Dynamic SQL: DBMS_SQL package

DBMS_SQL Package

SYS.DBMS_SQL is compiled with AUTHID CURRENT_USER (runs using the privileges of the current user)

• Defines a SQL cursor number (PL/SQL Integer): Can be passed across call boundaries and stored.
• Provides an interface to use dynamic SQL to parse ANY DML or DDL statement using PL/SQL.
• When HAS to be used?
• - you don't have the complete SELECT list
• - you don't know what placeholds in a SELECT or DML must be bound
• When you CANNOT use DBMS_SQL?
• - Dynamic SQL stmt retrieves rows into records
• - If you need to use cursor attributes (%FOUND, %ISOPEN, %NOTFOUND, or %ROWCOUNT)
• You can switch between native dynamic SQL AND DBMS_SQL package with:
• DBMS_SQL.TO_REFCURSOR function
• DBMS_SQL.TO_CURSOR_NUMBER function

Oracle Call Interface (OCI) x DBMS_SQL Package

• The ability to use dynamic SQL from within stored procedures generally follows the model of the Oracle Call Interface (OCI).
• Addresses (also called pointers) are NOT user-visible in PL/SQL.
• The OCI uses bind by address. DBMS_SQL package uses bind by value.
• With DBMS_SQL you must call VARIABLE_VALUE to retrieve the value of an OUT parameter for an anonymous block, and you must call COLUMN_VALUE after fetching rows to actually retrieve the values of the columns in the rows into your program.
• The current release of the DBMS_SQL package does not provide CANCEL cursor procedures.
• Indicator variables are not required, because NULLs are fully supported as values of a PL/SQL variable.

SYS.DBMS_SQL Subprograms
BIND_ARRAY (Ps) BIND_VARIABLE (Ps) CLOSE_CURSOR COLUMN_VALUE DEFINE_ARRAY DEFINE_COLUMN DESCRIBE_COLUMN (P) EXECUTE (F) EXECUTE_AND_FETCH (F) FETCH_ROWS (F) IS_OPEN (F) LAST_ERROR_POSITION (F) LAST_ROW_COUNT (F) LAST_ROW_ID LAST_SQL_FUNCTION_CODE OPEN_CURSOR (F) PARSE (Ps) TO_CURSOR_NUMBER (F) TO_REFCURSOR (F) VARIABLE_VALUE (Ps)

New security regime: Oracle 11gR1

• Stricter than in previous versions
• Checks are made when binding and executing.
• Failure lead to ORA-29470: Effective userid or roles are not the same as when cursor was parsed

Execution Flow:

1. OPEN_CURSOR
2. PARSE
3. BIND_VARIABLE or BIND_ARRAY
4. DEFINE_COLUMN, DEFINE_COLUMN_LONG, or DEFINE_ARRAY
5. EXECUTE
6. FETCH_ROWS or EXECUTE_AND_FETCH
7. VARIABLE_VALUE, COLUMN_VALUE, or COLUMN_VALUE_LONG
8. CLOSE_CURSOR

i.e. -- Procedure uses DBMS_SQL package to perform SELECT and DML statements.

Task: Delete all employees of a given deparmtent who earn more than a given value.

(A) DEL_EMP procedure is composed of the three parts below:
(1) Perform a dynamic SELECT to show the number of records that will be deleted
(2) Perform the dynamic DML.
(3) Perform a dynamic SELECT to show that the records were indeed deleted.

(B) an anonymous block calls del_emp procedure

CREATE OR REPLACE PROCEDURE del_emp (dept in number, salary in number) as 
 cursor_dml     integer;
 cursor_select  integer;
 rows_processed integer;
 v_numemp       integer;
 sel_str        varchar2(500);
 dml_str        varchar2(500);
 ignore         integer;

BEGIN
  -- Part 1: Select number BEFORE DML statement
  -- (1) Open cursor for SELECT.
  cursor_select := dbms_sql.open_cursor; 
  sel_str := 'Select count(*) nuemp from employees ' ||
            ' where department_id = :d';           

  -- (2) Parse SQL statement
  dbms_sql.parse(cursor_select, sel_str, dbms_sql.native);

  -- (3) Bind variables
  dbms_sql.bind_variable(cursor_select, ':d', dept);
  sel_str := 'Select count(*) nuemp from employees ' ||
            ' where department_id = '|| to_char(dept);            
  dbms_output.put_line('Select is: ' || sel_str); 

  -- (4) use define_column to specify the variables that 
  -- are to receive the SELECT values, much the way an 
  -- INTO clause does for a static query. 
  dbms_sql.define_column(cursor_select, 1, v_numemp);

