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Exceptions and Exception Handling
Oracle Tips by Burleson

PL/SQL was originally based on the Ada programming language (once widely used in software written for the Department of Defense). One of the primary reasons that Ada was chosen as the mother language for PL/SQL is the concept of exceptions. Figure 2.10 illustrates how exceptions are raised and handled.

When a PL/SQL block encounters an error condition, an exception is raised. Each PL/SQL block can have an exception handler prior to the END statement. In this section of the block, the developer specifies what actions should be taken for specific exceptions. If no exception handler is declared in a block, the exception is raised to the calling PL/SQL block.

Table 2.4 lists the standard exceptions that you will encounter most frequently.

Table 2.4 The five most commonly encountered exceptions.




You executed a query for which no rows were found.


You executed a query for which more than one row was found. Your query was only structured to receive a single row in the result set.


You attempted to insert a row into a table, but the row violates the table’s primary key or unique index.


You assigned a value to a variable that is too short to hold the value. This occurs most commonly with variables of type varchar2 or char, but this can also happen to variables of other types.


You referenced a value containing a character in an expression that attempted to convert the value to a number, either explicitly or implicitly.

If no exception is raised from a PL/SQL block (or stored PL/SQL object), the calling object assumes that the block completed successfully. Unlike a function written in C, there are no status values that are found in the database environment to indicate if a procedure or function call encountered an error.


Most of the code that you write will be executable code, with one notable exception. A PRAGMA is not executable code; instead, a PRAGMA is an instruction to the PL/SQL compiler. PL/SQL provides two distinct uses of the PRAGMA statement: exception_init and restrict_references.

The exception_init PRAGMA instructs Oracle to assign a name to a standard Oracle error message that does not have an associated named exception. In this example, the ORA-00942 error is renamed to a user-defined exception xTABLEDOESNOTEXIST:

   xTableDoesNotExist    EXCEPTION;
   PRAGMA exception_init (xTableDoesNotExist -942);

The RESTRICT_REFERENCES PRAGMA instructs Oracle about the purity level of a packaged PL/SQL function. In this example, the Conversions package instructs the database that the function Feet_To_Meters does not alter any database or package states:

PACKAGE Conversions
   FUNCTION Feet_To_Meters (nFeet IN     number) RETURN number;
END Conversions;

Once the function has been declared in this fashion, it can be execute inside a DML statement without receiving an error message, as shown in this example:

SELECT Feet_To_Meters (3.45)

There are four purity levels that can be asserted for packaged functions:

  • WNDS—The function does not write to any tables.

  • RNDS—The function does not read from any tables.

  • WNPS—The function does not modify any variables inside the package.

  • RNPS—The function does not read any variables inside the package.


This is an excerpt from the book "High Performance Oracle Database Automation" by Jonathan Ingram and Donald K. Burleson, Series Editor.


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