remaining topics in sql to be covered… null values in sql outer joins in sql constraints and...

Remaining Topics in SQL to be covered…

NULL values in SQL Outer joins in SQL Constraints and Triggers in SQL Embedded SQL.

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)

Cs184projects

student proj title date

boris null null

stefan null null

Need for Care in Using Nulls...

Using nulls for missing values may result in loss of information

Cs184projects


boris oodb 11/16/95

stefan oodb 11/16/95

monica par dbms 11/21/95

Cs184projects


boris null null

stefan null null

monica null null

• 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!

Sometimes Nulls very useful even if possible to avoid them…

office relation

employee office num

fax number

boris DCL 2111 333-2400 stefan DCL 3001 null monica DCL 3444 333-0067

• 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.

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

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?

3-Valued Logic

Think of true = 1; false = 0, and unknown = 1/2.

Then: AND = min. OR = max. NOT(x) = 1 - x

Truth Table

X Y X AND y X OR Y NOT X T T T T F T U U T F T F F T F U T U T U U U U U U U F F U U F T F T T F U F U T F F F F T

T = true F = false U = unknown

SQL …… constraints, assertions, triggers …

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.

Example

Bar beer price

Joe's bar Bud NULL

SELECT barFROM SellsWhere price < 2.00 OR price >= 2.00 UNKNOWN UNKNOWN UNKNOWN

Modifying Views How can we modify a view that is “virtual”? Many views cannot be modified Some views can be “modified,” called “updatable views”

Their definitions must satisfy certain requirements. A modification is translated to a modification to its base tables.

eName Dno Sal Jack 111 50K Alice 111 90K Lisa 222 80K Tom 333 70K Mary 333 60K

dno dname Mgr 111 Sells Alice 222 Toys Lisa 333 Electronics Mary


views

Updatable views

Tom 111 0

CREATE TABLE Emp(ename char(20), dno int, sal float default 0);

CREATE VIEW toyEmp ASSELECT ename, dnoFROM empWHERE dno = 111;

Insert a tuple to a view: Insert a corresponding tuple to its base table(s) Missing values will use NULL or default value Inserted tuples in base table(s) must generate the new view tuple.

toyEmp (ename, dno)

eName Dno Jack 111 Alice 111

INSERT INTO toyEmp VALUES (‘Tom’, 111);

Emp (ename, dno, sal)


Tom 111

Non-updatable views

Tom ?????? 0

CREATE TABLE Emp(ename char(20), dno int, sal float default 0);

CREATE VIEW toyEmp ASSELECT ename, dnoFROM empWHERE dno = 111;

Insert a tuple to a view: Not allowed: what do we insert into Emp? view not updatable! The system is not “smart” enough to know the value of “dno” is 111. If we fill “dno” with “NULL,” then this view tuple cannot be generated

toyEmp (ename, dno)

eName Jack Alice

INSERT INTO toyEmp VALUES (‘Tom’, 111);



Tom

Delete from Updatable Views When deleting a tuple from a view, should delete all tuples from

base table(s) that can produce this view tuple. Example:

DELETE FROM toyEmpWHERE ename = ‘Jack’

Will be translated to:DELETE FROM EmpWHERE ename = ‘Jack’ AND dno = 111;

toyEmp (ename, dno)

eName Dno Jack 111 Alice 111



Update Updatable Views Will update all tuples in the base relations that produce the

updated tuples in the view Example:

CREATE VIEW toyEmp ASSELECT ename, dno, salFROM Emp WHERE dno = 111;

UPDATE toyEmp SET sal = sal * 0.9 WHERE ename = ‘Jack’

Will be translated to: UPDATE Emp SET sal = sal * 0.9 WHERE ename = ‘Jack’ AND dno = 111;

toyEmp (ename, dno)

eName Dno Sal Jack 111 50K Alice 111 90K



Drop Views

DROP VIEW <name>; Example: DROP VIEW toyEmp; The base tables will NOT change.

