4 the sql_standard

Prof P Sreenivasa Kumar
Department of CS&E, IITM
1
The SQL Standard
• SQL – Structured Query Language
a ‘standard’ that specifies how
a relational schema is created
data is inserted / updated in the relations
data is queried
transactions are started and stopped
programs access data in the relations
and a host of other things are done
• Every relational database management system (RDBMS) is
required to support / implement the SQL standard.

2
History of SQL
SEQUEL
developed by IBM in early 70’s
relational query language as part of System-R project at
IBM San Jose Research Lab.
the earliest version of SQL
SQL evolution
SQL- 86/89
SQL- 92 - SQL2
SQL- 99/03 - SQL3
(includes object relational features)
And the evolution continues.

3
Components of SQL Standard(1/2)
Data Definition Language (DDL)
Specifies constructs for schema definition, relation definition,
integrity constraints, views and schema modification.
Data Manipulation Language (DML)
Specifies constructs for inserting, updating and querying the
data in the relational instances ( or tables ).
Embedded SQL and Dynamic SQL
Specifies how SQL commands can be embedded in a high-level
host language such as C, C++ or Java for programmatic access
to the data.

4
Transaction Control
Specifies how transactions can be started / stopped, how a set
of concurrently executing transactions can be managed.
Authorization
Specifies how to restrict a user / set of users to access only
certain parts of data, perform only certain types of queries etc.
Components of SQL Standard(2/2)

5
Data Definition in SQL
Defining the schema of a relation
create table r ( attributeDefinition-1, attributeDefinition-2,…,
attributeDefinition-n, [integrityConstraints-1],
[integrityConstraints-2],…,[integrityConstraints-m])
Attribute Definition –
attribute-name domain-type [NOT NULL] [DEFAULT v]
E.g.:
create table example1 ( A char(6) not null default “000000”,
B int, C char (1) default “F” );
name of the
relation

6
Domain Types in SQL-92 (1/2)
Numeric data types
• integers of various sizes – INT, SMALLINT
• real numbers of various precision – REAL, FLOAT,
DOUBLE PRECISION
• formatted numbers – DECIMAL ( i, j ) or NUMERIC ( i, j )
i – total number of digits ( precision )
j – number of digits after the decimal point ( scale )
Character string data types
• fixed length – CHAR(n) – n: no. of characters
• varying length – VARCHAR(n) – n: max.no. of characters
Bit string data types
• fixed length – BIT(n)
• varying length – BIT VARYING(n)

7
Date data type
DATE type has 10 position format – YYYY-MM-DD
Time data type
TIME type has 8 position format – HH : MM : SS
Others
There are several more data types whose details are
available in SQL reference books
Domain Types in SQL-92 (2/2)

8
Specifying Integrity Constraints in SQL
Also called Table Constraints
Included in the definition of a table
Key constraints
PRIMARY KEY (A1,A2,…,Ak)
specifies that {A1,A2,…,Ak} is the primary key of the table
UNIQUE (B1,B2,…,Bk)
specifies that {B1,B2,…,Bk} is a candidate key for the table
There can be more than one UNIQUE constraint but only one
PRIMARY KEY constraint for a table.

9
Specifying Referential Integrity Constraints
FOREIGN KEY (A1) REFERENCES r2 (B1)
specifies that attribute A1 of the table being defined, say r1, is a
foreign key referring to attribute B1 of table r2
recall that this means:
each value of column A1 is either null or is one of the
values appearing in column B1 of r2

10
Specifying What to Do if RIC Violation Occurs
RIC violation
can occur if a referenced tuple is deleted or modified
action can be specified for each case using qualifiers
ON DELETE or ON UPDATE
Actions
three possibilities can be specified
SET NULL, SET DEFAULT, CASCADE
these are actions to be taken on the referencing tuple
SET NULL – foreign key attribute value to be set null
SET DEFAULT – foreign key attribute value to be set to its
default value
CASCADE – delete the referencing tuple if the referenced
tuple is deleted or update the FK attribute if the
referenced tuple is updated

