1. C20.0046: Database Management Systems Lecture #20
M.P. Johnson
Stern School of Business, NYU
Spring, 2008
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

2. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Agenda
Stored procedures?
Triggers
Transactions
RAID?
Implementation?
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

3. Programming: step back
Recall basic problem: need SQL plus stronger programming lang
 need to connect the two langs
In all these cases (and in the web app case), idea is: put SQL in (traditional-lang) programs
Another way: put programs in SQL
i.e., store programs on the DBMS
“stored procedures”
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

4. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Next topic: SPs
“Persistent, Stored Modules” / “Stored Procedures / “PL/SQL programs” (in Oracle)
Another way to connect application programming language and SQL
Supports usual things:
Declare, set vars to vals of expressions
Print output
Define (optional) procedures, functions
Cursors
PL/SQL can compute n!
Help perform computations not possible using the SQL query language alone.
- For example, you can embed business logic in PSM instead of putting it in the programming language.
- The DBA can give authorizations for users to run a PSM but not to modify it – useful to enforce security.
- One can perform batch-updates using PSMs – where several tables need to be refreshed in a particular sequence.
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

5. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Integration with SQL
DECLARE
l_book_count INTEGER;
BEGIN
SELECT COUNT(*)
INTO l_book_count
FROM books
WHERE author LIKE '%FEUERSTEIN, STEVEN%';
DBMS_OUTPUT.PUT_LINE (
'Steven has written (or co-written) ' || l_book_count ||
' books.');
-- Oh, and I changed my name, so...
UPDATE books
SET author = REPLACE (author, 'STEVEN', 'STEPHEN')
END;
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

6. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
PL/SQL
“Procedural Language/SQL”
Oracle’s language for stored procedures
MySQL’s is similar
Simple, interpreted, procedural language
But Pascal-like:
BEGIN END, not { }
AND OR, not && ||
vars defined at top of procedure
how return works
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

7. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
PL/SQL
Generally speaking can be used wherever SQL can be
Sql command-line
embeded SQL
JDBC/PHP/etc
Can store programs in files (.sql), run later
CALL myprog.sql runs code in myprog.sql
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

8. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
PL/SQL: Hello, World
BEGIN
-- print out message
DBMS_OUTPUT.PUT_LINE('Hello World, from PL/SQL');
END; /
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

9. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Hello, World
Try again…
SET SERVEROUTPUT ON
BEGIN
-- print out message
DBMS_OUTPUT.PUT_LINE('Hello World, from PL/SQL');
END; /
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

10. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
How to run code
The code before ended with a forward slash
Not SQL and not PL/SQL – just for SQL*Plus to tell it to run the code entered
Must go on its own line
O.w., will be ignored and then interpreted as part of code, causing an error
To call a procedure in SQL*Plus, can also use execute/exec:
exec DBMS_OUTPUT.PUT_LINE('Hello World, from PL/SQL')
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

11. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
How to run code
EXEC is just short-hand:
SQL> exec dbms_output.put_line('hi '); dbms_output.put_line('there'
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

12. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
SP operators/symbols
; end statement
% attribute indicator (cursor attributes like %ISOPEN and indirect declaration attributes like %ROWTYPE
: host variable indicator
<> and != not-equal-to
= equal-to
:= assignment op
** exponentiation operator
-- , /* and */, rem comments
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

13. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Var names
identifiers limited to 30 alpha-num chars
Must start with letter, $, _, or #
E.g.: abc, $a$, $$$
PL/SQL is case INsensitive
abc, ABC, AbC all the same
Unless you use double-quotes…
Also supports constants:
Varname datatype CONSTANT := val;
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

14. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Literals
Numbers: 123, 12.3, 3.05E19, 12e-5, null
String: 'abc', 'AbC', null
String comparison is case-SENSitive
Boolean: true, false, null
true != ‘true’
No date literals, as in regular SQL
To_date('31-JAN-94')
Escape single-quotes in strings with two single-quotes
'it''s'  it's
''''''  ''
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

15. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Blocks
SP languages are block-structured
Block = seq. of instructions, with scope
Can have anonymous blocks
And named blocks
Procedures
Functions
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

16. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Structure of a block
header --if named
DECLARE --optional
--var declarations
BEGIN
--executable statements
--queries/updates, etc.
EXCEPTION --optional
--catch exceptions
END;
/ --to execute
As in Pascal, var declars precede body
Declarations – usually variables to be used to store parameters passed from outside, and/or to hold local values
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

17. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
PL/SQL code examples
One example:
Likes(drinker, beverage)
Another example:
BEGIN
INSERT INTO Likes VALUES(‘Izzy', ‘milk');
DELETE FROM Likes
WHERE drinker = ‘Izzy' AND
beverage = ‘Beaujolais Nouveau ';
COMMIT;
END; /
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

18. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Procedures
Stored database objects that use a PL/SQL statement(s) in their body
Create/drop similar to other SQL objects:
CREATE PROCEDURE <my-proc>
(<params>) AS
<procedure body as above>;
CREATE OR REPLACE PROCEDURE <my-proc>(<params>) AS
<procedure body as above>;
DROP PROCEDURE <my-proc>;
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

19. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Example procedure
Define the procedure:
Now we can call it:
CREATE PROCEDURE testProcedure AS
BEGIN
INSERT INTO Student VALUES (5, 'Joe');
END;
EXEC testProcedure
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

20. More details on procedures
Parameter list has name-mode-type triples:
Modes: IN, OUT, or IN OUT
Fulfills role similar to pass-by-value v. pass-by-reference
Default is IN
Types must match, so can get exact field type:
relation.attribute%TYPE
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

21. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Procedure I/O example
A procedure to take a beer and price and add it to Joe's menu: Sells(bar, beer, price)
CREATE PROCEDURE izzyMenu(
b IN char(20),
p IN double) AS
BEGIN
INSERT INTO Sells
VALUES('Izzy''s', b, p);
END; /
Are these the right types?
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

22. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Procedure I/O example
A procedure to take a beer and price and add it to Joe's menu: Sells(bar, beer, price)
CREATE PROCEDURE izzyMenu(
b IN Sells.beer%TYPE,
p IN Sells.price%TYPE) AS
BEGIN
INSERT INTO Sells
VALUES('Izzy''s', b, p);
END; /
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

23. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Larger procedure e.g.
CREATE or replace PROCEDURE hike_prices(old_price number,
new_price out number,
percent_hike number := 5) is
Begin
new_price := old_price + old_price * percent_hike/100;
End; /
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

24. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Call the procedure
Declare
currprice number := 20;
newprice number;
Begin
hike_prices(currprice,newprice,5);
dbms_output.put_line(newprice);
End;
But how to use to modify table data?
Convert to a function
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

25. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Functions
Like procedures but with return values
Big strength: can be called from SQL itself
CREATE FUNCTION <functionName>
(<paramList>) RETURN type AS
<localDeclarations>
BEGIN
<functionBody>
END;
DROP FUNCTION <functionName>;
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

26. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Function example
CREATE OR REPLACE FUNCTION maxval(a IN int, b IN int) RETURN int AS
BEGIN
IF a > b THEN RETURN a;
ELSE RETURN b;
END IF;
END maxval;
INSERT INTO R VALUES('abc', maxval(5,10));
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

27. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Hike function
CREATE or replace FUNCTION hike_pricesf(old_price number,
percent_hike number := 5) return number is
Begin
return old_price + old_price * percent_hike/100;
End; /
Now can use directly in update statements
NB: had to use different name for ftn
Same namespace for ftns & procs, although different
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

28. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
How to run scripts
Usually don’t type ftns into sqlplus by hand
Define them in a .sql file
In sqlplus, execute .sql file
Runs commands in file
Here, defines function
Now, we can call functions (they persist)
See
SQL> CALL maxval.sql
SQL> exec DBMS_OUTPUT.PUT_LINE (maxval(5,10))
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

29. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
ED command
Though define procedures at cmd-prompt
Forgot AS!
Type ED…
SQL> create or replace procedure hello2
begin
dbms_output.put_line('hi');
end; /
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

