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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 --

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remaining topics in sql to be covered
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
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)
need for care in using nulls
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!
sometimes nulls very useful even if possible to avoid them
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.
interpreting sql queries with null 1
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
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
3-Valued Logic
  • Think of true = 1; false = 0, and unknown = 1/2.
  • Then: AND = min. OR = max. NOT(x) = 1 - x
truth table
Truth Table

T = true F = false U = unknown

some key laws fail to hold in 3 valued logic
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
Example

Bar beer price

Joe's bar Bud NULL

SELECT bar

FROM Sells

Where price < 2.00 OR price >= 2.00

UNKNOWN UNKNOWN

UNKNOWN

modifying views
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.

views

updatable views
Updatable views

toyEmp (ename, dno)

CREATE TABLE Emp(ename char(20),

dno int,

sal float default 0);

CREATE VIEW toyEmp AS

SELECT ename, dno

FROM emp

WHERE dno = 111;

Emp (ename, dno, sal)

INSERT INTO toyEmp VALUES (‘Tom’, 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.
non updatable views
Non-updatable views

toyEmp (ename, dno)

CREATE TABLE Emp(ename char(20),

dno int,

sal float default 0);

CREATE VIEW toyEmp AS

SELECT ename, dno

FROM emp

WHERE dno = 111;

Emp (ename, dno, sal)

INSERT INTO toyEmp VALUES (‘Tom’, 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
delete from updatable views
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 toyEmp

WHERE ename = ‘Jack’

Will be translated to:

DELETE FROM Emp

WHERE ename = ‘Jack’ AND dno = 111;

toyEmp (ename, dno)

Emp (ename, dno, sal)

update updatable views
Update Updatable Views
  • Will update all tuples in the base relations that produce the updated tuples in the view
  • Example:

CREATE VIEW toyEmp AS

SELECT ename, dno, sal

FROM 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)

Emp (ename, dno, sal)

drop views
Drop Views
  • DROP VIEW <name>;
  • Example: DROP VIEW toyEmp;
  • The base tables will NOT change.
join expressions in sql
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 sql1
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>
other types of join expressions
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
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 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 sql1
Constraints in SQL
  • Constraints on tuples
  • Tuple based CHECK constraint:

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

another example of tuple based constraint
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.
attribute and tuple based constraints
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
  • 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
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 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
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

giving names to constraints
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
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.

triggers
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!
elements of triggers in sql3
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
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

statement level trigger
Statement Level Trigger

emp(dno…), dept(dept#, …)

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

CREATE TRIGGER deptExistTrig

AFTER INSERT ON emp

REFERENCING

OLD_TABLE AS OldStuff

NEW_TABLE AS NewStuff

WHEN (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
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

embedded sql
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.

programs with sql
Programs with SQL

Host language + Embedded SQL

Preprocessor

Preprocessor

Host Language + function calls

Host language compiler

Host language compiler

Host language program

the impedance mismatch problem
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
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 no

problem).

EXEC SQLBEGIN DECLARE SECTION;

char productName[30];

EXEC SQLEND DECLARE SECTION;

using shared variables
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 statements
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 */ }

cursors
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
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
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
Dynamic SQL (II)
  • Two special statements of embedded SQL:
    • PREPARE turns a character string into an SQL query.
    • EXECUTE executes that query.
example usage
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
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;