  -- (5) call the execute function to run STMT
  ignore := dbms_sql.execute(cursor_select);
  -- (6) Use fetch_rows to retrieve the query results. 
  -- Each successive fetch retrieves another set of rows, 
  -- until the fetch is unable to retrieve anymore rows.
  -- If SELECT returns only ONE row, you may prefer to use 
  -- EXECTUVE_AND_FETCH instead.
  If dbms_sql.fetch_rows(cursor_select) > 0 then 
    dbms_sql.column_value(cursor_select, 1, v_numemp);
  end if;
  dbms_output.put_line('Num emp in dept '|| to_char(dept) || ' before delete: ' 
                   || to_char(v_numemp)); 

  -- (7) Close the cursor.
  dbms_sql.close_cursor(cursor_select);
 
  -- Part 2: Now proceed with the DML Statement
  dml_str := 'delete from employees where department_id = :d' || 
            ' and salary > :x';
  cursor_dml := dbms_sql.open_cursor;
  dbms_sql.parse(cursor_dml, dml_str, dbms_sql.native);
  dbms_sql.bind_variable(cursor_dml, ':d', dept);
  dbms_sql.bind_variable(cursor_dml, ':x', salary);
  rows_processed := dbms_sql.execute(cursor_dml);
  dbms_output.put_line('Num rows deleted: ' || rows_processed);
  dbms_sql.close_cursor(cursor_dml);
  
  -- Part 3: Select the number AFTER the DML statement
  cursor_select := dbms_sql.open_cursor; 
  sel_str := 'Select count(*) nuemp from employees ' ||
            ' where department_id = :d';           
  dbms_sql.parse(cursor_select, sel_str, dbms_sql.native);
  dbms_sql.bind_variable(cursor_select, ':d', dept);
  dbms_sql.define_column(cursor_select, 1, v_numemp);
  sel_str := 'Select count(*) nuemp from employees ' ||
            ' where department_id = '|| to_char(dept);            
  dbms_output.put_line('Select is: ' || sel_str); 
  dbms_sql.define_column(cursor_select, 1, v_numemp);
  ignore := dbms_sql.execute(cursor_select);
  If dbms_sql.fetch_rows(cursor_select) > 0 then 
    dbms_sql.column_value(cursor_select, 1, v_numemp);
  end if;  
  dbms_output.put_line('Num emp in dept '|| to_char(dept) || ' after delete: ' 
                   || to_char(v_numemp)); 
  dbms_sql.close_cursor(cursor_select);
EXCEPTION
 when others then 
   if dbms_sql.is_open(cursor_select) then
      dbms_sql.close_cursor(cursor_select);
  end if;
  if dbms_sql.is_open(cursor_dml) then 
     dbms_sql.close_cursor(cursor_dml);
  end if;
   -- Use Function SQLCODE to return error number 
   dbms_output.put_line('Error code: ' || to_char(sqlcode));

   -- Use Function SQLERRM to return error Message 
   dbms_output.put_line('Error Message: ' || to_char(sqlerrm));
 
   -- Starting on 10gR2, Oracle recommends that you use 
   -- DBMS_UTILITY.FORMAT_ERROR_STACK() instead of SQLERRM.
   -- While SQLERRM is limited to 512bytes, the other function is not. 
   dbms_output.put_line('Error stack: ' || dbms_utility.format_error_stack());

   -- You can also use DBMS_UTILITY.FORMAT_ERROR_BACKTRACE()
   -- this function lists the complete error stack, including the line number 
   -- that generated the exception.
   dbms_output.put_line('Error backtrace: ' || dbms_utility.format_error_backtrace());
END del_emp;

(B) Anonymous block calls DEL_EMP.

set serveroutput on
declare
  nemp integer;
begin
 del_emp(110, 6000);
 commit;
end;
/

anonymous block completed
Select is: Select count(*) nuemp from employees  where department_id = 110
Num emp in dept 110 before delete: 2
Num rows deleted: 2
Select is: Select count(*) nuemp from employees  where department_id = 110
Num emp in dept 110 after delete: 0

-- If the table employees is dropped or renamed, execution of del_emp will fail:

set serveroutput on
declare
  nemp integer;
begin
 del_emp(110, 6000);
 commit;
end;
/

anonymous block completed
Error code: -942
Error Message: ORA-00942: table or view does not exist
Error stack: ORA-00942: table or view does not exist

Error backtrace: ORA-06512: at "SYS.DBMS_SQL", line 1199
ORA-06512: at "DEVEL.DEL_EMP", line 17

(15) Dynamic SQL

[ Oracle PL/SQL ]

"From all old seamy throats of elders, musty books, I've salvaged not a word." (Cormac McCarthy)

Dynamic SQL statements are not embedded in your source program and checked at compile time.
Instead, they are stored in character strings that are input to, or built by, the program at runtime.
(i.e. dynamic SQL lets you create a procedure that operates on a table whose name is not known until runtime.)

