Oracle8 Server Application Developer's Guide
Release 8.0
A54642_01

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5
Selecting a Datatype

This chapter discusses how to use Oracle built-in datatypes in applications. Topics include :

For information about user-defined datatypes, refer to Oracle8 Server Concepts and to Chapter 7, "User-Defined Datatypes -An Extended Example" in this manual.

Oracle Built-In Datatypes

A datatype associates a fixed set of properties with the values that can be used in a column of a table or in an argument of a procedure or function. These properties cause Oracle to treat values of one datatype differently from values of another datatype; for example, Oracle can add values of NUMBER datatype but not values of RAW datatype.

Oracle supplies the following built-in datatypes:

character datatypes

CHAR
NCHAR
VARCHAR2 and VARCHAR
NVARCHAR2
CLOB
NCLOB
LONG

NUMBER datatype
DATE datatype
binary datatypes

BLOB
BFILE
RAW
LONG RAW

Another datatype, ROWID, is used for values in the ROWID pseudocolumn, which represents the unique address of each row in a table.

Table 5-2 summarizes the information about each Oracle built-in datatype. See Oracle8 Server Concepts for general descriptions of these datatypes, and see Chapter 6, "Large Objects (LOBs)" in this Guide for information about the LOB datatypes.

Table 5-1: Summary of Oracle Built-In Datatypes

Datatype	Description	Column Length and Default
CHAR (size)	Fixed-length character data of length size bytes.	Fixed for every row in the table (with trailing blanks); maximum size is 2000 bytes per row, default size is 1 byte per row. Consider the character set (one-byte or multibyte) before setting size.
VARCHAR2 (size)	Variable-length character data.	Variable for each row, up to 4000 bytes per row. Consider the character set (one-byte or multibyte) before setting size. A maximum size must be specified.
NCHAR(size)	Fixed-length character data of length size characters or bytes, depending on the national character set.	Fixed for every row in the table (with trailing blanks). Column size is the number of characters for a fixed-width national character set or the number of bytes for a varying-width national character set. Maximum size is determined by the number of bytes required to store one character, with an upper limit of 2000 bytes per row. Default is 1 character or 1 byte, depending on the character set.
NVARCHAR2 (size)	Variable-length character data of length size characters or bytes, depending on national character set. A maximum size must be specified.	Variable for each row. Column size is the number of characters for a fixed-width national character set or the number of bytes for a varying-width national character set. Maximum size is determined by the number of bytes required to store one character, with an upper limit of 4000 bytes per row. Default is 1 character or 1 byte, depending on the character set.
CLOB	Single-byte character data.	Up to 2^32 - 1 bytes, or 4 gigabytes.
NCLOB	Single-byte or fixed-length multibyte national character set (NCHAR) data.	Up to 2^32 - 1 bytes, or 4 gigabytes.
LONG	Variable-length character data.	Variable for each row in the table, up to 2^31 - 1 bytes, or 2 gigabytes, per row. Provided for backward compatibility.
NUMBER (p, s)	Variable-length numeric data. Maximum precision p and/or scale s is 38.	Variable for each row. The maximum space required for a given column is 21 bytes per row.
DATE	Fixed-length date and time data, ranging from Jan. 1, 4712 B.C.E. to Dec. 31, 4712 C.E.	Fixed at 7 bytes for each row in the table. Default format is a string (such as DD-MON-YY) specified by NLS_DATE_FORMAT parameter.
BLOB	Unstructured binary data.	Up to 2^32 - 1 bytes, or 4 gigabytes.
BFILE	Binary data stored in an external file.	Up to 2^32 - 1 bytes, or 4 gigabytes.
RAW (size)	Variable-length raw binary data.	Variable for each row in the table, up to 2000 bytes per row. A maximum size must be specified. Provided for backward compatibility.
LONG RAW	Variable-length raw binary data.	Variable for each row in the table, up to 2^31 - 1 bytes, or 2 gigabytes, per row. Provided for backward compatibility.
ROWID	Binary data representing row addresses.	Fixed at 10 bytes (extended ROWID) or 6 bytes (restricted ROWID) for each row in the table.
MLSLABEL	Trusted Oracle datatype.	See the Trusted Oracle Server Administrator's Guide.

Using Character Datatypes

Use the character datatypes to store alphanumeric data.