11
Table Definition Example
create table students (
rollNo char(8) not null,
name varchar(15) not null,
degree char(5),
year smallint,
sex char not null,
deptNo smallint,
advisor char(6),
primary key(rollNo),
foreign key(deptNo) references
department(deptId)
on delete set null on update cascade,
foreign key(advisor) references
professor(empId)
on delete set null on update cascade
);

12
Modifying a Defined Schema
ALTER TABLE command can be used to modify a schema
Adding a new attribute
ALTER table student ADD address varchar(30);
Deleting an attribute
need to specify what needs to be done about views or
constraints that refer to the attribute being dropped
two possibilities
CASCADE – delete the views/constraints also
RESTRICT – do not delete the attributes if there are some
views/constraints that refer to it.
ALTER TABLE student DROP degree RESTRICT
Similarly, an entire table definition can be deleted

13
Data Manipulation in SQL
Basic query syntax
select A1,A2,…,Am a set of attributes
from relations R1,…,Rp that are
from R1,R2,…,Rp required in the output table.
the set of tables that
where θ contain the relevant
tuples to answer the query.
a boolean predicate that
specifies when a combined
tuple of R1,…,Rp contributes
to the output.
Equivalent to: Assuming that each attribute
name appears exactly once
in the table.
1 2 nA ,A ,....A θ 1 2 pπ (σ (R ×R ×.....×R ))

14
Meaning of the Basic Query Block
The cross product M of the tables in the from clause
would be considered.
Tuples in M that satisfy the condition θ are selected.
For each such tuple, values for the attributes A1,A2,….,Am
( mentioned in the select clause) are projected.
This is a conceptual description
- in practice more efficient methods are employed for
evaluation.
The word select in SQL should not be confused with select
operation of relational algebra.

15
SQL Query Result
The result of any SQL query
a table with select clause attributes as column names.
duplicate rows may be present.
- differs from the definition of a relation.
duplicate rows can be eliminated by specifying DISTINCT
keyword in the select clause, if necessary.
SELECT DISTINCT name
FROM student
duplicate rows are essential while computing aggregate
functions ( average, sum etc ).
removing duplicate rows involves additional effort and is
done only when necessary.

16
Example Relational Scheme
student (rollNo, name, degree, year, sex, deptNo, advisor)
department (deptId, name, hod, phone)
professor (empId, name, sex, startYear, deptNo, phone)
course (courseId, cname, credits, deptNo)
enrollment (rollNo, courseId, sem, year, grade)
teaching (empId, courseId, sem, year, classRoom)
preReq (preCourseId, courseId)

17
Example Relational Scheme with RIC’s shown
student (rollNo, name, degree, year, sex, deptNo, advisor)
department (deptId, name, hod, phone)
professor (empId, name, sex, startYear, deptNo, phone)
course (courseId, cname, credits, deptNo)
enrollment (rollNo, courseId, sem, year, grade)
teaching (empId, courseId, sem, year, classRoom)
preRequisite (preReqCourse, courseID)

18
Example Queries Involving a Single Table
Get the rollNo, name of all women students in the
dept no. 5.
select rollNo, name
from student
where sex = ‘F’ and deptNo = ‘5’;
Get the employee Id, name and phone number of
professors in the CS dept (deptNo = 3) who have
joined after 1999.
select empId, name, phone
from professor
where deptNo = 3 and startYear > 1999;

19
Examples Involving Two or More Relations (1/2)
Get the rollNo, name of students in the CSE
dept (deptNo = 3)along with their advisor’s
name and phone number.
select rollNo, s.name, f.name as advisorName,
phone as advisorPhone
from student as s, professor as f
where s.advisor = f.empId and
s.deptNo = ‘3’;
attribute
renaming in
the output
table aliases are
used to disambiguate
the common attributes
table aliases are required
if an attribute name
appears in more than
one table.
Also when same relation
appears twice in the from
clause.

20
Get the names, employee ID’s, phone numbers
of professors in CSE dept who joined
before 1995.
select empId, f.name, phone
from professor as f, department as d
where f.deptNo = d.deptId and
d.name = ‘CSE’ and
f.startYear < 1995
Examples Involving Two or More Relations (2/2)

21
Nested Queries or Subqueries
While dealing with certain complex queries
beneficial to specify part of the computation as a
separate query & make use of its result to formulate
the main query.
such queries – nested / subqueries.
Using subqueries
makes the main query easy to understand / formulate
sometimes makes it more efficient also
• sub query result can be computed once and
used many times.
• not the case with all subqueries.

