_typename     VARCHAR2(60);
  v_dummy        PLS_INTEGER; 
  v_n            NUMBER;
  v_employee     Employee_type; 
  non_null_anytype_for_NUMBER exception; 
  unknown_typename            exception; 
BEGIN 
  OPEN cur; 
  LOOP 
    FETCH cur INTO v_id, v_data; 
    EXIT WHEN cur%NOTFOUND;

/* The typecode is a number that signifies what type is represented by v_data.
   GetType also produces a value of type SYS.AnyType with methods you can call
   to find precision and scale of a number, length of a string, and so on. */

    v_typecode := v_data.GetType ( v_type /* OUT */ ); 

/* Now we compare the typecode against constants from DBMS_TYPES to see what
   kind of data we have, and decide how to display it. */

    CASE v_typecode 
      WHEN DBMS_TYPES.TYPECODE_NUMBER THEN 
        IF v_type IS NOT NULL 
-- This condition should never happen, but we check just in case.
          THEN RAISE non_null_anytype_for_NUMBER; END IF; 
-- For each type, there is a Get method.
        v_dummy := v_data.GetNUMBER ( v_n /* OUT */ ); 
        DBMS_OUTPUT.PUT_LINE ( 
          TO_CHAR(v_id) || ': NUMBER = ' || To_Char(v_n) ); 
      WHEN DBMS_TYPES.TYPECODE_OBJECT THEN 
        v_typename := v_data.GetTypeName(); 
-- An object type's name is qualified with the schema name.
        IF v_typename NOT IN ( 'HR.EMPLOYEE_TYPE' ) 
-- If we encounter any object type besides EMPLOYEE_TYPE, raise an exception.
          THEN RAISE unknown_typename; END IF; 
        v_dummy := v_data.GetObject ( v_employee /* OUT */  ); 
        DBMS_OUTPUT.PUT_LINE ( 
          To_Char(v_id) || ': user-defined type = ' || v_typename || 
            ' ( ' || v_employee.empno || ', ' || v_employee.ename || ' )' ); 
    END CASE; 
  END LOOP; 
  CLOSE cur; 
EXCEPTION 
  WHEN non_null_anytype_for_NUMBER THEN 
    RAISE_Application_Error ( -20000, 
      'Paradox: the return AnyType instance FROM GetType ' || 
      'should be NULL for all but user-defined types' ); 
  WHEN unknown_typename THEN 
    RAISE_Application_Error ( -20000, 'Unknown user-defined type ' ||
      v_typename || ' - program written to handle only HR.EMPLOYEE_TYPE' ); 
END; 
/

The query and procedure in Example 3-4 produce output like that shown in Example 3-5.

SQL> SELECT t.data.gettypename() AS "Type Name" FROM mytab t;
 
Type Name
--------------------------------------------------------------------------------
SYS.NUMBER
HR.EMPLOYEE_TYPE
 
SQL> EXEC p;
1: NUMBER = 5
2: user-defined type = HR.EMPLOYEE_TYPE ( 5555, john )

You can access the same features through the OCI interface by using the OCIType, OCIAnyData, and OCIAnyDataSet interfaces.

Representing Data with ANSI/ISO, DB2, and SQL/DS Datatypes

You can define columns of tables in Oracle Database by means of ANSI/ISO, DB2, and SQL/DS datatypes. Oracle Database internally converts such datatypes to Oracle datatypes.

The ANSI datatype conversions are shown in Table 3-7. The ANSI/ISO datatypes NUMERIC, DECIMAL, and DEC can specify only fixed-point numbers. For these datatypes, s defaults to 0.

Table 3-8 shows the DB2 and SQL/DS conversions.

The datatypes TIME, GRAPHIC, VARGRAPHIC, and LONG VARGRAPHIC of IBM products SQL/DS and DB2 have no corresponding Oracle datatype, and they cannot be used.

Querying the ROWID Pseudocolumn

Each table in Oracle Database has a pseudocolumn named ROWID. If the row is too large to fit within a single data block, then ROWID identifies the initial row piece. Although rowids are usually unique, different rows can have the same rowid if they are in the same data block but in different clustered tables.

