1. Remaining Topics in SQL to be covered…
NULL values in SQL
Outer joins in SQL
Constraints and Triggers in SQL
Embedded SQL.

2. Null’s in SQL
SQL supports a special value -- NULL in place of a value in a tuple's component
Null’s can be used for multiple purposes --
value exists, but we do not know what it is..
Information does not exist
Example:
boris registered for pass/fail and thus has no project assigned (nulls used to represent information is inapplicable)
stefan registered for letter grade but has no current project assignment (nulls used to represent unavailability of information)

3. Need for Care in using Nulls...
Using nulls for missing values may result in loss of information
information that boris and stefan are part of the same project team , and that monica is a team by herself is lost by using nulls!

4. Sometimes Nulls very useful even if possible to avoid them…
Say 98% of employees have a fax number and a query accessing office number and fax number is very common.
Storing information using a different schema (employee, office num) and (employee, fax number) will cause all such queries to perform a join!
Instead using nulls is a better idea in this case.

5. Interpreting SQL queries with Null (1)
Any arithmetic operation on Null and any other value results in Null.
E.g., x + 3 == Null, if x is Null
Comparison of Null with any value (including other Null) results in a value UNKNOWN
E.g., x > 3 results in UNKNOWN, if x is Null

6. Interpreting SQL with Null (2)
Earlier, we learnt that results of comparison was always T or F.
Logical operators: AND, OR, NOT combined these truth values in a natural way to return a T or a F.
However, comparison of Null to a value produces a third truth value -- UNKNOWN
How to interpret the logical operators in this case?

7. 3-Valued Logic Think of true = 1; false = 0, and unknown = 1/2.
Then: AND = min. OR = max. NOT(x) = 1 - x

8. Truth Table
T = true F = false U = unknown

9. Some Key Laws Fail to Hold in 3-Valued Logic
Example: Law of the excluded middle, i.e., p OR NOT p = TRUE
For 3-valued logic: if p = unknown, then left side = max(1/2,(1-1/2)) = 1/2  1.
there is no way known to make 3-valued logic conform to all the laws we expect for 2-valued logic.

10. Example Bar beer price Joe's bar Bud NULL SELECT bar FROM Sells
Where price < 2.00 OR price >= 2.00
UNKNOWN UNKNOWN
UNKNOWN

11. Join Expressions in SQL
Joins can be expressed implicitly in SQL using SELECT-FROM-WHERE clause.
Alternatively, joins can also be expressed using join expressions.
E.g.,
relations: emp (ssn, sal, dno), dept(dno,dname)
emp CROSS JOIN dept
produces a relation with 5 attributes which is a cross product of emp and dept.

12. Join Expressions in SQL
Join expressions can also be used in FROM clause
SELECT name
FROM emp JOIN dept ON emp.dno = dept.dno AND dept.dname = ‘toy’
Note the join expression
R JOIN S on <condition>

13. Other Types of Join Expressions
R NATURAL JOIN S
R NATURAL FULL OUTER JOIN S
R NATURAL LEFT OUTER JOIN S
R NATURAL RIGHT OUTER JOIN S
R FULL OUTER JOIN S ON <condition>
R LEFT OUTER JOIN S ON <condition>
R RIGHT OUTER JOIN S ON <condition>

14. Revisit to Specifying Integrity Constraints in SQL
We have already seen how to specify:
primary key and uniqueness constraints
constraint checked whenever we do insertion, or modification to the table
referential integrity constraints
constraint checked whenever we do insertion, or modification to the table, or deletion, or modification in referred table

15. Constraints in SQL constraints on attribute values:
these are checked whenever there is insertion to table or attribute update
not null constraint
attribute based check constraint
E.g., sex char(1) CHECK (sex IN (‘F’, ‘M’))
domain constraint
E.g., Create domain gender-domain CHAR (1) CHECK (VALUE IN (‘F’, ‘M’))
define sex in schema defn to be of type gender-domain

