Remaining Topics in SQL to be covered

1
Remaining Topics in SQL to be covered

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

2
Nulls in SQL

• SQL supports a special value -- NULL in place of
  a value in a tuple's component
• Nulls 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 gt 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
SQL constraints, assertions, triggers
10
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.

11
Example
Bar beer price Joe's bar Bud NULL
SELECT bar FROM Sells Where price lt 2.00 OR price
gt 2.00 UNKNOWN UNKNOWN
UNKNOWN
12
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
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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.

14
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

15
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)
16
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)
17
Drop Views

• DROP VIEW ltnamegt
• Example DROP VIEW toyEmp
• The base tables will NOT change.

18
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.

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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 ltconditiongt

20
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 ltconditiongt
• R LEFT OUTER JOIN S ON ltconditiongt
• R RIGHT OUTER JOIN S ON ltconditiongt

21
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

22
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

23
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 lt 100 AND age gt 20)
• CHECK (dno IN (SELECT dno FROM dept))
• )
• these are checked on insertion to relation or
  tuple update

24
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 lt 50000))
• )
• If someone made a manager whose salary is less
  than 50K that insertion/update to dept table will
  be rejected.
• However, if managers salary reduced to less than
  50K, the corresponding update to emp table will
  NOT be rejected.

25
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

26
Assertions

• Assertions are constraints over a table as a
  whole or multiple tables.
• General form
• CREATE ASSERTION ltnamegt CHECK ltcondgt
• 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.

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Example Assertion

• CREATE ASSERTION RichMGR CHECK
• (NOT EXISTS
• (SELECT
• FROM dept, emp
• WHERE emp.name dept.mgrname AND
• emp.salary lt 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 managers salary reduced to less
  than 50K, the corresponding update to emp table
  will be rejected.

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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
29
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 gt
100000000
AND VALUE lt 999999999 CONSTRAINT
rightage CHECK (age gt 0 OR status
dead)
30
Altering Constraints
ALTER TABLE Product DROP CONSTRAINT
positivePrice ALTER TABLE Product ADD
CONSTRAINT positivePrice CHECK (price gt
0) ALTER DOMAIN ssn ADD CONSTRAINT
no-leading-1s CHECK (value gt
200000000) DROP ASSERTION assert1.
31
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!

32
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.

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Example Row Level Trigger
CREATE TRIGGER NoLowerPrices AFTER UPDATE OF
price ON Product REFERENCING OLD AS
OldTuple NEW AS NewTuple WHEN
(OldTuple.price gt NewTuple.price) UPDATE
Product SET price OldTuple.price
WHERE name NewTuple.name FOR EACH ROW
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Statement Level Trigger
emp(dno), dept(dept, ) Whenever we insert
employees tuples, make sure that their dnos
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))
35
Bad Things Can Happen
CREATE TRIGGER Bad-trigger AFTER UPDATE OF
price IN Product REFERENCING OLD AS OldTuple
NEW AS NewTuple WHEN
(NewTuple.price gt 50) UPDATE Product
SET price NewTuple.price 2
WHERE name NewTuple.name FOR EACH ROW
36
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.
37
Programs with SQL
Host language Embedded SQL
Preprocessor
Preprocessor
Host Language function calls
Host language compiler
Host language compiler
Host language program
38
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.

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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 SQL BEGIN DECLARE SECTION
char productName30 EXEC SQL
END DECLARE SECTION
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Using Shared Variables
Void simpleInsert() EXEC SQL BEGIN
DECLARE SECTION char
productName20, company30 char
SQLSTATE6 EXEC SQL END DECLARE
SECTION / get values for
productName and company somehow
/ EXEC SQL INSERT INTO Product(name,
company) VALUES (productName,
company)
41
Single-Row Select Statements
Void getPrice() EXEC SQL BEGIN DECLARE
SECTION char productName20,
company30 integer price
char SQLSTATE6 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 /
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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
43
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).

44
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

45
Dynamic SQL (II)

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

46
Example Usage

• EXEC SQL BEGIN DECLARE SECTION char
  queryMAX_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 /

47
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