CHAR and NCHAR datatypes store fixed-length character strings.
VARCHAR2 and NVARCHAR2 datatypes store variable-length character strings. (The VARCHAR datatype is synonymous with the VARCHAR2 datatype.)
CLOB and NCLOB datatypes store single-byte and multibyte character strings of up to four gigabytes (see Chapter 6, "Large Objects (LOBs)").
The LONG datatype stores variable-length character strings containing up to two gigabytes (with many restrictions - see "Restrictions on LONG and LONG RAW Data" on page 5-9). This datatype is provided for backward compatibility with existing applications; in general, new applications should use CLOB and NCLOB datatypes to store large amounts of character data.

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, while the VARCHAR2 datatype does not blank-pad or store trailing blanks for column values.
Comparison Semantics
Use the CHAR datatype when you require ANSI compatibility in comparison semantics, that is, 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 and will always be fully supported. At this time, the VARCHAR datatype automatically corresponds to the VARCHAR2 datatype and is reserved for future use.

CHAR, VARCHAR2, and LONG data is automatically converted from the database character set to the character set defined for the user session by the NLS_LANGUAGE parameter, where these are different.

Column Lengths for Single-Byte and Multibyte Character Sets

The lengths of CHAR and VARCHAR2 columns are specified in bytes rather than characters, and are constrained as such. The lengths of NCHAR and NVARCHAR2 columns are specified either in bytes or in characters, depending on the national character set being used.

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, the number of bytes and the number of characters in a column is the same. If it is multibyte, 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.

See the Oracle8 Server Reference Manual for information about National Language Support features of Oracle and support for different character encoding schemes.

Comparison Semantics

Oracle compares CHAR and NCHAR values using blank-padded comparison semantics. If two values have different lengths, Oracle adds blanks at the end of the shorter value, until the two values are the same length. Oracle then compares the values character-by-character up to the first character that differs. The value with the greater character in the first differing position is considered greater. Two values that differ only in the number of trailing blanks are considered equal.

Oracle compares VARCHAR2 and NVARCHAR2 values using non-padded comparison semantics. Two values are considered equal only if they have the same characters and are of equal length. Oracle compares the values character-by-character up to the first character that differs. The value with the greater character in that position is considered greater.

Because Oracle blank-pads values stored in CHAR columns but not in VARCHAR2 columns, a value stored in a VARCHAR2 column may take up less space than if it were stored 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, you might be able to improve performance by storing this data in VARCHAR2 columns rather than in CHAR columns.

However, performance is not the only factor to consider when deciding which of these datatypes to use. Oracle 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 to ignore trailing blanks when comparing character values, you must store these values in CHAR columns.

See Also: For more information on comparison semantics for these datatypes, see the Oracle8 Server SQL Reference.

Using the NUMBER Datatype

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 platforms, and offer up to 38 decimal digits of precision. You can store positive and negative numbers of magnitude 1 x 10^-130 to 9.99...x10^125, as well as zero, in a NUMBER column.

For numeric columns you can specify the column as a floating-point number:

column_name NUMBER

or you can specify a precision (total number of digits) and scale (number of digits to right of decimal point):

column_name NUMBER (precision, scale)

Although not required, specifying the precision and scale for numeric fields provides extra integrity checking on input. If a precision is not specified, the column stores values as given. Table 5-2 shows examples of how data would be stored using different scale factors.

Table 5-2: How Scale Factors Affect Numeric Data Storage

Input Data	Stored As	Specified As
7,456,123.89	NUMBER	7456123.89
7,456,123.89	NUMBER (9)	7456124
7,456,123.89	NUMBER (9,2)	7456123.89
7,456,123.89	NUMBER (9,1)	7456123.9
7,456,123.89	NUMBER (6)	(not accepted, exceeds precision)
7,456,123.89	NUMBER (7, -2)	7456100

See Also: For information about the internal format for the NUMBER datatype, see Oracle8 Server Concepts.

Using the DATE Datatype

Use the DATE datatype to store point-in-time values (dates and times) in a table. The DATE datatype stores the century, year, month, day, hours, minutes, and seconds.

Oracle uses its own internal format to store dates. Date data is stored in fixed-length fields of seven bytes each, corresponding to century, year, month, day, hour, minute, and second. See the Programmer's Guide to the Oracle Call Interface, Volume I: OCI Concepts for a complete description of the Oracle internal date format.