16. Constraints in SQL Tuple Based CHECK contraint:
CREATE TABLE Emp (
name CHAR(30) UNIQUE
gender CHAR(1) CHECK (gender in (‘F’, ‘M’)
age int
dno int
CHECK (age < 100 AND age > 20)
CHECK (dno IN (SELECT dno FROM dept)) )
these are checked on insertion to relation or tuple update

17. Another Example of Tuple Based Constraint
CREATE TABLE dept (
mgrname CHAR(30)
dno int
dname CHAR(20)
check (mgrname NOT IN (SELECT name
FROM emp
WHERE emp.sal < 50000)) )
If someone made a manager whose salary is less than 50K that insertion/update to dept table will be rejected.
However, if manager’s salary reduced to less than 50K, the corresponding update to emp table will NOT be rejected.

18. Assertions
Assertions are constraints over a table as a whole or multiple tables.
General form:
CREATE ASSERTION <name> CHECK <cond>
An assertion must always be true at transaction boundaries. Any modification that causes it to become false is rejected.
Similar to tables, assertions can be dropped by a DROP command.

19. Example Assertion CREATE ASSERTION RichMGR CHECK (NOT EXISTS (SELECT *
FROM dept, emp
WHERE emp.name = dept.mgrname AND
emp.salary < 50000))
This assertion correctly guarantees that each manager makes more than
If someone made a manager whose salary is less than 50K that insertion/update to dept table will be rejected.
Furthermore, if manager’s salary reduced to less than 50K, the corresponding update to emp table will be rejected.

20. Different Constraint Types
Type Where Declared When activated Guaranteed
to hold?
Attribute with attribute on insertion not if contains
CHECK or update subquery
Tuple relation schema insertion or not if contains
CHECK update to subquery
relation
Assertion database schema on change to Yes
any relation
mentioned

21. Giving Names to Constraints
Why give names? In order to be able to alter constraints.
Add the keyword CONSTRAINT and then a name:
ssn CHAR(50) CONSTRAINT ssnIsKey PRIMARY KEY
CREATE DOMAIN ssnDomain INT
CONSTRAINT ninedigits CHECK (VALUE >=
AND VALUE <=
CONSTRAINT rightage
CHECK (age >= 0 OR status = “dead”)

22. Altering Constraints ALTER TABLE Product DROP CONSTRAINT positivePrice
ALTER TABLE Product ADD CONSTRAINT
positivePrice CHECK (price >= 0)
ALTER DOMAIN ssn ADD CONSTRAINT no-leading-1s
CHECK (value >= )
DROP ASSERTION assert1.

23. Triggers Enable the database programmer to specify:
when to check a constraint,
what exactly to do.
A trigger has 3 parts:
An event (e.g., update to an attribute)
A condition (e.g., a query to check)
An action (deletion, update, insertion)
When the event happens, the system will check the constraint, and
if satisfied, will perform the action.
NOTE: triggers may cause cascading effects.
Triggers not part of SQL2 but included in SQL3… however,
database vendors did not wait for standards with triggers!

24. Elements of Triggers (in SQL3)
Timing of action execution: before, after or instead of triggering
event
The action can refer to both the old and new state of the database.
Update events may specify a particular column or set of columns.
A condition is specified with a WHEN clause.
The action can be performed either for
once for every tuple, or
once for all the tuples that are changed by the database operation.

25. Example: Row Level Trigger
CREATE TRIGGER NoLowerPrices
AFTER UPDATE OF price ON Product
REFERENCING
OLD AS OldTuple
NEW AS NewTuple
WHEN (OldTuple.price > NewTuple.price)
UPDATE Product
SET price = OldTuple.price
WHERE name = NewTuple.name
FOR EACH ROW

26. Statement Level Trigger
CREATE TRIGGER average-price-preserve
INSTEAD OF UPDATE OF price ON Product
REFERENCING
OLD_TABLE AS OldStuff
NEW_TABLE AS NewStuff
WHEN (1000 <
(SELECT AVG (price)
FROM ((Product EXCEPT OldStuff) UNION NewStuff))
DELETE FROM Product
WHERE (name, price, company) IN OldStuff;
INSERT INTO NewStuff
(SELECT * FROM NewStuff)