22
Nested Query Example
Get the rollNo, name of students who have
a lady professor as their advisor.
select s.rollNo, s.name
from student s
where s.advisor IN
(select empId
from professor
where sex = ‘F’);
NOT IN can be used in the above query to get details of students
who don’t have a lady professor as their advisor.
IN Operator: One of the
ways of making use of
the subquery result
Subquery computes
the empId’s of
lady professors

23
Set Comparison Operators
SQL supports several operators to deal with subquery results or
in general with collection of tuples.
Combination of { =, <, ≤, ≥, >, < > } with keywords
{ ANY, ALL } can be used as set comparison operators.
Get the empId, name of the senior-most Professor(s):
select p.empId, p.name
from professors p
where p.startYear <= ALL ( select distinct startYear
from professor );

24
Semantics of Set Comparison Operators
v op ANY S
true if for some member x of S, v op x is true
false if for no member x of S, v op x is true
v op ALL S
true if for every member x of S, v op x is true
false if for some member x of S, v op x is not true
IN is equivalent to = ANY
NOT IN is equivalent to < > ALL
v is normally a single attribute, but while using IN or
NOT IN it can be a tuple of attributes
op is one of <, ≤, >, ≥, =, < >
S is a subquery

25
Correlated Nested Queries
If the nested query result is independent of the current tuple
being examined in the outer query,
nested query is called uncorrelated,
otherwise, nested query is called correlated.
Uncorrelated nested query
nested query needs to be computed only once.
Correlated nested query
nested query needs to be re-computed for each row
examined in the outer query.

26
Example of a Correlated Subquery
Get the roll number and name of students
whose gender is same as their advisor’s.
select s.rollNo, s.name
from student s
where s.sex = ALL ( select f.sex
from professor f
where f.empId = s.advisor );

27
EXISTS Operator
Using EXISTS, we can check if a subquery result is non-empty
EXISTS ( S ) is true if S has at least one tuple / member
is false if S contain no tuples
Get the employee Id and name of professors
who advise at least one women student.
select f.empId, f.name
from professors f
where EXISTS ( select s.rollNo
from student s
where s.advisor = f.empId and
s.sex = ‘F’ );
SQL does not have an operator for universal quantification.
a correlated
subquery

28
Obtain the department Id and name of
departments that do not offer any 4 credit courses.
select d.deptId, d.name
from department d
where NOT EXISTS ( select courseId
from course c
where c.deptNo = d.deptId and
c.credits = ‘4’ );
Queries with existentially quantified predicates can be easily
specified using EXISTS operator.
Queries with universally quantified predicates can only be
specified after translating them to use existential quantifiers.
NOT EXISTS Operator

29
Example Involving Universal Quantifier
Obtain the department Id and name of departments
whose courses are all 3-credit courses.
Equivalently, obtain the department Id and name of departments
that do not offer a single course that is not 3-credit course
select d.deptNo, d.name
from department d
where NOT EXISTS ( select c.courseId
from course c
where c.deptNo = d.deptId and
c.credits ≠ 3);

30
Missing where Clause
If the where clause in an SQL query is not specified, it is treated
as the where condition is true for all tuple combinations.
Essentially no filtering is done on the cross product of from
clause tables.
Get the name and contact phone of all Departments.
select name,phone
from department

31
Union, Intersection and Difference Operations
In SQL, using operators UNION, INTERSECT and EXCEPT,
one can perform set union, intersection and difference
respectively.
Results of these operators are sets –
i.e duplicates are automatically removed.
Operands need to be union compatible and also have same
attributes in the same order.