Date Format

For input and output of dates, the standard Oracle default date format is DD-MON-YY, as in:

'13-NOV-92'

To change this default date format on an instance-wide basis, use the NLS_DATE_FORMAT parameter. To change the format during a session, use the ALTER SESSION statement. To enter dates that are not in the current default date format, use the TO_DATE function with a format mask, as in:

TO_DATE ('November 13, 1992', 'MONTH DD, YYYY')

Note: Oracle Julian dates might not be compatible with Julian dates generated by other date algorithms. For inrformation about Julian dates, see Oracle8 Server Concepts.

If the date format DD-MON-YY is used, YY indicates the year in the 20th century (for example, 31-DEC-92 is December 31, 1992). If you want to indicate years in any century other than the 20th century, use a different format mask, as shown above.

Time Format

Time is stored in 24-hour format#HH:MM:SS. By default, the time in a date field is 12:00:00 A.M. (midnight) if no time portion is entered. 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 in:

INSERT INTO birthdays (bname, bday) VALUES

('ANNIE',TO_DATE('13-NOV-92 10:56 A.M.','DD-MON-YY HH:MI A.M.'));

To compare dates that have time data, use the SQL function TRUNC if you want to ignore the time component. Use the SQL function SYSDATE to return the system date and time. The FIXED_DATE initialization parameter allows you to set SYSDATE to a constant; this can be useful for testing.

Centuries and the Year 2000

Oracle stores year data with the century information. For example, the Oracle database stores 1996 or 2001, and not just 96 or 01. The DATE datatype always stores a four-digit year internally, and all other dates stored internally in the database have four digit years. Oracle utilities such as import, export, and recovery also deal properly with four-digit years.

However, some applications might be written with an assumption about the year (such as assuming that everything is 19xx). The application might hand over a two-digit year to the database, and the procedures that Oracle uses for determining the century could be different from what the programmer expects. Application programmers should therefore review and test their code with regard to the year 2000.

The RR date format element of the TO_DATE and TO_CHAR functions allows a database site to default the century to different values depending on the two-digit year, so that years 50 to 99 default to 19xx and years 00 to 49 default to 20xx. This can help applications make the conversion to the new century easily.

The CC date format element of the TO_CHAR function sets the century value to one greater than the first two digits of a four-digit year (for example, '20' from '1900'). For years that are a multiple of 100, this is not the true century. Strictly speaking, the century of '1900' is not the twentieth century (which began in 1901) but rather the nineteenth century.

The following workaround computes the correct century for any Common Era (CE, formerly known as AD) date. If userdate is a CE date for which you want the true century, use the expression:

DECODE (TO_CHAR (userdate, 'YY'), 

       '00', TO_CHAR (userdate - 366, 'CC'), 

       TO_CHAR (userdate, 'CC'))

This expression works as follows.

Get the last two digits of the year. If it is '00', then it is a year in which the Oracle century is one year too large so compute a date in the preceding year (whose Oracle century is the desired true century). Otherwise, use the Oracle century.

For more information about date format codes, see Oracle8 Server SQL Reference.

Using the LONG Datatype

Note: The LONG datatype is provided for backward compatibility with existing applications. For new applications, you should use the CLOB and NCLOB datatypes for large amounts of character data. See Chapter 6, "Large Objects (LOBs)" for information about the CLOB and NCLOB datatypes.

The LONG datatype can store variable-length character data containing up to two gigabytes of information. The length of LONG values might be limited by the memory available on your computer.

You can use columns defined as LONG in SELECT lists, SET clauses of UPDATE statements, and VALUES clauses of INSERT statements. LONG columns have many of the characteristics of VARCHAR2 columns.

Restrictions on LONG and LONG RAW Data

Although LONG (and LONG RAW; see below) columns have many uses, their use has some restrictions:

Only one LONG column is allowed per table.
LONG columns cannot be indexed.
LONG columns cannot appear in integrity constraints.
LONG columns cannot be used in WHERE, GROUP BY, ORDER BY, or CONNECT BY clauses or with the DISTINCT operator in SELECT statements.
LONG columns cannot be referenced by SQL functions (such as SUBSTR or INSTR).
LONG columns cannot be used in the SELECT list of a subquery or queries combined by set operators (UNION, UNION ALL, INTERSECT, or MINUS).
LONG columns cannot be used in SQL expressions.
LONG columns cannot be referenced when creating a table with a query (CREATE TABLE . . . AS SELECT . . .) or when inserting into a table or view with a query (INSERT INTO . . . SELECT . . .).
A variable or argument of a PL/SQL program unit cannot be declared using the LONG datatype.
Variables in database triggers cannot be declared using the LONG or LONG RAW datatypes.
References to :NEW and :OLD in database triggers cannot be used with LONG or LONG RAW columns.
LONG and LONG RAW columns cannot be used in distributed SQL statements.
LONG and LONG RAW columns cannot be replicated.

