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Lecture 4 on Structural Query Language

To study Structural Query Language (SQL) as a non-procedural computer language to access relational database in data definitional language such as Create, Drop and Alter statements and in data manipulation language such as Insert, Update and Delete statements.

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Structural Query Language (SQL)

• SQL is a transform oriented relational language.

• SQL can be run as a query/update language by itself or SQL command can be embedded in application programs.

• SQL include commands for data definition, data manipulation and data control.

• It is non-procedural and includes no reference to access paths, links or navigation such that the user only need to specify what he wants to retrieve/update rather than how to do it.

• It has a high level of independence from physical data storage.

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SQL syntax

SELECT [Distinct] select-list

FROM from-list

WHERE qualification

GROUP BYgrouping-list

HAVING group-qualification

ORDER attribute-list

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Where

The Where clause limits the rows that are returned from Query:

For example:

Select * from users where userid = 2

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Order by

This command can sort by any column type: alphabetical, chronological or numeric in either ascending or descending order by placing asc or desc:

For example:

Select * from users order by lname, fname

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Having

The having predicate restricts the rows displayed by a group by (whereas the where clause restricts the rows that are used in the group by).

For example:

Select avg(contribution) as avg_contrib, state from contributions group by state having avg(contribution) > 500

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Sample database for SQL

SID NAME MAJOR GRADE_LEVEL

AGE

100 JONES HIST GR 21

150 PARKS ACCT SO 19

200 BAKER MATH GR 50

250 GLASS HIST SN 50

300 BAKER ACCT SN 41

350 RUSELL MATH JR 20

400 RYE ACCT FR 18

450 JONES HIST SN 24

Student Relation

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STUDENT_NUMBER

CLASS_NAME

POSITION_ NUMBER

100 BD445 1

150 BA200 1

200 BD445 2

200 CS250 1

300 CS150 1

400 BA200 2

400 BF410 1

400 CS250 2

450 BA200 3

Class_NAME

TIME ROOM

BA200 MF9 SC110

BD445 MWF3 SC213

BF410 MWF8 SC213

CS150 MWF3 EA304

CS250 MWF12

EB210

Class Relation

Enrollment Relation

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SQL Projections

The projection Student[Sid, Name, Major] is created by

• Select Sid, Name, Major from Student

100 Jones Hist150 Parks Acct200 Baker Math250 Glass Hist300 Baker Acct350 Russell Math400 Rye Acct450 Jones Hist

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• Select statement with Unique:

Select Unique Major from Student

=> HistAcctMath

• Select statement with where clause:

Select * from student where Major = ‘Math’

200 Baker Math Gr 50350 Russell Math Jr 20

Note: The * in the SQL statement means that all attributes of the relation are to be obtained.

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Select statement with several conditions:Select Name, Age from Student where Major = ‘Math’ and Age > 21 Baker 50

Select statement with an In clause:Select Name from Student where Major in [‘Math’, ‘Acct’]=> Parks

BakerBakerRussellRye

Select statement with an Not In clause:Select Name from Student where Major not in [‘Math’, ‘Acct’]=> Jones

GlassJones

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Select statement with LIKE conditions

Like operation allows a rich set of regular expressions to be used as patterns while searching text.

For example,

SELECT AgeFROM studentWHERE Name = ‘R_%E’

The only such student is Rye and his age is 18.

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SQL Built-in functionsSome standard built-in functions are available in SQL as follows:

• Count: computes the number of tuples in a relation• Sum: totals numeric attributes• Avg: computes the average value• Max: obtain the maximum value of an attribute• Min: obtain the minimum value of an attribute

For example:Select Count(Major) from Student 8

Select Count(Unique Major) from Student 3

Select Sid, Name from Student where Age > Avg(Age) 200 Baker

250 Glass300 Baker

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Built-in functions and groupingBuilt-in functions can be applied to groups of tuples within a relation. Such groups are for

med by collecting tuples together that have the same value of a specific attribute. The SQL keyword Group By instructs the DBMS to group tuples together that have the same value of an attribute.

For example, the count function sums the number of tuples in each group/

Select Major, Count(*) from Student Group By Major

=> Hist 3Acct 3Math 2

For example, use SQL Having clause to identify the subset of groups we want to consider.

Select Major, Avg(Age) from Student Group By Major Having Count(*)>2

Hist 31.67Acct 26

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Union, Intersect, Except

• Union set operation is similar to “or” condition in where clause.

• Intersect set operation is similar to “and” condition in where clause.

• Except set operation is a difference set operation in relational algebra.

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Select Name from student where age < 20 or age > 40

Can be rewritten as

Select Name from student where age < 20 Union

Select Name from student where age > 40

The answer is Rye, Baker, and Glass.

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Select Name from student where age < 20 and Grade = SN

Can be rewritten as

Select Name from student where age < 20

INTERSECT

Select Name from student where Grade = SN

The answer is Jones

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Select Name from student where Major not = ‘Hist’

Can be rewritten as

Select name from student

EXCEPT

Select name from student where Major = ‘Hist’

The answer is: Parks, Baker, Rusell and Rye.