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.

Join Expressions in SQL Join expressions can also be used in FROM clauseSELECT nameFROM emp JOIN dept ON emp.dno = dept.dno AND

dept.dname = ‘toy’ Note the join expression

R JOIN S on <condition>

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>

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

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

Constraints in SQL

Constraints on tuples Tuple based CHECK constraint:

CREATE TABLE Emp (name CHAR(30) UNIQUEgender CHAR(1) CHECK (gender in (‘F’, ‘M’)age intdno intCHECK (age < 100 AND age > 20)CHECK (dno IN (SELECT dno FROM dept)))

these are checked on insertion to relation or tuple update

Another Example of Tuple Based Constraint

CREATE TABLE dept (mgrname CHAR(30)dno intdname 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.

Attribute and Tuple Based Constraints

If refer to (attributes from) another relation then DBMS ignores any changes to the other relations Even if constraint condition violated

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.

Example Assertion CREATE ASSERTION RichMGR CHECK

(NOT EXISTS (SELECT *FROM dept, empWHERE emp.name = dept.mgrname AND

emp.salary < 50000))

This assertion correctly guarantees that each manager makes more than 50000.

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.

Different Constraint Types

Type Where Declared When activated Guaranteed to hold?

Attribute with attribute on insertion not if containsCHECK or update subquery

Tuple relation schema insertion or not if containsCHECK update to subquery relation

Assertion database schema on change to yes !! any relation mentioned

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 >= 100000000 AND VALUE <= 999999999

CONSTRAINT rightage CHECK (age >= 0 OR status = “dead”)

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 >= 200000000)

DROP ASSERTION assert1.

TriggersEnable 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!

Elements of Triggers (in SQL3)• Timing of action execution

• before • after • instead of

…. the 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.

Example: Row Level Trigger

CREATE TRIGGER NoLowerPrices

AFTER UPDATE OF price ON ProductREFERENCING OLD AS OldTuple NEW AS NewTupleWHEN (OldTuple.price > NewTuple.price) UPDATE Product SET price = OldTuple.price WHERE name = NewTuple.name

FOR EACH ROW

Statement Level Trigger emp(dno…), dept(dept#, …)

“Whenever we insert employees tuples, make sure that their dno’s exist in Dept.”

CREATE TRIGGER deptExistTrigAFTER INSERT ON empREFERENCING OLD_TABLE AS OldStuff NEW_TABLE AS NewStuffWHEN (EXSITS (SELECT * FROM NewStuff

WHERE dno NOT IN (SELECT dept# FROM dept)))

DELETE FROM NewStuff WHERE dno NOT IN (SELECT dept# FROM dept));

Bad Things Can Happen

CREATE TRIGGER Bad-trigger

AFTER UPDATE OF price IN ProductREFERENCING OLD AS OldTuple NEW AS NewTuple

WHEN (NewTuple.price > 50)

UPDATE Product SET price = NewTuple.price * 2 WHERE name = NewTuple.name

FOR EACH ROW

Embedded SQL

Direct SQL is rarely used: usually, SQL is embedded in someapplication code.

We need some method to reference SQL statements.

But: there is an impedance mismatch problem

So: we use cursors.

Programs with SQL

Host language + Embedded SQL

Preprocessor

Host Language + function calls

Host language compiler

Host language program

Preprocessor

Host language compiler

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.

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., 00000 says that the operation completed with noproblem).

EXEC SQL BEGIN DECLARE SECTION; char productName[30];EXEC SQL END DECLARE SECTION;

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); }

Single-Row Select StatementsVoid 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 */ }

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;

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).

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

Dynamic SQL (II)

Two special statements of embedded SQL: PREPARE turns a character string into an SQL query. EXECUTE executes that query.

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 */

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;

remaining topics in sql to be covered… null values in sql outer joins in sql constraints and...

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