Suggestion: If you design tables containing LONG or LONG RAW data, you should place each LONG or LONG RAW column in a table separate from any other data associated with it, rather than storing the LONG or LONG RAW column and its associated data together in the same table. You can then relate the two tables with a referential integrity constraint. This design allows SQL statements that access only the associated data to avoid reading through LONG or LONG RAW data.

Example of LONG Datatype

To store information on magazine articles, including the texts of each article, create two tables:

CREATE TABLE article_header   (id               NUMBER  

                                                PRIMARY KEY                 

                               title            VARCHAR2(200),

                               first_author     VARCHAR2(30),

                               journal          VARCHAR2(50),

                               pub_date         DATE)

CREATE TABLE article_text     (id               NUMBER

                                                REFERENCES

                                                article_header,

                               text             LONG)

The ARTICLE_TEXT table stores only the text of each article. The ARTICLE_HEADER table stores all other information about the article, including the title, first author, and journal and date of publication. The two tables are related by the referential integrity constraint on the ID column of each table.

This design allows SQL statements to query data other than the text of an article without reading through the text. If you want to select all first authors published in Nature magazine during July 1991, you can issue this statement that queries the ARTICLE_HEADER table:

SELECT first_author
    FROM article_header
    WHERE journal = 'NATURE'
          AND TO_CHAR(pub_date, 'MM YYYY') = '07 1991')

If the text of each article were stored in the same table with the first author, publication, and publication date, Oracle would have to read through the text to perform this query.

Using RAW and LONG RAW Datatypes

Note: The RAW and LONG RAW datatypes are provided for backward compatibility with existing applications. For new applications, you should use the BLOB and BFILE datatypes for large amounts of binary data. See Chapter 6, "Large Objects (LOBs)" for information about the BLOB and BFILE datatypes.

The RAW and LONG RAW datatypes store data that is not to be interpreted by Oracle (that is, not to be converted when moving data between different systems). These datatypes are intended for binary data and byte strings. For example, LONG RAW can be used to store graphics, sound, documents, and arrays of binary data; the interpretation is dependent on the use.

Net8 and the Export and Import utilities do not perform character conversion when transmitting RAW or LONG RAW data. When Oracle automatically converts RAW or LONG RAW data to and from CHAR data (as is the case when entering RAW data as a literal in an INSERT statement), the data is represented as one hexadecimal character representing the bit pattern for every four bits of RAW data. For example, one byte of RAW data with bits 11001011 is displayed and entered as 'CB'.

LONG RAW data cannot be indexed, but RAW data can be indexed. For more information about restrictions on LONG RAW data, see "Restrictions on LONG and LONG RAW Data" on page 5-9.

ROWIDs and the ROWID Datatype

Every row in a nonclustered table of an Oracle database is assigned a unique ROWID that corresponds to the physical address of a row's row piece (initial row piece if the row is chained among multiple row pieces). In the case of clustered tables, rows in different tables that are in the same data block can have the same ROWID.

Each table in an Oracle database internally has a pseudocolumn named ROWID. See Oracle8 Server Concepts for general information about the ROWID pseudocolumn and the ROWID datatype.

Extended ROWID Format

The Oracle Server uses an extended ROWID format, which supports features such as table partitions, index partitions, and clusters.

The extended ROWID includes the following information:

data object (segment) identifier
datafile identifier
block identifier
row identifier

The data object identifier is an identification number that Oracle assigns to schema objects in the database, such as nonpartitioned tables or partitions. For example, the query

SELECT DATA_OBJECT_ID FROM DBA_OBJECTS

  WHERE OWNER = 'SCOTT' AND OBJECT_NAME = 'EMP';

returns the data object identifier for the EMP table in the SCOTT schema. "The DBMS_ROWID Package" on page 10-74 describes other ways to get the data object identifier, using the DBMS_ROWID package functions.

Different Forms of the ROWID

Oracle documentation uses the term ROWID in different ways, depending on context. These uses are explained in this section.

Internal ROWID

The internal ROWID format is an internal structure which holds information that the server code needs to access a row. The restricted internal ROWID is 6 bytes on most platforms; the extended ROWID is 10 bytes on these platforms.