30. Calling functions and procedures
Procedures can executed, ftns can’t
How to just call a ftn?
Can use dbms_output, as seen
Can also select the ftn value from dual
SQL> select(wordcount('hi there') from mytable;
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

31. Stored ftns & procs persist
Once a function or procedure is created, it persists until it’s dropped
Stored procs are stored in the DB itself
In user_procedures in Oracle
Also, can describe ftns and procs:
CREATE OR REPLACE FUNCTION …
SELECT object_name from user_procedures;
SQL> describe wordcount
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

32. Look up procedures, functions
In Oracle, functions & procedures in user_procedures:
Also, can describe ftns and procs:
SELECT object_name from user_procedures;
SQL> describe wordcount
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

33. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Getting errors
Simply says:
To get actual errors, say SHOW ERR(ORS)
Can also get errors per object:
Warning: must get object type right!
Can also look at user_errors tbl directly
Warning: Function created with compilation errors.
SQL> show errors function wordcount
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

34. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Branching
IF–THEN statements use THEN
Must end with END IF
Use ELSIF in place of ELSE IF
Example:
IF <condition> THEN
<statement(s)>
ELSIF
END IF;
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

35. Simple case statements
CASE expression
WHEN result1 THEN
statements1
WHEN result2 THEN
statements2
...
ELSE
statements_else
END CASE;
General form:
ELSE is optional
expression and results are scalars
numbers, chars, strings, etc.; not tables
Literals or vars
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

36. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Simple case e.g.
CASE employee_type
WHEN 'S' THEN
award_salary_bonus(employee_id);
WHEN 'H' THEN
award_hourly_bonus(employee_id);
WHEN 'C' THEN
award_commissioned_bonus(employee_id);
ELSE
RAISE invalid_employee_type;
END CASE;
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

37. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
CASE statements in SQL
By the way: CASE statements are now supported in Oracle SQL itself
SELECT
CASE
WHEN comm is null THEN 0
ELSE comm
END
FROM emp;
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

38. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Loop example
DECLARE    
i NUMBER := 1;
BEGIN    
LOOP        
INSERT INTO T1VALUES(i,i);
i := i+1;        
EXIT WHEN i>100;    
END LOOP;
END; /
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

39. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
More loops
Numerical for loop:
Cursor for loop:
FOR for_index IN low_value .. high_value
LOOP
executable_statements;
END LOOP;
FOR record_index IN my_cursor
LOOP
executable_statements;
END LOOP;
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

40. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Word count program
CREATE OR REPLACE FUNCTION wordcount (str IN VARCHAR2)
RETURN PLS_INTEGER AS
/* words PLS_INTEGER := 0; ***Commented out for intentional error*** */
len PLS_INTEGER := NVL(LENGTH(str),0);
inside_a_word BOOLEAN;
BEGIN
FOR i IN 1..len LOOP
IF ASCII(SUBSTR(str, i, 1)) < 33 OR i > len
THEN
IF inside_a_word
words := words + 1;
inside_a_word := FALSE;
END IF;
ELSE
inside_a_word := TRUE;
END LOOP;
RETURN words;
END;
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

41. Explicit cursors v. for loop cursors
DECLARE
CURSOR occupancy_cur IS
SELECT pet_id, room_number
FROM occupancy WHERE occupied_dt = TRUNC (SYSDATE);
occupancy_rec occupancy_cur%ROWTYPE;
BEGIN
OPEN occupancy_cur;
LOOP
FETCH occupancy_cur INTO occupancy_rec;
EXIT WHEN occupancy_cur%NOTFOUND;
update_bill
(occupancy_rec.pet_id, occupancy_rec.room_number);
END LOOP;
CLOSE occupancy_cur;
END;
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

42. Explicit cursors v. for loop cursors
DECLARE
CURSOR occupancy_cur IS
SELECT pet_id, room_number
FROM occupancy WHERE occupied_dt = TRUNC (SYSDATE);
BEGIN
FOR occupancy_rec IN occupancy_cur
LOOP
update_bill (occupancy_rec.pet_id, occupancy_rec.room_number);
END LOOP;
END;
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