27. Bad Things Can Happen CREATE TRIGGER Bad-trigger
AFTER UPDATE OF price IN Product
REFERENCING OLD AS OldTuple
NEW AS NewTuple
WHEN (NewTuple.price > 50)
UPDATE Product
SET price = NewTuple.price * 2
WHERE name = NewTuple.name
FOR EACH ROW

28. Embedded SQL
Direct SQL is rarely used: usually, SQL is embedded in some
application code.
We need some method to reference SQL statements.
But: there is an impedance mismatch problem
So: we use cursors.

29. Host language compiler
Programs with SQL
Host language + Embedded SQL
Preprocessor
Preprocessor
Host Language + function calls
Host language compiler
Host language compiler
Host language program

30. The Impedance Mismatch Problem
The host language manipulates variables, values, pointers
SQL manipulates relations.
There is no construct in the host language for manipulating
relations.
Why not use only one language?
Forgetting SQL: definitely not a good idea!
SQL cannot do everything that the host language can do.

31. Interface: SQL / Host Language
Values get passed through shared variables.
Colons precede shared variables when they occur within the
SQL statements.
EXEC SQL: precedes every SQL statement in the host language.
The variable SQLSTATE provides error messages and status
reports (e.g., says that the operation completed with no
problem).
EXEC SQL BEGIN DECLARE SECTION;
char productName[30];
EXEC SQL END DECLARE SECTION;

32. Using Shared Variables
Void simpleInsert() {
EXEC SQL BEGIN DECLARE SECTION;
char productName[20], company[30];
char SQLSTATE[6];
EXEC SQL END DECLARE SECTION;
/* get values for productName and company
somehow */
EXEC SQL INSERT INTO Product(name, company)
VALUES (:productName, :company); }

33. Single-Row Select Statements
Void getPrice() {
EXEC SQL BEGIN DECLARE SECTION;
char productName[20], company[30];
integer price;
char SQLSTATE[6];
EXEC SQL END DECLARE SECTION;
/* read value of product name */
EXEC SQL SELECT price
INTO :price
FROM Product
WHERE Product.name = :productName;
/* print out value of price */ }

34. Cursors EXEC SQL DECLARE cursorName CURSOR FOR
SELECT …. FROM …. WHERE …. ;
EXEC SQL OPEN cursorName;
while (true) {
EXEC SQL FETCH FROM cursorName INTO :variables;
if (NO_MORE_TUPLES) break;
/* do something with values */ }
EXEC SQL CLOSE cursorName;

35. More on Cursors cursors can modify a relation as well as read it.
We can determine the order in which the cursor will get
tuples by the ORDER BY keyword in the SQL query.
Cursors can be protected against changes to the
underlying relations.
The cursor can be a scrolling one: can go forward, backward
+n, -n, Abs(n), Abs(-n).

36. Dynamic SQL
So far we have only considered embedding ‘static’ SQL in programming languages.
Static SQL embedding is fine for fixed applications when we wish to execute a specific SQL query from a programming language, e.g., a program that is used by a sales clerk to book an airline seat.
What if the SQL query that we wish to embed is itself not known in advance at compile time?
For example, the code that implements dbaccess takes a user query at run time and submits it to the database.
Dynamic SQL allows for the query to be specified at run-time 12

37. Dynamic SQL (II) Two special statements of embedded SQL:
PREPARE turns a character string into an SQL query.
EXECUTE executes that query. 13

38. Example Usage
EXEC SQL BEGIN DECLARE SECTION; char query[MAX_QUERY_LENGTH]; EXEC SQL END DECLARE SECTION; /* read user's text into array query */ EXEC SQL PREPARE q FROM :query; EXEC SQL EXECUTE q;
/* program that reads an SQL query and executes it */ 14

39. Example Usage (II) Once prepared, a query can be executed many times.
Alternatively, PREPARE and EXECUTE can be combined into: EXEC SQL EXECUTE IMMEDIATE :query; 15