ROWID Pseudocolumn

Each table and nonjoined view has a pseudocolumn called ROWID. Statements such as

CREATE TABLE T1 (col1 ROWID);



INSERT INTO T1 SELECT ROWID FROM EMP WHERE empno = 7499;

return the ROWID pseudocolumn of the row of the EMP table that satisfies the query, and insert it into the T1 table.

External Character ROWID

The extended ROWID pseudocolumn is returned to the client in the form of an 18-character string (for example, "AAAA8mAALAAAAQkAAA"), which represents a base 64 encoding of the components of the extended ROWID in a four-piece format, OOOOOOFFFBBBBBBRRR:

OOOOOO: The data object number identifies the database segment (AAAA8m in the example). Schema objects in the same segment, such as a cluster of tables, have the same data object number.
FFF: The datafile that contains the row (file AAL in the example). File numbers are unique within a database.
BBBBBB: The data block that contains the row (block AAAAQk in the example). Block numbers are relative to their datafile, not tablespace. Therefore, two rows with identical block numbers could reside in two different datafiles of the same tablespace.
RRR: The row in the block (row AAA in the example).

There is no need to decode the external ROWID; you can use the functions in the DBMS_ROWID package to obtain the individual components of the extended ROWID. For more information, see "The DBMS_ROWID Package" on page 10-74.

The restricted ROWID pseudocolumn is returned to the client in the form of an 18-character string with a hexadecimal encoding of the datablock, row, and datafile components of the ROWID.

External Binary ROWID

Some client applications use a binary form of the ROWID. For example, OCI and some precompiler applications can map the ROWID to a 3GL structure on bind or define calls. The size of the binary ROWID is the same for extended and restricted ROWIDs. The information for the extended ROWID is included in an unused field of the restricted ROWID structure.

The format of the extended binary ROWID, expressed as a C struct, is:

struct riddef {

  ub4    ridobjnum; /* data obj#--this field is 

                       unused in restricted ROWIDs */

  ub2    ridfilenum;

  ub1    filler;

  ub4    ridblocknum;

  ub2    ridslotnum;

}

ROWID Migration and Compatibility Issues

For backward compatibility, the restricted form of the ROWID is still supported. These ROWIDs exist in massive amounts of Oracle7 data, and the extended form of the ROWID is required only in global indexes on partitioned tables (see Oracle8 Server Administrator's Guide). New tables always get extended ROWIDs.

It is possible for an Oracle7 client to access an Oracle8 database. Similarly, an Oracle8 client can access an Oracle7 Server. A client in this sense can include a remote database accessing a server using database links, as well as a client 3GL or 4GL application accessing a server.

See Also: For additional migration information, see the description of "ROWID_TO_EXTENDED Function" on page 10-80. Also see Oracle8 Server Migration.

Accessing an Oracle7 Database from an Oracle8 Client

The ROWID values that are returned are always restricted ROWIDs. Also, Oracle8 uses restricted ROWIDs when returning a ROWID value to an Oracle7 or earlier server.

The following ROWID functionality works when accessing an Oracle7 Server:

selecting a ROWID and using the obtained value in a WHERE clause
WHERE CURRENT OF cursor operations
storing ROWIDs in user columns of ROWID or CHAR type
interpreting ROWIDs using the hexadecimal encoding (not recommended, use the DBMS_ROWID functions)

Accessing an Oracle8 Database from an Oracle7 Client

Oracle8 returns ROWIDs in the extended format. This means that you can only:

select a ROWID and use it in a WHERE clause
use WHERE CURRENT OF cursor operations
store ROWIDs in user columns of CHAR(18) datatype

Import and Export

It is not possible for an Oracle7 client to import an Oracle8 table that has a ROWID column (not the ROWID pseudocolumn), if any row of the table contains an extended ROWID value.

Trusted Oracle MLSLABEL Datatype

Trusted Oracle provides the MLSLABEL datatype, which stores Trusted Oracle's internal representation of labels generated by multilevel secure (MLS) operating systems. Trusted Oracle uses labels to control database access.

You can define a column using the MLSLABEL datatype for compatibility with Trusted Oracle applications, but the only valid value for the column in Oracle8 is NULL.

When you create a table in Trusted Oracle, a column called ROWLABEL is automatically appended to the table. This column contains a label of the MLSLABEL datatype for every row in the table.