Programming methodology for generating and running SQL statements at run time. Good for:

• Writing general-purpose and flexible programs like ad hoc query systems
• Writing programs that must run DDL statements
• When number and datatypes of IN/OUT variables are unknown at compilation time
• When the full text of a SQL stmt needs to be obtained at run-time only.
• i.e. a SELECT stmt that includes an identifier (a table name) that is unknown at compile time.
• i.e. a WHERE clause in which the number of subclauses is unknown at compile time.

How to write dynamic SQL ? (two ways)

• (1) Native dynamic SQL: PL/SQL language feature for building and running dynamic SQL statements
• (2) DBMS_SQL package: , an API for building, running, and describing dynamic SQL statements

Native dynamic SQL x DBMS_SQL package:

• native dynamic SQL: Faster, and more readable (even more when optimized by compiler)
• Native dynamic SQL: requires complete specification of IN/OUT vars (number and datatype) at compile time.
• Switch between native dynamic SQL and DBMS_SQL package using
  DBMS_SQL.TO_REFCURSOR and DBMS_SQL.TO_CURSOR_NUMBER.

Using EXECUTE IMMEDIATE (Advantages over DBMS_SQL)

• DBMS_SQL: Oracle 8 and earlier
• EXECUTE IMMEDIATE: Oracle 8i and later
• Native dynamic SQL:
• Integral part of the SQL language that is built into Pl/SQL, thus making it simpler to use and faster than the DBMS_SQL package.
• Native dynamic SQL also supports the usage of object types and collections not supported by the DBMS_SQL package
• Faster than DBMS_SQL because it parses, binds, and executes the statements immediately.
• DBMS_SQL requires calls to procedures and functions.

Question: Can I use TCL or DDL within a PL/SQL Block? A: No. You HAVE to use DYNAMIC SQL for that. (Native Dyn SQL or DBMS_SQL). Static SQL is limited to DML (SELECT, INSERT, UPDATE, DELETE, MERGE), TCL (COMMIT, ROLLBACK, SAVEPOINT, SET TRANSACTION) and LOCK TABLE.

Native Dynamic SQL: EXECUTE IMMEDIATE

• EXECUTE IMMEDIATE: Builds and runs a dynamic SQL statement in a single operation.
• If dynamic_sql_stmt has a RETURNING INTO clause, using_clause can contain only IN bind arguments. The bind arguments in the RETURNING INTO clause are OUT bind arguments by definition.
• When using dynamic SQL, beware of SQL injection, a security risk.
• The value of bind_argument cannot be TRUE, FALSE, or NULL

Type of dynamic SQL statement	USE of EXECUTE IMMEDIATE
SELECT returns at most one row	OUT-bind arguments (defines): in the INTO clause. IN-bind arguments: in the USING clause. EXECUTE IMMEDIATE SELECT ... INTO <OUT-vars> USING <bind_arg>;
SELECT returns multiple rows	OUT-bind arguments: in the BULK COLLECT INTO clause. IN-bind arguments: in the USING clause. EXECUTE IMMEDIATE SELECT ... BULK COLLECT INTO <OUT-vars> USING <bind_arg>;
DML with RETURNING INTO clause	IN-bind arguments: in the USING clause. OUT-bind arguments: in the RETURNING INTO clause.
DMLw/o RETURNING INTO clause	all bind arguments in the USING clause

Using dynamic SQL with EXECUTE IMMEDIATE:

-- create procedure to insert new record on departments table
create or replace procedure new_dept
 (deptid in out number,
  dname  in varchar2,
  mgrid  in varchar2,
  locid  in varchar2) as 
begin 
 deptid:= departments_seq.nextval;
 insert into departments (
    department_id,
    department_name,
    manager_id,
    location_id)
  values (deptid, dname, mgrid, locid);
end;

-- Use dynamic SQL to call the procedure new_dept
set serveroutput on
declare
 plsql_block varchar2(500);
 new_deptid  number(4);
 new_dname   varchar2(30)    := 'Advertising';
 new_mgrid   number(6)       := 200;
 new_locid   number(4)       := 1700;
 numdep      number;
begin
 select count(*) into numdep  from departments;
 dbms_output.put_line('# depts: ' || numdep);

 -- enter the dynamic SQL statement
 -- note that the statement is complete. Only dynamic binding of the 
 -- procedure parameters occur.
 -- note that the SQL stmt has placeholders for bind arguments.
 plsql_block := 'begin new_dept(:a, :b, :c, :d); end;';