32
Example using UNION
Obtain the roll numbers of students who are currently
enrolled for either CS230 or CS232 courses.
(SELECT rollNo
FROM enrollment
WHERE courseId = ‘CS230’ and
sem = odd and year = 2005 ) UNION
(SELECT rollNo
FROM enrollment
sem = odd and year = 2005 );
Equivalent to:
(SELECT rollNo
FROM enrollment
WHERE (courseId = ‘CS230’ or courseID = ‘CS232’)
and sem = odd and year = 2005 )

33
Obtain the roll numbers of students who are currently
enrolled for both CS230 and CS232 Courses.
select rollNo
from enrollment
where courseId = ‘CS230’ and
sem = odd and
year = 2005
INTERSECT
select rollNo
from enrollment
where courseId = ‘CS232’ and
sem = odd and year = 2005;
Example using INTERSECTION

34
Example using EXCEPT
Obtain the roll numbers of students who are
currently not enrolled for CS230 course.
(SELECT rollNo
FROM enrollment
WHERE sem = odd and year = 2005 )
EXCEPT
(SELECT rollNo
FROM enrollment
sem = odd and year = 2005);

35
Aggregation of Data
Data analysis
need for computing aggregate values for data
total value, average value etc
Aggregate functions in SQL
five aggregate function are provided in SQL
AVG, SUM, COUNT, MAX, MIN
can be applied to any column of a table
can be used in the select clause of SQL queries

36
Aggregate functions
AVG ( [DISTINCT]A):
computes the average of (distinct) values in column A
SUM ( [DISTINCT]A):
computes the sum of (distinct) values in column A
COUNT ( [DISTINCT]A):
computes the number of (distinct) values in column A or no.
of tuples in result
MAX (A): computes the maximum of values in column A
MIN (A): computes the minimum of values in column A
Optional
keyword

37
Examples involving aggregate functions (1/2)
Suppose data about Gate in a particular year is available in a table
with schema
gateMarks(regNo,name,sex,branch,city,state,marks)
Obtain the number of students who have taken GATE in
CS and their average marks
Select count(regNo) as CsTotal avg(marks) as CsAvg
from gateMarks
where branch = ‘CS’
Get the maximum, minimum and average marks obtained
by Students from the city of Hyderabad
Select max(marks), min(marks), avg(marks)
from gateMarks
where city = ‘Hyderabad’;
CStotal CSavg
Output

38
Examples involving aggregate functions (2/2)
Get the names of students who obtained the
maximum marks in the branch of EC
Select name, max(marks)
from gateMarks
where branch = ‘EC’
Will not work
Only aggregate functions can be specified here. It does not
make sense to include normal attributes ! (unless they are
grouping attributes – to be seen later)
Select regNo, name, marks
from gateMarks
where branch = ‘EC’ and marks =
(select max(marks)
from gateMarks
where branch = ‘EC’);
Correct way of
specifying the query

39
Date Aggregation and Grouping
Grouping
Partition the set of tuples in a relation into groups based on
certain criteria and compute aggregate functions for each group
All tuples that agree on a set of attributes (i.e have the same
value for each of these attributes ) are put into a group
The specified aggregate functions are computed for each group
Each group contributes one tuple to the output
All the grouping attributes must also appear in the select clause
the result tuple of the group is listed along with the values of the
grouping attributes of the group
Called the grouping
attributes

40
Examples involving grouping(1/2)
Determine the maximum of the GATE CS marks
obtained by students in each city, for all
cities.
Select city, max(marks) as maxMarks
from gateMarks
where branch = ‘CS’
group by city;
Grouping attributes
must appear in the
select clause
Grouping
attribute
Result:
City maxMarks
Hyderabad
Chennai
Mysore
Bangalore
87
84
90
82

41
In the University database, for each department,
obtain the name, deptId and the total number of four
credit courses offered by the department
Select deptId, name, count(*) as totalCourses
from department, course
where deptId = deptNo and credits = 4
group by deptId, name;
Examples involving grouping(2/2)

42
Having clause
After performing grouping, is it possible to report information
about only a subset of the groups ?
Yes, with the help of having clause which is always used in
conjunction with Group By clause
Report the total enrollment in each course in the 2nd
semester of 2004; include only the courses with a
minimum enrollment of 10.
Select courseId, count(rollNo) as Enrollment
from enrollment
where sem = 2 and year = 2004
group by courseId
having count(rollNo) ≥ 10;