See the Trusted Oracle Server Administrator's Guide for more information about the MLSLABEL datatype, the ROWLABEL column, and Trusted Oracle.

ANSI/ISO, DB2, and SQL/DS Datatypes

You can define columns of tables in an Oracle database using ANSI/ISO, DB2, and SQL/DS datatypes. Oracle internally converts such datatypes to Oracle datatypes.

The ANSI datatype conversions to Oracle datatypes are shown in Table 5-3. The ANSI/ISO datatypes NUMERIC, DECIMAL, and DEC can specify only fixed-point numbers. For these datatypes, s defaults to 0.

Table 5-3: ANSI Datatype Conversions to Oracle Datatypes

ANSI SQL Datatype	Oracle Datatype
CHARACTER (n), CHAR (n)	CHAR (n)
NUMERIC (p,s), DECIMAL (p,s), DEC (p,s)	NUMBER (p,s)
INTEGER, INT, SMALLINT	NUMBER (38)
FLOAT (p)	FLOAT (p)
REAL	FLOAT (63)
DOUBLE PRECISION	FLOAT (126)
CHARACTER VARYING(n), CHAR VARYING(n)	VARCHAR2 (n)

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

Table 5-4 shows the DB2 and SQL/DS conversions.

Table 5-4: SQL/DS, DB2 Datatype Conversions to Oracle Datatypes

DB2 or SQL/DS Datatype	Oracle Datatype
CHARACTER (n)	CHAR (n)
VARCHAR (n)	VARCHAR2 (n)
LONG VARCHAR	LONG
DECIMAL (p,s)	NUMBER (p,s)
INTEGER, SMALLINT	NUMBER (38)
FLOAT (p)	FLOAT (p)
DATE	DATE

Data Conversion

In some cases, Oracle allows data of one datatype where it expects data of a different datatype. Generally, an expression cannot contain values with different datatypes. However, Oracle can use the following functions to automatically convert data to the expected datatype:

TO_NUMBER()
TO_CHAR()
TO_DATE()
HEXTORAW()
RAWTOHEX()
ROWIDTOCHAR()
CHARTOROWID()

Implicit datatype conversions work according to the rules explained below.

If you are using Trusted Oracle, see "Data Conversion for Trusted Oracle" on page 5-20 for information about data conversions and the MLSLABEL datatype.

Rule 1: Assignments

For assignments, Oracle can automatically convert the following:

VARCHAR2 or CHAR to NUMBER
NUMBER to VARCHAR2
VARCHAR2 or CHAR to DATE
DATE to VARCHAR2
VARCHAR2 or CHAR to ROWID
ROWID to VARCHAR2
VARCHAR2 or CHAR to MLSLABEL
MLSLABEL to VARCHAR2
VARCHAR2 or CHAR to HEX
HEX to VARCHAR2

The assignment succeeds if Oracle can convert the datatype of the value used in the assignment to that of the assignment's target.

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

var1  CHAR(5); 

CREATE TABLE table1 (col1 NUMBER);

variable := expression

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

VAR1 := 0

INSERT INTO table 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 table. For example, Oracle automatically converts the data provided in the following INSERT statement for TABLE1 (see table definition above):

INSERT INTO table1 VALUES ('19');

UPDATE table SET column = expression

The datatype of expression must be either the same as or convertible to the datatype of column. For example, Oracle automatically converts the data provided in the following UPDATE statement issued against TABLE1:

UPDATE table1 SET col1 = '30';

SELECT column INTO variable FROM table

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

SELECT col1 INTO var1 FROM table1 WHERE col1 = 30;

Rule 2: Expression Evaluation

For expression evaluation, Oracle 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 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

comm + '500'

Boolean expressions, such as

bonus > sal / '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 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 first evaluates expression using the conversions covered by Rule 2; expression can be as simple or complex as desired. If it succeeds, the evaluation of expression results in a single value and datatype. Then, Oracle tries to assign this value to the assignment's target using Rule 1.

Data Conversion for Trusted Oracle

In Trusted Oracle, labels are stored internally as compact binary structures. Trusted Oracle provides the TO_LABEL function that enables you to convert a label from its internal binary format to an external character format. To convert a label from character format to binary format in Trusted Oracle, you use the TO_CHAR function.

The TO_LABEL function is provided for compatibility with Trusted Oracle applications. It returns the NULL value in Oracle8.

See your Trusted Oracle Server Administrator's Guide for more information about using the TO_LABEL and TO_CHAR functions to convert label formats.