The following SQL statements return the ROWID pseudocolumn of the row of the hr.employees table that satisfies the query, and inserts it into the t_tab table:

CREATE TABLE t_tab (col1 ROWID);
INSERT INTO t_tab 
  SELECT ROWID 
  FROM hr.employees
  WHERE employee_id = 7499;

Accessing the ROWID Datatype

In tables that are not index-organized and foreign tables, the values of the ROWID pseudocolumn have the datatype ROWID. The format of this datatype is either extended or restricted.

struct riddef {
    ub4    ridobjnum; /* data obj#--this field is 
                         unused in restricted ROWIDs */
    ub2    ridfilenum;
    ub1    filler;
    ub4    ridblocknum;
    ub2    ridslotnum;
}

Accessing the UROWID Datatype

The rows of some tables have addresses that are not physical or permanent or were not generated by Oracle Database. For example, the row addresses of index-organized tables are stored in index leaves, which can move. Oracle provides these tables with logical row identifiers, called logical rowids. Rowids of foreign tables, such as DB2 tables accessed through a gateway, are not standard Oracle Database rowids. Oracle provides foreign tables with identifiers called foreign rowids.Oracle Database uses universal rowids (urowids) to store the addresses of index-organized and foreign tables. Both types of urowid are stored in the ROWID pseudocolumn, as are the physical rowids of heap-organized tables.Oracle creates logical rowids based on the primary key of the table. The logical rowids do not change as long as the primary key does not change. The ROWID pseudocolumn of an index-organized table has a datatype of UROWID. You can access this pseudocolumn as you would the ROWID pseudocolumn of a heap-organized table (that is, using a SELECT ... ROWID statement). If you want to store the rowids of an index-organized table, then you can define a column of type UROWID for the table and retrieve the value of the ROWID pseudocolumn into that column.

Datatype Conversion During Assignments

The datatype conversion for an assignment succeeds if Oracle Database can convert the datatype of the value used in the assignment to that of the assignment target.

For the examples in the following list, assume a package with a public variable and a table declared as in the following statements:

CREATE PACKAGE Test_Pack AS var1 CHAR(5); END;
CREATE TABLE Table1_tab (col1 NUMBER); 

• variable := expression

  The datatype of expression must be either the same as, or convertible to, the datatype of variable. For example, Oracle Database automatically converts the data provided in the following assignment within the body of a stored procedure:

  VAR1 := 0;
  
  

• INSERT INTO Table1_tab VALUES (expression1, expression2, ...)

  The datatypes of expression1, expression2, and so on, must be either the same as, or convertible to, the datatypes of the corresponding columns in Table1_tab. For example, Oracle Database automatically converts the data provided in the following INSERT statement for Table1_tab:

  INSERT INTO Table1_tab VALUES ('19'); 
  
  

• UPDATE Table1_tab SET column = expression

  The datatype of expression must be either the same as, or convertible to, the datatype of column. For example, Oracle Database automatically converts the data provided in the following UPDATE statement issued against Table1_tab:

  UPDATE Table1_tab SET col1 = '30'; 
  
  

• SELECT column INTO variable FROM Table1_tab

  The datatype of column must be either the same as, or convertible to, the datatype of variable. For example, Oracle Database automatically converts data selected from the table before assigning it to the variable in the following statement:

  SELECT Col1 INTO Var1 FROM Table1_tab WHERE Col1 = 30;
  
  

Datatype Conversion During Expression Evaluation

For expression evaluation, Oracle Database can automatically perform the same conversions as for assignments. An expression is converted to a type based on its context. For example, operands to arithmetic operators are converted to NUMBER, and operands to string functions are converted to VARCHAR2.

Oracle Database can automatically convert the following:

• VARCHAR2 or CHAR to NUMBER

• VARCHAR2 or CHAR to DATE

Character to NUMBER conversions succeed only if the character string represents a valid number. Character to DATE conversions succeed only if the character string satisfies the session default format, which is specified by the initialization parameter NLS_DATE_FORMAT.

Some common types of expressions follow:

• Simple expressions, such as:

  commission + '500'
  
  

• Boolean expressions, such as:

  bonus > salary / '10'
  
  

• Function and procedure calls, such as:

  MOD (counter, '2')
  
  

• WHERE clause conditions, such as:

  WHERE hiredate = TO_DATE('1997-01-01','yyyy-mm-dd')
  
  

• WHERE clause conditions, such as:

  WHERE rowid = 'AAAAaoAATAAAADAAA'
  
  

In general, Oracle Database uses the rule for expression evaluation when a datatype conversion is needed in places not covered by the rule for assignment conversions.

In assignments of the form:

variable := expression

Oracle Database first evaluates expression using the conversion rules for expressions; expression can be as simple or complex as desired. If it succeeds, then the evaluation of expression results in a single value and datatype. Then, Oracle Database tries to assign this value to the target variable using the conversion rules for assignments.