43
Where clause versus Having clause
• Where clause
• Performs tests on rows and eliminates rows not satisfying
the specified condition
• Performed before any grouping of rows is done
• Having clause
• Always performed after grouping
• Performs tests on groups and eliminates groups not
satisfying the specified condition
• Tests can only involve grouping attributes and aggregate
functions
Select courseId, count(rollNo) as Enrollment
from enrollment
where sem = 2 and year = 2004
group by courseId
having count(rollNo) ≥ 10;

44
String Operators in SQL
Specify strings by enclosing them in single quotes
e.g., ‘Chennai’
Common operations on strings –
• pattern matching – using ‘LIKE’ comparison operator
Specify patterns using special characters –
• character ‘%’ (percent) matches any Substring
e.g., ‘Jam%’ matches any string starting with “Jam”
• character ‘_’ (underscore) matches any single character
e.g., (a) ‘_ _ press’ matches with any string ending
with “press”, with any two characters before that.
(b) ‘_ _ _ _’ matches any string with exactly four
characters

45
Using the ‘LIKE’ operator
Obtain roll numbers and names of all students
whose names end with ‘Mohan’
Select rollNo, name
from student
where name like ‘%Mohan’;
Patterns are case sensitive.
Special characters (percent, underscore) can be included in
patterns using an escape character ‘’ (backslash)

46
Join Operation
In SQL, usually joining of tuples from different relations is
specified in ‘where’ clause
Get the names of professors working in CSE dept.
Select f.name
where f.deptNo = d.deptId and
d.name = ‘CSE’;
The above query specifies joining of professor and department
relations on condition f.deptNo = d.deptId and d.name = ‘CSE’

47
Explicit Specification of Joining in ‘From’ Clause
select f.name
from (professor as f join department as d on
f.deptNo = d.deptId)
where d.name = ‘CSE’;
Join types:
1. inner join (default):
from (r1 inner join r2 on <predicate>)
use of just ‘join’ is equivalent to ‘inner join’
2. left outer join:
from (r1 left outer join r2 on <predicate>)
3. right outer join:
from (r1 right outer join r2 on <predicate>)
4. full outer join:
from (r1 full outer join r2 on <predicate>)

48
Natural join
The adjective ‘natural’ can be used with any of the join types to
specify natural join.
FROM (r1 NATURAL <join type> r2 [USING <attr. list>])
• natural join by default considers all common attributes
• a subset of common attributes can be specified in an
optional using <attr. list> phrase
REMARKS
• Specifying join operation explicitly goes against the spirit of
declarative style of query specification
• But the queries may be easier to understand
• The feature is to be used judiciously

49
Views
Views provide virtual relations which contain data spread across
different tables. Used by applications.
• simplified query formulations
• data hiding
• logical data independence
Once created, a view is always kept up-to-date by the RDBMS
View is not part of conceptual schema
• created to give a user group, concerned with a certain aspect
of the information system, their view of the system
Storage
• Views need not be stored as permanent tables
• They can be created on-the-fly whenever needed
• They can also be materialized
Tables involved in the view definition – base tables

50
Creating Views
CREATE VIEW v AS <query expr>
creates a view ‘v’, with structure and data defined by the
outcome of the query expression
Create a view which contains name, employee Id and
phone number of professors who joined CSE dept
in or after the year 2000.
create view profAft2K as
(Select f.name, empId, phone
where f.depNo = d.deptId and
d.name = ‘CSE’ and
f.startYear >= 2000);
If the details of a new CSE professor are entered into professor table,
the above view gets updated automatically
name of the view

51
Queries on Views
Once created a view can be used in queries just like any other
table.
e.g. Obtain names of professors in CSE dept,
who joined after 2000 and whose name
starts with ‘Ram’
select name
from profAft2K
where name like ‘Ram%’;
The definition of the view is stored in DBMS, and executed to
create the temporary table (view), when encountered in query

52
Operations on Views
Querying is allowed
Update operations are usually restricted
because – updates on a view may modify many base tables
– there may not be a unique way of updating the
base tables to reflect the update on view
– view may contain some aggregate values
– ambiguity where primary key of a base table is not
included in view definition.

