Tài liệu PL/SQL User’s Guide and Reference phần 5

User-Defined Exceptions
PL/SQL lets you define exceptions of your own. Unlike predefined exceptions,
user-defined exceptions must be declared and must be raised explicitly by RAISE
statements.
Declaring Exceptions
Exceptions can be declared only in the declarative part of a PL/SQL block,
subprogram, or package. You declare an exception by introducing its name,
followed by the keyword EXCEPTION. In the following example, you declare an
exception named past_due:
DECLARE
past_due EXCEPTION;
Exception and variable declarations are similar. But remember, an exception is an
error condition, not a data item. Unlike variables, exceptions cannot appear in
assignment statements or SQL statements. However, the same scope rules apply to
variables and exceptions.
Scope Rules
You cannot declare an exception twice in the same block. You can, however, declare
the same exception in two different blocks.
Exceptions declared in a block are considered local to that block and global to all its
sub-blocks. Because a block can reference only local or global exceptions, enclosing
blocks cannot reference exceptions declared in a sub-block.
If you redeclare a global exception in a sub-block, the local declaration prevails. So,
the sub-block cannot reference the global exception unless it was declared in a
labeled block, in which case the following syntax is valid:
block_label.exception_name