Data type conversion, PL-SQL Programming

Datatype Conversion

At times it is necessary to convert a value from one datatype to another. For e.g. if you want to inspect a rowid, you should convert it to a character string. The PL/SQL supports both the explicit and implicit (automatic) datatype conversion.

Explicit Conversion

To convert the values from one datatype to other, you use the built-in functions. For e.g. to convert a CHAR  value to a DATE   or NUMBER   value, use the function TO_DATE  or TO_NUMBER, respectively. On the contrary, to convert a DATE or NUMBER value to the CHAR value, you use the function TO_CHAR.

Implicit Conversion

When it makes sense, the PL/SQL can convert the datatype of a value implicitly. This permits you to use the variables, literals, and parameters of one type where the other type is expected. In the example shown below, the CHAR variables start_time and finish_ time hold string values representing the number of seconds in the past midnight. The variation between those values must be assigned to the NUMBER variable elapsed_time. And hence, the PL/SQL converts the CHAR values to the NUMBER values automatically.

DECLARE

start_time CHAR(5);

finish_time CHAR(5);

elapsed_time NUMBER(5);

BEGIN

/* Get system time as seconds past midnight. */

SELECT TO_CHAR(SYSDATE,'SSSSS') INTO start_time FROM sys.dual;

-- do something

/* Get system time again. */

SELECT TO_CHAR(SYSDATE,'SSSSS') INTO finish_time FROM sys.dual;

/* Compute elapsed time in seconds. */

elapsed_time := finish_time - start_time;

INSERT INTO results VALUES (elapsed_time, ...);

END;

Before assigning a selected column value to a variable, the PL/SQL will, if necessary then convert the value from the datatype of the source column to the datatype of the variable. This happens, for e.g.  If you select a DATE column value into a VARCHAR2 variable.

Similarly, before assigning the value of a variable to a database column, the PL/SQL will, if necessary then convert the value from the datatype of the variable to the datatype of the target column. If the PL/SQL cannot determine that implicit conversion is required, you get a compilation error. In such situation, you should use a datatype conversion function. The table shows that implicit conversions PL/SQL can do.

566_data type conversion.png

It is your duty to ensure that the values are convertible. For illustration, the PL/SQL can convert the CHAR value '02-JUN-92' to a DATE value but cannot convert the CHAR value 'YESTERDAY' to a DATE value. Likewise, the PL/SQL cannot convert a VARCHAR2 value containing the alphabetic characters to a NUMBER value.

Implicit versus Explicit Conversion

Normally, to rely on the implicit datatype conversions is a poor programming practice as they can hamper the performance and might change from one software release to the next. Also, the implicit conversions are context sensitive and hence not always predictable. Rather, use datatype conversion functions. In that way, your applications will be easier and reliable to maintain.

Posted Date: 10/2/2012 2:41:38 AM | Location : United States







Related Discussions:- Data type conversion, Assignment Help, Ask Question on Data type conversion, Get Answer, Expert's Help, Data type conversion Discussions

Write discussion on Data type conversion
Your posts are moderated
Related Questions
Using TRIM This process has two forms. The TRIM removes an element from the end of the collection. The TRIM(n) removes the n elements from the end of the collection. For e.g.

Processing Transactions This part describes how to do the transaction processing. You learn the fundamental techniques that safeguard the consistency of your database, involvin

What Is a Package? The package is a schema object that group logically related PL/SQL items, types, and subprograms. The Packages usually have 2 parts, a specification & a bo

How Exceptions Are Raised By the run-time system, the internal exceptions are raised implicitly as are user-defined exceptions that you have related with an Oracle error number

IS NULL Operator The IS NULL operator returns the Boolean value TRUE whenever its operand is null or FALSE if it is not null. The comparisons including the nulls always yield NU

BETWEEN and NOT BETWEEN Operator in SQL Example: Restricting exam marks to between 0 and 100 CREATE ASSERTION Marks_between_0_and_100 CHECK (NOT EXISTS (SELECT * FROM

Restriction in SQL Syntax: SELECT DISTINCT StudentId FROM IS_CALLED WHERE Name = 'Boris' The WHERE clause operates on the result of the FROM clause in analogo

Manipulating Individual Elements Faraway you have manipulated an entire collection. Within the SQL, to manipulate the individual elements of the collection, and then use the ope

Project Description: I want a small relational database to be built. I want the database to have the subsequent information tables: Employee Information Document storage

Disjunction (OR, ∨) Again we have nine rows instead of just four and again, when unknown is not involved, the rows are as for 2VL. Also, when anything is paired with true, t