53
Restrictions on Updating Views
Updates on views defined on joining of more than one table
are not allowed
For example, updates on the following view are not allowed
create a view Professor_Dept with professor
ID, department Name and department phone
create view profDept(profId,DeptName,DPhone) as
(select f.empId, d.name, d.phone
from professor f, department d
where f.depNo = d.depId);

54
Updates on views defined with ‘group by’ clause and aggregate
functions is not permitted, as a tuple in view will not have a
corresponding tuple in base relation.
Create a view deptNumCourses which contains the
number of courses offered by a dept.
create view deptNumCourses(deptNo,numCourses)
as select deptNo, count(*)
from course
group by deptNo;

55
Updates on views which do not include Primary Key of
base table, are also not permitted
Create a view StudentPhone with Student name and
phone number.
create view StudentPhone (sname,sphone) as
(select name, phone
from student);
View StudentPhone does not include Primary key of the
base table.

56
Updates to views are allowed only if
defined on single base table
not defined using ‘group by’ clause and aggregate functions
include Primary Key of base table
Allowed Updates on Views

57
Inserting data into a table
Specify a tuple(or tuples) to be inserted
INSERT INTO student VALUES
(‘CSO5D014’,‘Mohan’,‘PhD’,2005,‘M’,3,‘FCS008’),
(‘CSO5S031’,‘Madhav’,‘MS’,2005,‘M’,4,‘FCE009’);
Specify the result of query to be inserted
INSERT INTO r1 SELECT … FROM … WHERE …
Specify that a sub-tuple be inserted
INSERT INTO student(rollNo, name, sex)
VALUES (CS05M022, ‘Rajasri’, ‘F’),
(CS05B033, ‘Kalyan’, ‘M’);
the attributes that can be NULL or have
declared default values can be left-out to be
updated later

58
Deleting rows from a table
Deletion of tuples is possible ; deleting only part of a tuple is
not possible
Deletion of tuples can be done only from one relation at a time
Deleting a tuple might trigger further deletions due to
referentially triggered actions specified as part of RIC’s
Generic form: delete from r where <predicate>;
Delete tuples from professor relation with start year
as 1982.
delete from professor
where startYear = 1982;
If ‘where’ clause is not specified, then all the tuples of that
relation are deleted ( Be careful !)

59
A Remark on Deletion
The where predicate is evaluated for each of the tuples in the
relation to mark them as qualified for deletion before any
tuple is actually deleted from the relation
Note that the result may be different if tuples are deleted as and
when we find that they satisfy the where condition!
An example:
Delete all tuples of students that scored the least marks in the
CS branch:
DELETE
FROM gateMarks
WHERE branch = “CS” and
marks = ANY ( SELECT MIN(marks)
FROM gateMarks
WHERE branch = “CS”)

60
Updating tuples in a relation
update r
set <<attr = newValue> list>
where <predicates>;
Change phone number of all professors working in CSE
dept to “94445 22605”
update professors
set phone = ‘9444422605’
where deptNo = (select deptId
from department
where name = ‘CSE’);
If ‘where’ clause is not specified, values for the specified
attributes in all tuples is changed.

61
Miscellaneous features in SQL (1/3)
Ordering of result tuples can be done using ‘order by’ clause
e.g., List the names of professors who joined
after 1980, in alphabetic order.
select name
from professor
where startYear > 1980
order by name;
Use of ‘null’ to test for a null value, if the attribute can take null
e.g., Obtain roll numbers of students who
don’t have phone numbers
select rollNo
from student
where phoneNumber is null;

62
Use of ‘between and’ to test the range of a value
e.g., Obtain names of professors who have
joined between 1980 and 1990
select name
from professor
where startYear between 1980 and 1990;
Change the column name in result relation
e.g.,
select name as studentName, rollNo as studentNo
from student;

63
Use of ‘distinct’ key word in ‘select’ clause to determine
duplicate tuples in result.
Obtain all distinct branches of study for students
select distinct d.name
from student as s, department as d
where s.deptNo = d.deptId;
Use of asterisk (*) to retrieve all the attribute values of
selected tuples.
Obtain details of professors along with their
department details.
select *
where f.deptNo = d.deptId;

64
Application Development Process
Host language (HL) – the high-level programming language in
which the application is developed (e.g., C, C++, Java etc.)
Database access – using embedded SQL is one approach
• SQL statements are interspersed in HL program.
Data transfer –
takes place through specially declared HL variables
Mismatch between HL data types and SQL data types
• SQL 92 standard specifies the corresponding SQL types for
many HLs.