 -- use EXECUTE IMMEDIATE to run the SQL stmt and specify the 
 -- binding variables.
 EXECUTE IMMEDIATE plsql_block
   USING IN OUT new_deptid, new_dname, new_mgrid, new_locid;
 select count(*) into numdep
 from departments;
 dbms_output.put_line('# depts: ' || numdep);
end;

anonymous block completed
# depts: 27
# depts: 28

Using SELECT, DDL and DML with Execute Immediate Using Conditional Compilation

What the example below does (3 procedures):
(a) Check whether a table exists in the database (SELECT)
(b) DROP and Recreate or CREATE table test and GRANT access on the table to another user (DDL)
(c) Insert data on the created table (DML)


(1) Procedure check_object:

• (SELECT returns one row only): Check whether a table named TESTE already exists in the database
• procedure call: check_object('TESTE', 'TABLE');

CREATE OR REPLACE Procedure check_object (p_objname in varchar2, p_objtype in varchar2, 
                        p_created OUT boolean) IS
 query varchar2(200);
 vcount varchar2(40);
BEGIN
  -- using SELECT with EXECUTE IMMEDIATE
  query := 'select count(*) from user_objects '||
          'where object_type = :b '|| 
          'and object_name = :c ';
  -- If the dynamic SQL statement is a SELECT statement that can return 
  -- at most one row, put out-bind arguments (defines) in the INTO clause 
  -- and in-bind arguments in the USING clause.
  EXECUTE IMMEDIATE query into vcount  USING IN p_objtype, p_objname;
  IF vcount = 0 THEN
    $IF $$my_debug $THEN
       dbms_output.put_line(p_objtype || ' ' || p_objname || ' does not yet exist on the database.');
    $END
    p_created := FALSE;
  ELSE
    $IF $$my_debug $THEN
       dbms_output.put_line(p_objtype || ' ' || p_objname || ' already exist on the database.');
    $END
    p_created := TRUE;
  END IF; 
EXCEPTION
 When others then 
   dbms_output.put_line('Error Procedure check_object');
   dbms_output.put_line(sqlerrm);
   dbms_output.put_line(dbms_utility.format_error_backtrace());
END;

alter session set plsql_ccflags = 'my_debug:TRUE';
set serveroutput on
declare
 v_created boolean;
begin
check_object('TESTE', 'TABLE',v_created);
end;
/

Anonymous block completed
TABLE TESTE already exist on the database.

(2) Procedure create_table:

• (DDL): If table exists, drop table and recreate it.
• (DDL): Grant SELECT on table TESTE to userA.
• procedure call: cr_tbl('TRUE', 'create table TESTE (name varchar2(20), salary number)';');
• For the procedure to succeed, the procedure owner MUST have been granted the CREATE TABLE privilege directly, not via a role.
• Role privileges are not applied in procedures

CREATE OR REPLACE procedure create_table (p_created in boolean, p_stmt in varchar2) IS
 e_leave_drop exception;
 e_leave_create exception;
 stmt varchar2(200);
 v_created boolean;
BEGIN
-- USING DDL with EXECUTE IMMEDIATE  
  IF p_created then
    -- table exist. Drop before creating.
    stmt := 'drop table teste';
    begin
      execute immediate stmt;
    exception 
      when others then 
        dbms_output.put_line('Error dropping table');
        dbms_output.put_line(sqlerrm);
        raise e_leave_drop;
    end;
    dbms_output.put_line('Table dropped.');
  END IF;
  stmt := p_stmt; 
  begin
    execute immediate stmt;
  exception 
    when others then 
      dbms_output.put_line('Error creating table');
      dbms_output.put_line(sqlerrm);
      raise e_leave_create;
  end;
  dbms_output.put_line('Table teste created.'); 
  v_created := TRUE;
  dbms_output.put_line('Calling check_object..'); 
  check_object('TESTE', 'TABLE', v_created);

  stmt :=  'grant select on teste to userb';
  execute immediate stmt;
  dbms_output.put_line('grant select completed');
EXCEPTION
 when e_leave_drop then 
  dbms_output.put_line('leaving procedure...');
 when e_leave_create then 
  dbms_output.put_line('leaving procedure...');
END;

alter procedure check_object compile;
alter session set PLSQL_CCFLAGS = 'my_debug:TRUE';
set serveroutput on
declare
stmt varchar2(200);
vcreated boolean default TRUE;
begin
 stmt := 'create table UserA.teste (name varchar2(20), salary number)';
 create_table(vcreated, stmt);
end;
/

Anonymous block completed
Table dropped.
Table teste created.
Calling check_object..
TABLE TESTE already exist on the database.
grant select completed

(3) Procedure insert_tbl:

• (DML): Insert values on table TESTE
• If the dynamic SQL statement is a DML statement WITHOUT a RETURNING INTO clause, other than SELECT, put all bind arguments in the USING clause.