43. Using query results: SELECT … INTO
create or replace function getprod(manuf varchar) return varchar as
pn varchar(255);
begin
select prodname into pn
from products
where mfg = manuf;
return pn;
end; /
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

44. SELECT … INTO and exceptions
create or replace function getprod(manuf varchar) return varchar as
pn varchar(255);
begin
select prodname into pn
from products
where mfg = manuf;
return pn;
Exception
When TOO_MANY_ROWS then
dbms_output.put_line('got too many');
end; /
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

45. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Programs and rights
By default, only the creator of a program may run it (apart from the admin)
If others should run, must GRANT them permission:
Permissions can be revoked:
Can also grant to particular roles or everyone:
Wider/narrower grant ops are independent…
SQL> GRANT EXECUTE ON wordcount TO george;
SQL> REVOKE EXECUTE FROM wordcount TO george;
SQL> GRANT EXECUTE ON wordcount TO dba_role;
SQL> GRANT EXECUTE ON wordcount TO public;
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

46. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Packages
Functions and procedures (and vars) can be grouped in packages
Like Java packages, C++ namespaces, etc.
A pkg has a specification and a body
Somewhat like C++ class definitions
Specification: declares public functions
“public” means: can be run by a user with EXECUTE authority on this pkg
Body: defines all functions
Vars defined here are visible to the pkg’s programs
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

47. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Dynamic PL/SQL
Saw “dynamic SQL” in the cases of Pro*C and JDBC
Ability to run ad-hoc (non-hard-coded) SQL in programs/scripts
Can also do this in PL/SQL
The string can be passed in, created from concatenation, etc.
EXECUTE IMMEDIATE <string>;
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

48. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Dynamic PL/SQL
E.g.: write function to return number rows in an arbitrary table
CREATE OR REPLACE FUNCTION rowCount (
tabname IN VARCHAR2) return integer as
retval integer;
begin
execute immediate 'select count(*) from ' || tabname into retval;
return retval;
end; /
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

49. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Dynamic PL/SQL for DDL
Ordinarily can’t do DDL in PL/SQL
But you can in dynamic PL/SQL
Here’s an e.g.:
CREATE OR REPLACE procedure dropproc(procname in varchar2) as
begin
execute immediate 'drop procedure ' || procname;
end; /
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

50. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Live examples
Factorial function:
Converting between bases:
Directory of examples:
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

51. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
SPs in MySQL (5.0)
Generally similar, though technical diffs
Need to temporarily redefine ; delimiter
mysql> drop procedure if exists hello;
myslq> delimiter /
mysql> create procedure hello()
-> begin
-> select 'hi';
-> end;
-> /
mysql> delimiter ;
mysql> call hello();
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

52. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
New topic: Triggers
PL/SQL programs that run automatically (are “triggered”) when a certain event occurs
E.g.: on insert to some table
On system start-up
On delete from table
Big benefit: need not be called explicitly
However row in table x is deleted, the trigger gets called
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

53. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Trigger events
Trigger code may be “triggered” by many kinds of events:
Oracle start-up/shut-down
Triggers may replace initialization scripts
Data updates:
Delete: maybe delete related rows
Inserts
Updates: maybe make other rows consistent
Delete: maybe prevent
DDL statements
Log creation of all objects, e.g.
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

54. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Triggers
Constraints state what must remain true
DBMS decides when to check
Triggers are instructions to perform at explicitly specified times
Three aspects:
An event (e.g., update to an attribute)
A condition (e.g., a test of that update value)
An action (the trigger’s effect) (deletion, update, insertion)
When the event occurs, DBMS checks the constraint, and if it is satisfied, performs the action
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

55. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
DML trigger options
The trigger may be:
Statement-level (e.g., a DELETE WHERE statement) or
Row-level (e.g., for each row deleted)
The trigger may run
BEFORE
AFTER or
INSTEAD OF the statement (in Oracle, not in others)
It may be triggered by
INSERTs
UPDATEs
DELETEs
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

56. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Trigger form
CREATE [OR REPLACE] TRIGGER trigger name
{BEFORE | AFTER | INSTEAD OF}
{INSERT | DELETE | UPDATE | UPDATE OF column list} ON table name
[FOR EACH ROW]
[WHEN (...)]
[DECLARE ... ]
BEGIN
... executable statements ...
[EXCEPTION ... ]
END [trigger name];
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

57. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Trigger type examples
First run copy_tables.sql
statement_vs_row.sql
INSERT INTO to_table SELECT * FROM from_table;
before_vs_after.sql
one_trigger_per_type.sql
INSERT INTO to_table VALUES (1);
UPDATE to_table SET col1 = 10;
DELETE to_table;
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

58. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
DML Trigger e.g.
CREATE OR REPLACE TRIGGER validate_employee_changes
BEFORE INSERT OR UPDATE
ON employee
FOR EACH ROW
BEGIN
check_age (:NEW.date_of_birth);
check_resume (:NEW.resume);
END;
Q: Why is this (maybe) better than client-side validation?
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

59. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Triggers with WHEN
CREATE OR REPLACE TRIGGER check_raise
AFTER UPDATE OF salary, commission
ON employee
FOR EACH ROW
WHEN ((OLD.salary != NEW.salary OR
(OLD.salary IS NULL AND NEW.salary IS NULL))
OR (OLD.commission != NEW.commission OR
(OLD.commission IS NULL AND NEW.commission IS NULL)))
BEGIN
...
END;
NB: WHEN applies only to row-level triggers
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

60. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Triggers with WHEN
Parentheses are required
Can only call built-in functions in when
Packages like DBMS_OUTPUT are not allowed
CREATE OR REPLACE TRIGGER valid_when_clause
BEFORE INSERT ON frame
FOR EACH ROW
WHEN ( TO_CHAR(SYSDATE,'HH24') BETWEEN 9 AND 17 )
...
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

61. Simple trigger example
R(id, data, last-modified)
data is a large string
Last-modified is a newly added date field
Goal: whenever data is modified, update last-modified date
Could modify all scripts/programs that touch this table
Bad idea
Better: user a trigger
CREATE TRIGGER UpdateDateTrigger
AFTER UPDATE OF data ON R
REFERENCING
NEW ROW AS NewTuple
FOR EACH ROW
BEGIN
NewTuple.last-modified = sysdate;
END;
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

62. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Multiple DML actions
DML actions may be ORed together
CREATE OR REPLACE TRIGGER three_for_the_price_of_one
BEFORE DELETE OR INSERT OR UPDATE ON account_transaction
FOR EACH ROW
BEGIN
IF INSERTING
THEN
:NEW.created_by := USER;
:NEW.created_date := SYSDATE;
ELSIF DELETING
audit_deletion(USER,SYSDATE);
END;
To find actual action, check:
INSERTING
DELETING
UPDATING
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

63. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
More on UPDATING
UPDATING may be called for partic. columns
CREATE OR REPLACE TRIGGER validate_update
BEFORE UPDATE ON account_transaction
FOR EACH ROW
BEGIN
IF UPDATING ('ACCOUNT_NO')
THEN
errpkg.raise('Account number cannot be updated');
END IF;
END;
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

64. Extended auditing example
Tables: grades, grades_audit
Run: grades_tables.sql, grades_audit.sql
Cases: hacker changes grades, deletes others
UPDATE grades
SET grade = 'A+'
WHERE student_id = 1
AND class_id = 101;
DELETE grades
WHERE student_id = 2
AND class_id = 101;
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

65. Extended auditing example
Run: grades_tables.sql, grades_audit2.sql
Cases: hacker changes student or class ids
UPDATE grades SET student_id = 3
WHERE student_id = 1 AND class_id = 101;
UPDATE grades SET student_id = 1
WHERE student_id = 2 AND class_id = 101;
UPDATE grades SET student_id = 2
WHERE student_id = 3 AND class_id = 101;
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

66. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
DDL Triggers
Respond to DDL events
Creating/dropping tables, indices, etc.
ALTER TABLE etc.
General form:
CREATE [OR REPLACE] TRIGGER trigger name
{BEFORE | AFTER| {DDL event} ON
{DATABASE | SCHEMA}
DECLARE
Variable declarations
BEGIN
... some code...
END;
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

67. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
DDL trigger e.g.
Town crier examples triggered by creates:
uninformed_town_crier.sql
informed_town_crier.sql
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

68. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Available DDL events
CREATE, ALTER, DROP, GRANT, RENAME, REVOKE, TRUNCATE
DDL: any DDL event
Q: Does this work??
CREATE OR REPLACE TRIGGER no_create
AFTER CREATE ON SCHEMA
BEGIN
RAISE_APPLICATION_ERROR (-20000,
'ERROR : Objects cannot be created in the production database.');
END;
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

69. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
DB Event triggers
Form similar to DDL triggers:
Triggering events: STARTUP, SHUTDOWN, SERVERERROR, LOGON, LOGOFF
CREATE [OR REPLACE] TRIGGER trigger name
{BEFORE | AFTER} {database event} ON {DATABASE | SCHEMA}
DECLARE
Variable declarations
BEGIN
... some code...
END;
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

70. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
DB event restrictions
Have BEFORE and AFTER as above, but they don’t always apply:
No BEFORE STARTUP/LOGON/SERVERERROR
No AFTER SHUTDOWN/LOGOFF
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

71. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
DB Trigger e.g.
Gather stats before shutdown:
Log error messages
CREATE OR REPLACE TRIGGER on_shutdown
BEFORE SHUTDOWN ON DATABASE
BEGIN
gather_system_stats;
END;
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

72. New-old topic: Transactions
So far, have simply issued commands
Ignored xacts
Recall, though: an xact is an operation/set of ops executed atomically
In one instant
ACID test:
Xacts are atomic
Each xact (not each statement) must leave the DB consistent
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

73. Default xact behavior (in Oracle)
An xact begins upon login
By default, xact lasts until logoff
Except for DDL statements
They automatically commit
Examples with two views of tbl…
But with TYPE=innodb !
mysql> set autocommit = 0
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

74. Direct xact instructions
At any point, may explicitly COMMIT:
SQL> COMMIT;
Saves all statements entered up to now
Begins new xact
Conversely, can ROLLBACK
SQL> ROLLBACK;
Cancels all statements entered since start of xact
Example: delete from emp; or delete junk;
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

75. Direct xact instructions
Remember, DDL statements are auto-committed
 They cannot be rollbacked
Examples:
Q: Why doesn’t rollback “work”?
drop table junk;
rollback;
truncate table junk;
rollback;
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

76. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Savepoints
Xacts are atomic
Can rollback to beginning of current xact
But might want to rollback only part way
Make 10 changes, make one bad change
Want to: roll back to before last change
Don’t have Word-like multiple undo
But do have savepoints
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

77. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
Savepoints
Create a savepoint:
emp example:
SAVEPOINT savept_name;
--changes
SAVEPOINT sp1;
SAVEPOINT sp2;
SAVEPOINT sp3
ROLLBACK TO SAVEPOINT sp2;
ROLLBACK TO SAVEPOINT sp1;
Can skip savepoints
But can ROLLBACK only backwards
Can ROLLBACK only to last COMMIT
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

78. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
AUTOCOMMIT
Finally, can turn AUTOCOMMIT on:
Oralce: SQL> SET AUTOCOMMIT ON;
Mysql: mysql> SET AUTOCOMMIT=1;
Can put this in your config file
Can specify through JDBC, etc.
Then each statement is auto-committed as its own xact
Not just DDL statements
M.P. Johnson, DBMS, Stern/NYU, Spring 2008

79. M.P. Johnson, DBMS, Stern/NYU, Spring 2008
RAID levels
RAID level 1: each disk gets a mirror
RAID level 4: one disk is xor of all others
Each bit is sum mod 2 of corresponding bits
E.g.:
Disk 1:
Disk 2:
Disk 3:
Disk 4:
How to recover?
What’s the disadvantage of R4?
Various other RAID levels in text…
M.P. Johnson, DBMS, Stern/NYU, Spring 2008