65
Declaring Variables
Variables that need to be used in SQL statements are declared in a
special section as follows:
EXEC SQL BEGIN DECLARE SECTION
char rollNo[9]; // HL is C language
char studName[20], degree[6];
int year; char sex;
int deptNo; char advisor[9];
EXEC SQL END DECLARE SECTION
Note that schema for student relation is
student(rollNo, name, degree, year, sex, deptNo, advisor)
Use in SQL statements: variable name is prefixed with a colon(:)
e.g., :ROLLNO in an SQL statement refers to rollNo variable

66
Handling Error Conditions
The HL program needs to know if an SQL statement has
executed successfully or otherwise
Special variable called SQLSTATE is used for this purpose
SQLSTATE is set to appropriate value by the RDBMS
run-time after executing each SQL statement
non-zero values indicate errors in execution
• different values indicate different types of error situations
SQLSTATE variable must be declared in the HL program and
HL program needs to check for error situations and handle them
appropriately.

67
Embedding SQL statements
Suppose we collect data through user interface into variables
rollNo, studName, degree, year, sex, deptNo, advisor
A row in student table can be inserted –
EXEC SQL INSERT INTO STUDENT
VALUES (:rollNo,:studName,:degree,
:year,:sex,:deptNo,:advisor);

68
Impedance mismatch and cursors
Occurs because, HL languages do not support set-of-records
as supported by SQL
A‘cursor’ is a mechanism which allows us to retrieve one row
at a time from the result of a query
We can declare a cursor on any SQL query
Once declared, we use open, fetch, move and close commands
to work with cursors
We usually need a cursor when embedded statement is
a SELECT query
INSERT, DELETE and UPDATE don’t need a cursor.

69
We don’t need a cursor if the query results in a single row.
e.g., EXEC SQL SELECT s.name, s.sex
INTO :name, :sex
FROM student s
WHERE s.rollNo = :rollNo;
Result row values name and phone are assigned to HL variables
:name and :phone, using ‘INTO’ clause
Cursor is not required as the result always contains only
one row ( rollNo is a key for student relation)
Embedded SQL (1/2)

70
If the result contains more than one row, cursor declaration
is needed
e.g., select s.name, s.degree
from student s
where s.sex = ‘F’;
Query results in a collection of rows
HL program has to deal with set of records.
The use of ‘INTO’ will not work here
We can solve this problem by using a ‘cursor’.
Embedded SQL (2/2)

71
Declaring a cursor on a query
declare studInfo cursor for
select name, degree
from student
where sex = ‘F’;
Command OPEN studInfo; opens the cursor and makes it point
to first record
To read current row of values into HL variables, we use the
command FETCH studInfo INTO :name, :degree;
After executing FETCH statement cursor is pointed to next
row by default
Cursor movement can be optionally controlled by the
programmer
After reading all records we close the cursor using the
CLOSE studInfo command.
Cursor name

72
Dynamic SQL
Useful for applications to generate and run SQL statements,
based on user inputs
Queries may not be known in advance
e.g., char sqlstring [ ] = {“select * from student”};
EXEC SQL PREPARE runQ FROM sqlstring;
EXEC SQL EXECUTE runQ;
‘Sqlstring’ is a ‘C’ variable that holds user submitted query
‘runQ’ is an SQL variable that holds the SQL statements.

73
Connecting to Database from HL
ODBC (Open Database Connectivity) and
JDBC (Java Database Connectivity)
accessing database and data is through an API
many DBMSs can be accessed
no restriction on number of connections
appropriate drivers are required
steps in accessing data from a HL program
• select the data source
• load the appropriate driver dynamically
• establish the connection
• work with database
• close the connection.

4 the sql_standard

More Related Content

Viewers also liked (20)

Similar to 4 the sql_standard (20)

Recently uploaded (20)

4 the sql_standard