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Querying multiple relations by using SubquerySuppose we need to know the names of students enrolled in the class BD445. If we kno

w that only students 100 and 200 are enrolled in this class, then the following will produce the correct names.

Select Name from Student Where Sid in [100, 200]

=> JonesBaker

Select Student_number from Enrollment where Class_name = ‘BD445’

=> 100200

By combining the above 2 SQL statements, the SQL subquery can be easier to understand as follows:

Select Name from Student where Sid inSelect Student_number from Enrollment where Class_name=‘BD445’

=> JonesBaker

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Suppose we want to know the names of the students enrolled in classes on Monday, Wednesday and Friday at 3 o’clock.

First, we need the name of classes that meet at that time.Select Class.Name from Class where Time=‘MWF3’

=> BD445CS150

Now, what are the identifying numbers of relations in these classes? We can specify:

Select Student_number from Enrollment where Enrollment.Class_name in Select Class_name from Class where Time = ‘MWF3’

=> 100200300

Now, to obtain the names of those students, we specify:

Select Student.name from Student where Student.Sid inSelect Enrollment.Student_number from Enrollment where Enrollment.Class_name in

Select Class_name from Class where Time = ‘MWF3’

=> JonesBakerBaker

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Querying multiple relations using JoinA join is the combination of a product operation, followed by a selection and a

projection. For example, the From statement expresses the product of Student and Enrollment. The Where statement expresses the selection. The projection of student number, name and class name is taken.

Select Student.Sid, Student.Name, Enrollment.Class_Name From Student, Enrollment Where Student.Sid=Enrollment.Student_Number

=> 100 Jones BD445150 Parks BA200200 Baker BD445200 Baker CS250300 Baker CS150400 Rye BA200300 Rye BF410400 Rye CS250450 Jones BA200

In this example, table Student and Enrollment are joined.

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Comparison of SQL Subquery and JoinJoin expressions can substitute for subquery expressions, the converse is not

true. Subqueries cannot always be substituted for joins. When using a subquery, the displayed attributes can come from only the relation name in the from expression in the first select.

For example, if we want to know the names of the students enrolled in classes on Monday, Wednesday, and Friday at 3 o’clock using Join.

Select Student.Namefrom Student, Enrollment, ClassWhere Student.Sid = Enrollment.Student_Number

and Enrollment.Class_Name = Class.Nameand Class.Time = ‘MWF3’

If we want to know both the names of the classes and the grade levels of the undergraduate students, then we must use a join. A subquery will not suffice because the desired results arise from two different relations of Enrollment and Student.

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Exists and Not ExistsExists and Not Exists are logical operators; their value is either tru

e or false depending on the presence of absence of tuples that fit qualifying conditions.

For example, suppose we want to know the numbers of students enrolled in more than one class. The meaning of the subquery expression is “Find 2 tuples in enrollment having the same student number but different class names”.

Select unique Student_number from Enrollment Awhere Exists

Select *from Enrollment Bwhere A.Student_number = B.Student_numberand A.Class_name not = B.Class_name

=> 200400

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Another example is that we want to know the names of students taking all classes. An expression is to say we want the names of students such that there are no classes that the student did not take.

Select Student.Namefrom Studentwhere not exists

Select *from Enrollmentwhere not exists

Select * from Classwhere Class.Name = Enrollment.Class_nameand Enrollment.Student_number = Student.Sid

Note: This query has three parts. In the bottom part, we try to find classes the student did not take. The middle part determines if any classes were found that the student did not take. If not, then the student is taking all classes, and the student’s name is displayed.

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Inserting dataTuples can be inserted into a relation one at a time or in groups

To insert a single tuple, we state

Insert into Enrollment [400, ‘BD445’, 44]

Or if we do not know all of this data, we could say

Insert into Enrollment (Student_number, Class_name) [400, ‘BD445’]

Note: If the student’s information is not available, this insertion will cause an referential integrity problem such that a child relation’s tuple can only be inserted if its corresponding parent relation’s tuple exists.

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Deleting dataTuples can be deleted one at a time or in groups.

For example, we want to delete the tuple for student number 100

Delete Studentwhere Student.Sid = 100

Note: If student 100 is enrolled in classes, this deletion will cause an referential integrity problem such that a parent relation’s tuple can be deleted only if its corresponding child relation’s tuple has been deleted.

Another example is to delete groups of tuples.

Delete Enrollmentwhere Enrollment.Student_number IN

Select Student.Sidfrom Studentwhere Student.Major = ‘ACCT’

Delete Studentwhere Student.Major = ‘ACCT’

Note: The order of these two deletion is important. If the order were reversed, none of the Enrollment tuples would be deleted because the matching Student tuples have already been deleted.

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Modifying dataTuples can be modified one at a time or in groups. The keyword Set is used to change an attribute value. After S

et, the name of the attribute to be changed is specified and then the new value or way of computing the new value.

Update EnrollmentSet Position_number = 44where Student_number = 400

Update EnrollmentSet Position_number = Max(Position_number) + 1where Student_number = 400

Note: the value of the attribute will be calculated using the Max built-in function.