CREATE OR REPLACE procedure insert_tbl (p_name in varchar2, n in integer, 
                                        p_salinit number) as
 vsal number;
 stmt varchar2(200);
 vname varchar2(20);
BEGIN
  stmt := 'INSERT into TESTE values (:b, :c)';
  vsal := p_salinit;
  vname := p_name;
  for i in 1..n loop
    vname := vname || to_char(i);
    vsal := 10000 + i;
    execute immediate stmt USING vname, vsal;
    dbms_output.put_line('Inserted row '|| i);
  end loop;
EXCEPTION
 when others then 
   dbms_output.put_line('Insert error...');
   dbms_output.put_line(sqlerrm);
END;

set serveroutput on
begin
insert_tbl('Mary', 4, 5000);
end;
/

anonymous block completed
Inserted row 1
Inserted row 2
Inserted row 3
Inserted row 4

SQL> Select * from teste;

NAME                 SALARY                 
-------------------- ---------------------- 
Mary1                10001                  
Mary12               10002                  
Mary123              10003                  
Mary1234             10004                  

What the example below does:
(a) Check the columns of a given table in the data dictionary (Mulitple-row SELECT: BULK COLLECT INTO)
(b) Delete a row from the table. Returns data from the deleted row using RETURNIG clause (DML)

(1) Procedure list_tbl

• (SELECT): Get column name list with BULK COLLECT INTO.
• If the dynamic SQL statement is a SELECT statement that can return MULTIPLE rows,
  put out-bind arguments (defines) in the BULK COLLECT INTO clause and in-bind arguments in the USING clause.
• You CANNOT use bind values to pass in the names of schema objects, such as table names or column names.
• SO, names of schema objects have to be added to the dynamic sql statement, instead of passed as bind vars.

CREATE OR REPLACE procedure list_tbl (p_tblname in varchar2) as 
 type t_recdata is RECORD (nm varchar2(20), salary number);
 type t_aadata  is TABLE of t_recdata index by pls_integer;
 vdata t_aadata;
 type t_colnm is table of varchar2(20) index by pls_integer;
 vcolnm t_colnm;
 vb varchar2(30);
 stmt varchar2(200);
BEGIN
  stmt := 'SELECT column_name FROM user_tab_columns '||
          'WHERE table_name = :b';
  vb := p_tblname;
  EXECUTE IMMEDIATE stmt BULK COLLECT INTO vcolnm USING p_tblname;
   for i IN vcolnm.first..vcolnm.last loop
     if i = 1 then 
        vb := vcolnm(i);
     else
       vb := vb || ', '|| vcolnm(i); 
     end if;       
   end loop;
   dbms_output.put_line('columns are: '|| vb);  
    -- You CANNOT use bind values to pass in the names of schema objects, 
    -- such as table names or column names.
    -- SO, names of schema objects have to be added to the dynamic sql 
    -- statement, instead of passed as bind vars.
   stmt := 'select '|| vb || ' from teste'; 
   BEGIN
     EXECUTE IMMEDIATE stmt BULK COLLECT INTO vdata;
   EXCEPTION
     when others then
       dbms_output.put_line('Execute immediate: error SELECT');          
       dbms_output.put_line(sqlerrm);
  END;
  
  for i IN vcolnm.first..vcolnm.last loop
     dbms_output.put(vcolnm(i) || ' -- ');
  end loop;
  dbms_output.new_line();
  for i in vdata.first..vdata.last loop
     dbms_output.put_line(vdata(i).nm || ' '|| vdata(i).salary);  
  end loop;        
END list_tbl;

set serveroutput on
begin
list_tbl('TESTE');
end;
/

columns are: NAME, SALARY
NAME -- SALARY -- 
Ximenes12 10002
Ximenes123 10003

(2) Procedure delete_table:

• procedure call: delete_table('Mary');
• If the dynamic SQL statement is a DML statement with a RETURNING INTO clause,
  put in-bind arguments in the USING clause and out-bind arguments in the RETURNING INTO clause.

set serveroutput on
CREATE OR REPLACE procedure del_tbl(p_name in varchar2) as 
 type t_recdata is RECORD (nm varchar2(20), salary number);
 vrec t_recdata;
 stmt varchar2(200); 
BEGIN
  stmt := 'DELETE FROM teste '||
          'WHERE name = :b ' || 
          'RETURNING name, salary INTO :c, :d';
  execute immediate stmt USING IN p_name returning into vrec;
  dbms_output.put_line('Deleted rec: ' || vrec.nm || ' ' || vrec.salary);
  dbms_output.new_line;
  
  dbms_output.put_line('Listing table...');
  list_tbl('TESTE');
EXCEPTION
    when others then
       dbms_output.put_line(sqlerrm);
        dbms_output.put_line(dbms_utility.format_error_backtrace());
END;

set serveroutput on
begin
  del_tbl('Mary123');
end;
/

anonymous block completed
Deleted rec: Mary123 10003
Listing table...
columns are: NAME, SALARY
NAME -- SALARY -- 
Mary1 10001
Mary12 10002
Mary1234 10004

