cs640 information systems dr deepak b phatak subrao nilekani chair professor kanwal rekhi building,...

CS640 Information Systems

Dr Deepak B PhatakSubrao Nilekani Chair Professor

Kanwal Rekhi Building, Department of CSEIIT Bombay

Session 7, SQL DML and transactions

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CS634-Session 7, SQL-transactions Dr. Deepak B Phatak 2

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Session overview

• Views• Data manipulation

• Changes in database contents• Transactions

• Course Project


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View definition

• To create a view, we use:

Create view v as <query expression>;

• <Query expression> is any legal expression

• The view name is represented by ‘v’


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Sample view

Create view h8students as Select (sroll, sname, sroom, scpi) From student Where shostel = 8;


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Sample query a view

Select sname From h8studentsWhere sroom > 300And scpi > 9.00;


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Sample query a view

Select sname From h8studentsWhere sroom > 300And scpi > 9.00;

The create view statement only creates the schema of the view, It is materialized only when a query is executed


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Derived relation

• Temporary views created as a result of a query

Select sh, cpiavg from (select shostel,

avg(scpi) From student Group by shostel As result (sh, cpiavg) );


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Modification of values

• Database tables can be modified by any one of the following• Insert• Update• Delete


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Insertion of rows

• Insert record of a new student joining the institute

Insert into student Values (‘02D01234’, ‘alekh’, 02,

123,);

Insert into student (sroll, shostel)Values (‘01007052’, 08);


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Another insertion example

Insert into reg Select sroll, ‘CS771’ From reg where ccode = ‘CS634’ And sroll in (select sroll from student Where scpi > 9.5);


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• Change the grades of all CS634 students to ‘AA’

Update reg Set grade = ‘AA’ Where ccode = ‘CS634’;


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• Increase faculty salaries by 50%

Update faculty Set salary = salary * 1.50;


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Deletion

• Delete registration records of ‘CS634’ for students with CPI < 6.5

Delete from reg Where sroll in (select sroll from student natural join

reg Where scpi < 6.5 and ccode =

‘CS634’);


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Another example of deletion

- Remove all students of Hostel 8 Delete From Student Where shostel = 8;

- Fire all faculty

Delete from Faculty;


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Authorization

• It is obvious that the powerful SQL features for data manipulation may be misused to corrupt the contents of our database

• Data Base Administrator (DBA) has full rights of access

• Other users are given selective permissions using ‘Grant’ statement.

• These can be revoked later


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Insertion into views

• Modification of values in a view actually means modification of values in the base table (s)

• Modifications permitted only in simple views


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Schema modifications

• Data changes occur regularly• Schema changes are infrequent

• Must be handled with great care• Alter table command

• When the table structure is changed, what happens to values of existing and new columns?


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Authorization

• It is obvious that the powerful SQL features for data manipulation may be misused to corrupt the contents of our database

• Data Base Administrator (DBA) has full rights of access

• Other users are given select permissions using ‘Grant’ statement, which can be revoked later


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SQL transactions

• Transaction concept• ACID properties• SQL features supporting transactions• Special cases

• Long transactions• Distributed transactions• Replication


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Transaction concept

• A transaction is a unit of program execution that accesses and possibly updates various data items.

• A transaction is normally expected to support ACID properties to preserve data integrity


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Transaction concept (contd.)

• A transaction must see a consistent database. It must also leave behind a consistent database• During transaction execution the

database may be inconsistent.• When the transaction is

committed, the database must be consistent.


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Transaction concept (contd.)

• Two main issues to deal with:• Failures of various kinds

•hardware failures•system crashes

• Concurrent execution of multiple transactions•Different transactions trying to

read/update same rows/columns


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Example of fund transfer

• Transaction to transfer Rs. 50 from account A to account B:

1. Read(A)2. A := A – 50 3. Write(A)4. Read(B)5. B := B + 506. Write(B)


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Example of fund transfer

1. Read(A)2. A := A – 50 3. Write(A)

4. Read(b)5. B := B + 506. Write(B)

Update AccountsSet Bal = Bal - 50Where acode = ‘A’;

Update AccountsSet Bal = Bal + 50Where acode = ‘B’;


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ACID properties

• Atomicity. Either all operations of the transaction are properly reflected in the database or none are.

• Consistency. Execution of a transaction in isolation preserves the consistency of the database.


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Atomicity and consistency

• Atomicity requirement — if the transaction fails after step 3 and before step 6, the system should ensure that its updates are not reflected in the database, else an inconsistency will result.

• Consistency requirement – the sum of a and b is unchanged by the execution of the transaction.


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Acid properties (contd.)

• Isolation. Although multiple transactions may execute concurrently, each transaction must be unaware of other concurrently executing transactions.


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Isolation

• Intermediate transaction results must be hidden from other concurrently executed transactions.

• That is, for every pair of transactions ti and tj, it appears to ti that, either tj finished execution before ti started, or tj started execution after ti finished.


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Possible wrong effects of concurrency

1. Read(A)

4. A := A – 50 6. Write(A)7. Read(B)8. B := B + 509. Write(B)

2. Read (A)

5. Write (A)

Assume Deepak has Rs 14500 in ‘A’

T1 (Deepak) T2 (Pratibha)

3. A := A - 14500


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Isolation

• If, during the transaction T1, another transaction T2 reads the value of ‘A’ and attempts to modify it• Wrong balance for ‘A’ will result

at the end of these transactions

• Transaction may be illegal


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Isolation

• Isolation requirement — if another transaction is allowed to access the partially updated database, it will see an inconsistent database (the sum A + B will be less than what it should be)

• Can be ensured trivially by running transactions serially, that is one after the other. However, executing multiple transactions concurrently has significant benefits.


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Acid properties (contd.)

• Durability. After A transaction completes successfully, the changes it has made to the database persist, even if there are system failures.


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Durability

• Durability requirement — once the user has been notified that the transaction has completed (i.e., The transfer of the Rs. 50 has taken place), the updates to the database by the transaction must persist despite failures.


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SQL transactions

• Normal syntax• Special prescription


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SQL transactions (contd.)

• In most database systems, each SQL statement that executes successfully is automatically committed. • Each transaction would then consist

of only a single statement• Automatic commit can usually be

turned off, allowing multi-statement transactions, but how to do so depends on the database system


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SQL transactions

• A transaction is A sequence of queries and update statements executed as A single unit• Transactions are started

implicitly and terminated by one of•Commit work: makes all

updates of the transaction permanent in the database

•Rollback work: undoes all updates performed by the transaction.


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SQL transactions

• If any step of a transaction fails, all work done by the transaction can be undone by rollback work.

• Rollback of incomplete transactions is done automatically, in case of system failures


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SQL transactions (contd.)

• Another option in SQL, enclose statements within

begin atomic …

… end;


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Course project

• Campus Activity Information System• Hostel activities• Sports activities• Cultural activities• Academic activities

• Educational• Research and Development

• Events, for example• Mood Indigo• Techfest• Ecell Activities

• Other activities


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Activities

• Each group will give its choice by January 30

• Project allocation, and formation of the teams will be announced after the tutorial on 31st January

• Submission Schedule for project work• System Requirement Specification

• Draft SRS due on 5 February• Final SRS due on 15 February

• Mid Semester Examination?? • Monday 19 February, 1700 Hrs

cs640 information systems dr deepak b phatak subrao nilekani chair professor kanwal rekhi building,...

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