Another example for mass update is to change a course from BD445 to BD564. In this case, to prevent referential integrity problem, we perform as follows:

alter table Enrollment Add Constraint FK foreign key (Class_name) references Class(Class_name) on update cascade;

Update EnrollmentSet Class_name = ‘BD564’where Class_name = ‘BD445’

Update ClassSet Class_name = ‘BD564’where Class_name = ‘BD445’

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Database Programming Approaches

• Embedded commands:– Database commands are embedded in a

general-purpose programming language

• Library of database functions:– Available to the host language for database

calls; known as an API• API standards for Application Program Interface

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Embedded SQL in MS SQL Server Stored Procedure

Stored Procedure:

create proc show_course @incourse_no integer as select * from course where course_no = @incourse_noreturn

Execution of Stored Procedure:

execute show_course 4

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Embedded SQL

• Most SQL statements can be embedded in a general-purpose host programming language such as COBOL, C, Java

• An embedded SQL statement is distinguished from the host language statements by enclosing it between EXEC SQL or EXEC SQL BEGIN and a matching END-EXEC or EXEC SQL END (or semicolon)– Syntax may vary with language– Shared variables (used in both languages) usually

prefixed with a colon (:) in SQL

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Example: Variable Declarationin Language C

Variables inside DECLARE are shared and can appear (while prefixed by a colon) in SQL statements

SQLCODE is used to communicate errors/exceptions between the database and the program

int loop;

EXEC SQL BEGIN DECLARE SECTION;

varchar dname[16], fname[16], …;

char ssn[10], bdate[11], …;

int dno, dnumber, SQLCODE, …;

EXEC SQL END DECLARE SECTION;

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Example for SQL Commands forConnecting to a Oracle Database

In the “Microsoft ODBC for Oracle Setup” dialog, enter the following:

Database Source Name: ora9i

Description: <optional>

Username: <optional>

Server Name: w2ksc

Start “Oracle -> OraHome90 -> Application Development -> SQL Plus

User Name: grp##

Password: abcd1234

Host String: w2ksc

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Example of a non-menu driven embedded SQL Stored Procedure to access an Oracle Database

create procedure upd_per (in_id in integer)

as begin

update person set name = 'abc' where id_no = in_id;

end;

execute upd_per(1);

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Embedded SQL in CProgramming Examples

loop = 1;

while (loop) {

prompt (“Enter SSN: “, ssn);

EXEC SQL

select FNAME, LNAME, ADDRESS, SALARY

into :fname, :lname, :address, :salary

from EMPLOYEE where SSN == :ssn;

if (SQLCODE == 0) printf(fname, …);

else printf(“SSN does not exist: “, ssn);

prompt(“More SSN? (1=yes, 0=no): “, loop);

END-EXEC

}

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Embedded SQL in CProgramming Examples

• A cursor (iterator) is needed to process multiple tuples

• FETCH commands move the cursor to the next tuple

• CLOSE CURSOR indicates that the processing of query results has been completed

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Embedded SQL in CProgramming Examples with Cursor

Prompt (“Enter the Department Name:”, dname);

EXEC SQL

Select Dnumber into :dnumber

from DEPARTMENT where Dname = :dname;

EXEC SQL DECLARE EMP CURSOR FOR

Selec Ssn, Fname, Minit, Lname, Salary

from EMPLOYEE where Dno = :dnumber

FOR UPDATE OF Salary;

EXEC SQL OPEN EMP;

EXEC SQL FETCH from EMP into :ssn, :fname, :minit, :lname, :salary;

while (SQLCODE == 0) {

printf (“Employee name is:”, Fname, Minit, Lname);

prompt (“Enter the raise amount:”, raise);

EXEC SQL

update EMPLOYEE

set Salary = Salary + :raise

where CURRENT OF EMP;

EXEC SQL FETCH from EMP into :ssn, :fname, :minit, :lname, :salary;

}

EXEC SQL CLOSE EMP;

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Lecture Summary

SQL is a high level relational database language. It is much user friendly than other database languages, and becomes the most popular database language since 90’s. Nevertheless, it is not an end user language such as Natural language, and is subject to be changed by individual relational database vendors and academic committee.

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Review Question

What are the difference between Query and Nested Query in SQL with respect to performance and why?

How does Entity Integrity affect an Insert operation of SQL?

How does Referential Integrity affect Insert and Update operations of SQL?

What is the condition for using UNION, INTERSECT and EXCEPT clauses in SQL?

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Tutorial Question Given the following relations, write SQL statements to answer the questions

below:

Customer (Customer_id, Customer_name, Customer_Address)Order (Order_id, *Customer_id, Date, Cost)Payment (Payment_id, *Customer_id, Date, Amount)

(a) List out all the customer names who paid more than $300 on the 15th March 1997.

(i) without using sub-query (30%)

(ii) using sub-query (30%)

(b) Remove a customer with customer_id ######## from the customer list where ######## is unique student id in 8 digits

(40%)

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Reading Assignment

Chapter 8 SQL-99: Schema Definition, Constraints, Queries, and Views “Fundamentals of Database Systems” by Elmasri & Navathe fifth edition, Pearson, 2007.