Running all together:

set serveroutput on
declare
  vcreated boolean default TRUE;
begin
  check_object('TESTE', 'TABLE', vcreated);
  create_table(vcreated, 'create table teste (name varchar2(20), salary number)');
  insert_table('Ximenes', 3, 9700);
  list_tbl('TESTE');
  delete_table('Ximenes1');
end;

nonymous block completed
TABLE TESTE already exist on the database.
Table dropped.
Table teste created.
Calling check_object..
TABLE TESTE already exist on the database.
grant select completed
Inserted row 1
Inserted row 2
Inserted row 3
columns are: NAME, SALARY
NAME -- SALARY -- 
Ximenes1 10001
Ximenes12 10002
Ximenes123 10003
Deleted rec: Ximenes1 10001

Listing table...
columns are: NAME, SALARY
NAME -- SALARY -- 
Ximenes12 10002
Ximenes123 10003

Native Dynamic SQL: OPEN FOR, FETCH and CLOSE

DBMS_SQL Package

Other examples using DBMS_SQL

SQL Injection

(9) Exception Handling

[ Oracle PL/SQL ]

Handling exceptions

• Exceptions can be either an Oracle error or a user-defined error.
• Oracle errors include predefined exceptions enclosed in the STANDARD package.
• There are also unamed Oracle errors. It is possible to associate a name with an unamed Oracle error using the Pragma EXCEPTION_INIT
• User-defined exceptions are raised using
  (a) the RAISE statement, or
  (b) through calls to RAISE_APPLICATION_ERROR procedure.

Exception Handling block syntax:

WHEN {predefined_exception | user_defined_exception | OTHERS} THEN
   exception_handling_statement;
[RETURN | EXIT ];

---i.e.---
EXCEPTION
  WHEN ex_name_1 THEN statements_1               -- Exception handler
  WHEN ex_name_2 OR ex_name_3 THEN statements_2  -- Exception handler
  WHEN OTHERS THEN statements_3                  -- Exception handler
END;

[ WHEN OTHERS ]

• Catches all exceptions not explicitly handled in the previous exception-handling parts of the block.
• Recommendation: The last statement in the OTHERS exception handler should be either
  RAISE or an call of the RAISE_APPLICATION_ERROR procedure.
• If you do not follow this practice and PL/SQL warnings are enabled, you get PLW-06009.
• WHEN OTHERS is optional. It can appear only once, as the last exception handler in the exception-handling part of the block.

Exception Propagation

• Handlers in the Exception section catch exceptions raised in the EXECUTABLE section of the block.
• Exception raised in Declaration section
• Errors in the Declaration section are usually assignment errors, but can still be caught, not locally, but in the enclosing block.
• Exception raised in a Declaration section propagates immediately to the enclosing block (or to the invoker or host environment if there is no enclosing block).
• Therefore, the exception handler must be in an enclosing or invoking block, not in the same block as the declaration.
• It is probably better coding practice to avoid assignment in the Declaration section altogether.
• Exceptions raised in the Exception Section:
• Exception raised in an Exception section propagates immediately to the enclosing block (or to the invoker or host environment if there is no enclosing block).
• Exceptions in calls between subprograms

Handling exceptions:

• Case 1: An exception handler cannot catch an exception raised in the exception section of the same block.
• Such exceptions propagate to the outer block and can only be caught there or in further outer blocks.

set serveroutput on         
declare
  A exception;
  B exception;
begin
  raise a;
exception
  when a then 
     dbms_output.put_line('Raising b from A handler.');     -- exception b raised here CANNOT be 
                                                            -- caught by a handler in the same block. 
     raise b;
  when b then 
     dbms_output.put_line('exception b caught');
end;
/
                                                            -- Here the exception is not caught.
ORA-06510: PL/SQL: unhandled user-defined exception
ORA-06512: at line 9
ORA-06510: PL/SQL: unhandled user-defined exception
06510. 00000 -  "PL/SQL: unhandled user-defined exception"
*Cause:    A user-defined exception was raised by PL/SQL code, but
           not handled.
*Action:   Fix the problem causing the exception or write an exception
           handler for this condition. Or you may need to contact your
           application administrator or DBA.

• Case 2: The exception raised within an Exception section can only be caught in an outer block.

set serveroutput on         
DECLARE                                                           -- outer block
  A exception;
  B exception;
BEGIN
  begin                                                    -- inner block
    raise a;
  exception
    when a then 
       dbms_output.put_line('Raising b from A handler.');         -- exception b raised here is caught in the inner block 
       raise b;
    when b then 
       dbms_output.put_line('exception b caught in inner_block');
   end;
EXCEPTION
 when b then 
       dbms_output.put_line('exception B caught in out_block');
END;
/
Anonymous block completed
Raising b from A handler.
exception B caught in out_block

Exception Categories:
Internally defined:

• (ORA-n errors). automatically raised. Do not have names, unless explicitly given (user). Naming an ORA-n allows that error to be handled on its own WHEN section.Otherwise, you can handle them only with OTHERS exception handlers.

--- i.e. Naming an internal error for explict handling:
DECLARE
 -- (1) declare a name of "type" EXCEPTION.
 deadlock_detected EXCEPTION;
 -- (2) Associate the name with the internally defined error code
 PRAGMA EXCEPTION_INIT(Deadlock_detected, -60);
BEGIN
  ...
EXCEPTION
  WHEN deadlock_detected THEN
    ...
END;

PL/SQL Predefined Exceptions
Exception Name	Error Code
ACCESS_INTO_NULL	-6530
CASE_NOT_FOUND	-6592
COLLECTION_IS_NULL	-6531
CURSOR_ALREADY_OPEN	-6511
DUP_VAL_ON_INDEX	-1
INVALID_CURSOR	-1001
INVALID_NUMBER	-1722
LOGIN_DENIED	-1017
NO_DATA_FOUND	+100
NO_DATA_NEEDED	-6548
NOT_LOGGED_ON	-1012
PROGRAM_ERROR	-6501
ROWTYPE_MISMATCH	-6504
SELF_IS_NULL	-30625
STORAGE_ERROR	-6500
SUBSCRIPT_BEYOND_COUNT	-6533
SUBSCRIPT_OUTSIDE_LIMIT	-6532
SYS_INVALID_ROWID	-1410
TIMEOUT_ON_RESOURCE	-51
TOO_MANY_ROWS	-1422
VALUE_ERROR	-6502
ZERO_DIVIDE	-1476

Predefined Exceptions:

• An internally defined exception that PL/SQL has given a name (22 total). You can write exception handlers specifically for them.
• They are defined in the SYS.STANDARD package

• SQLCODE Function:
• Returns the numeric code of the exception being handled
• For predefined errors: Returns a negative number that maps to the Oracle predefined exceptions. (for NO_DATA_FOUND returns a positive #. (why?))
• For User-defined errors: Returns a positive 1 or the error code associated with the exception by the EXCEPTION_INIT PRAGMA. (a valid number in the range of negative 20001 to negative 20999.)

• SQLERRM Function
• returns the error message associated with an error code
• Returns max 512 bytes, which is the max length of an Oracle error message
• DBMS_UTILTY.FORMAT_ERROR_STACK is recommended over SQLERRM, unless you use the FORALL statement with its SAVE EXCEPTIONS clause.
• DBMS_UTILTY.FORMAT_ERROR_STACK returns the full error stack (up to 2kb)

• DBMS_UTILITY Package
• DBMS_UTILTY.FORMAT_ERROR_STACK is recommended over SQLERRM, unless you use the FORALL statement with its SAVE EXCEPTIONS clause.
• DBMS_UTILTY.FORMAT_ERROR_STACK() returns the full error stack (up to 2kb)
• DBMS_UTILTY.FORMAT_ERROR_BACKTRACE() lists the complete error stack, including the line number that generated the exception.
• See example here

User-defined Exceptions:

• Two-Step process:
• 1. Declaration Section:
  Declare exception_name EXCEPTION;
• 2. Declaration Section:
  Declare a PRAGMA to associate a number to the exception.
• A PRAGMA is a compiler directive. You use a PRAGMA to direct the compiler to perform something differently. PL/SQL supports a number of PRAGMA directives.
• You use the EXCEPTION_INIT directive to map an exception to an error code.
• Executable Section: Explicitly raise exception using: RAISE or RAISE_APPLICATION_ERROR.
• RAISE_APPLICATION_ERROR is defined in the DBMS_STANDARD package.
• can only be invoked from a stored subprogram or method.
• Syntax: RAISE_APPLICATION_ERROR (error_code, message[, {TRUE | FALSE}]);

Raising a user-defined Exception (with RAISE):

create or replace procedure account_status (
  due_date date,
  today    date ) authid definer is
 
  -- declare user-defined exception.
  past_due exception;
begin
  if due_date < today then 
    raise past_due;
  end if;
exception
 WHEN past_due THEN
   dbms_output.put_line('Error: Account past due...');
end;

SQL> set serveroutput on
SQL> begin
account_status('1-JUL-10', '2-AUG-10');
end;
/

anonymous block completed
Error: Account past due...

Declaration Block Errors:

Runtime assignment errors in the declaration block are not captured by the local exception block.

-- dynamic assignments in the declaration section of a block
-- may lead to run-time errors that are not captured by the 
-- local EXCEPTION block. 
set serveroutput on
declare
  emp_id  employees.employee_id%type := '&1';
  vsal    employees.salary%type;
begin
  dbms_output.put_line('Dynamic value captured: '|| emp_id);
  select salary into vsal
  from employees
  where employee_id = emp_id;
  dbms_output.put_line('Emp '|| emp_id || '-- salary: '|| vsal);
exception
 when others then 
   dbms_output.put_line('local exception caught');
end;

-- case 1: Value entered matches the datatype.
--         Procedure executes successfully
Enter value for 1: 100
old   2:   emp_id  employees.employee_id%type := '&1';
new   2:   emp_id  employees.employee_id%type := '100';
Dynamic value captured: 100
Emp 100-- salary: 24000

PL/SQL procedure successfully completed.

-- case 2: Invalid value entered.
-- Exception occurs and is not caught by the local 
-- error handling section.
Enter value for 1: 1G0
old   2:   emp_id  employees.employee_id%type := '&1';
new   2:   emp_id  employees.employee_id%type := '1G0';
declare
*
ERROR at line 1:
ORA-06502: PL/SQL: numeric or value error: character to number conversion error
ORA-06512: at line 2

• IMP: Stored programs (i.e. functions and procedures) behave similarly.
• Procedures require wrapping their calls, but functions do not.
• Thus, for functions called directly from SQL, if a declaration exception occurs, their local EXCEPTION section will not catch and an unhandled exception will result.
• Make dynamic assignments in execution blocks because PL/SQL doesn’t catch dynamic assignment errors in local exception handlers.

Raising a user-defined Exception with RAISE_APPLICATION_ERROR:

-- create a stored procedure to insert new employee
-- the procedure raises an exception if the hire date is 
-- a future date.
create or replace procedure new_emp 
  (empid  employees.employee_id%type,
   lname  employees.last_name%type,
   fname  employees.first_name%type,
   dept   employees.department_id%type,
   jobid  employees.job_id%type,
   sal    employees.salary%type,
   hiredt employees.hire_date%type
   ) authid definer is
begin
  if hiredt > sysdate then 
     raise_application_error(-20000, 'Future hire dates not allowed.');
  else
     insert into employees 
       (employee_id, last_name, first_name, department_id,
        job_id, salary, hire_date)
     values(empid, lname, fname, dept, jobid, sal, hiredt);
  end if;
end new_emp;
/

Stored procedure created.

-- autonomous block invokes the procedure new_emp
-- it defines the INVALID_HIRE_DATE exception
-- and associate an error number with PRAGMA EXCEPTION_INIT
SQL> set serveroutput on
SQL> declare
  invalid_hire_date exception;
  pragma exception_init(invalid_hire_date, -20000);
begin
  new_emp(200, 'Marty', 'Moe', 30, 'SA_REP', 3000, '1-JAN-14');
exception
  when invalid_hire_date then
    dbms_output.put_line(to_char(sqlerrm(-20000)));
end;
/

ORA-20000: Future hire dates not allowed.

PL/SQL procedure successfully completed.
SQL> 

Handling Exceptions: Guidelines

• (1) Use error-checking code and exception handlers.
• error-checking code: check for possible bad input data. (i.e. nulls when they should not be there, more rows than you expected, strings when you should have numbers, etc..)
• (2) Add exception handlers wherever errors can occur. (i.e. arithmetic calculations, string manipulation, and database operations. Also disk storage, memory hardware failure, etc).
• (3) Design your programs to work when the database is not in the state you expect. (i.e. a table might have a column added. don't do select *.... You can also declare variables with %TYPE and %ROWTYPE)
• (4) Use named exceptions instead of using OTHERS exception handlers.
• (5) Have your exception handlers output debugging information. You can use autonomous transactions (PRAGMA AUTONOMOUS_TRANSACTION) to commit your debug info).
• (6)For each exception handler, decide whether to commit, rollback or escalate.
• (7)Make sure to leave the db in a consistent state.
• (5)Include WHEN OTHERS THEN to avoid unhandled exceptions.
• (5) Make sure the last statement in the OTHERS handler either RAISE or an invocation of the RAISE_APPLICATION_ERROR procedure. (or you´ll get PLW-06009.)

Exception Stack

• Exception stack: the sequencing of errors from the triggering event to the calling block of
  code.
• When a failure occurs: (a) An exception is raised in the execution block and the the code in the local exception block executed. (b) If the block is nested or a referenced PL/SQL block, it (a.1) runs the local exception handler and then (a.2) runs the calling program unit’s exception handler, and (a.3, a.4, ...) It continues running available exception blocks or returning errors to the error stack until it returns control to the outermost PL/SQL block.
• Beginning in Oracle 10g, you can use an exception block and the DBMS_UTILITY package to get line number and error codes.

Exception handling in Triggers

Exceptions in Bulk Statements (FORALL)