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Archive

Posts Tagged ‘RDBMS’

What is SQL, PL/SQL, T-SQL and difference between them

May 3, 2015 14 comments

Today I got an email from a student who is pursuing his Bachelors degree in Computer Application (BCA). He visited my blog and sent me an email regarding his confusion with terms like SQL, T-SQL, PL/SQL, and asked me what is differences between them and how are they related? I had a chat with him and told the basic differences, but he further asked me how they are related to Microsoft SQL Server, Oracle, MySQL, etc? As he is studying SQL only based upon Oracle in his course curriculum, these all terms were not clear to him, so I cleared all his doubts while chatting with him.

After a while I had a same reminiscence that when I was a student I also had these doubts and confusions, and there was nobody to guide me, but I gradually came to know about this and it took some time. Thus, I’am taking this opportunity to put all these things together here in a single blog post for my readers (specially students) and for my reference as well.
 

–> SQL: stands for Structured Query Language and is pronounced as Sequel, and in early days it was also known as SEQUEL only.

– IBM in early 1970s developed SEQUEL which stands for Structured English QUEry Langauge for their RDBMS. The acronym was later changed to SQL, as SEQUEL was already trademarked by some UK based aircraft company.
 

–> PL/SQL: stands for Procedural Language/Structured Query Language and is used with Oracle database to create PL/SQL units such as Procedures, Functions, Packages, Types, Triggers, etc. which are stored in the database for reuse by applications that use any of the Oracle Database programmatic interfaces.

– Oracle in 1970s known as “Relational Software” saw SQL potential and influenced by Boyce, Codd and Chamberlin developed their own RDBMS product which was commercially available as Oracle Database. Oracle is supported on many Operating Systems like Windows, Linux, Solaris, AIX, OpenVMS, etc. [Oracle Database]
 

–> DB2: IBM during early 1980s made SQL commercially available with its product known as IBM DB2 from its prototype “System R”. [IBM DB2]
 

–> SQL Standardization: Later in 1986 SQL was adopted as a Standard by ANSI (American National Standards Institute) as SQL-86, and today the latest Standard is known as SQL:2011

✔ As a standarg SQL should support following:

– Language elements: Clauses, Expressions, Predicates, Queries, Statements.
– Operators: =, , >, =, <=, BETWEEN, LIKE, IN, NOT IN, IS, IS NOT, AS, etc.
– Conditional expressions: CASE, IF ELSE
– Queries: which include SELECT, FROM, WHERE, GROUP BY, HAVING, ORDER BY, etc.
– Data Types: Numeric, Char, Bit, Date and Time
– NULL or 3VL (Three value Logic)
– DDL, DML, TCL
 

–> T-SQL: stands for Transact-SQL, and is Sybase & Microsoft’s proprietary extension. T-SQL is very similar to PL/SQL, one can create T-SQL units such as Procedures, Functions, Types, Triggers, etc. as mentioned above.

– In 1987 Sybase shipped their first RDBMS product known as Sybase SQL Server. [Sybase]

– In 1988-89 Microsoft had an agreement with Sybase and ported the Sybase RDBMS to OS/2 platform and marketed it as Microsoft SQL Server 1.0, which was equivalent to Sybase SQL Server 3.0. [Microsoft SQL Server]

– In 1993 Sybase and Microsoft dissolved their partnership, and Microsoft bought the SQL Server code base from Sybase and both went different streams to develop their own product.

– Till SQL Server 2000 Microsoft has Sybase code base, and this was completely written in SQL Server 2005.

– Microsoft SQL Server is only supported on Windows Operating Systems.
 

–> MySQL: was founded by a Swedish Company MySQL AB in 1995 and is the most widely used open-source RDBMS. The MySQL development project has made its source code available under the terms of the GNU General Public License. MySQL does not currently comply with the full SQL standard. In 2008 Oracle corporation completely acquired MySQL. MySQL is supported on many Operating Systems like Windows, Linux, Solaris, OS X and FreeBSD. [MySQL.com]
 

–> FoxPRO and dBase: were very popular DataBase Management System (DBMS) products in mid 1980s and 1990s. They lack some RDBMS features and are out of support now, but are still being used in various legacy systems.

– FoxPRO is supported by Microsoft and was a text-based Procedural programming language and DBMS, for MD DOS, Windows, and UNIX. Microsoft Visual FoxPRO 9.0 being the latest and probably the last version published in 2007. [FoxPRO msdn]

– dBase was a very popular DBMS package including core Database engine, a Query system, a Forms engine and a xBase programming language with *.dbf file format. [dBase.com]
 

–> Here is a complete timeline that shows SQL and how it got evolved as different Products by different Vendors/Companies (click on the image to expand):
 

RDBMS_timeline.svg
 

–> There are many other popular SQL Products/System softwares available in market, and major of them are:
1. Oracle database
2. Microsoft SQL Server
3. IBM DB2
4. MySQL
5. PostgreSQL
6. Teradata
 


 

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DB Basics – What are ACID properties of a Transaction in an RDBMS?

September 28, 2014 3 comments

In order to perform a Transaction in a database system and to make sure it works without any issues, there are few rules a Database Transaction should follow. These rules are the standards across all Relational Database systems (RDBMS) and are called ACID rules.
 

ACID stands for Atomicity, Consistency, Isolation and Durability. So let’s check what all these Rules states.
 

–> A: Atomicity states that every Transaction should be atomic in nature. A Transaction in a Relational Database can contain either a single SQL statement or multiple SQL statements. Thus by Atomic Transaction it means “all or none”. Either all SQL statements/steps execute successfully in a transaction, or fail as a single unit and none of them should be treated as executed and the system should be returned to its original state.

For example: If account-A & account-B both having $2000 balance, you have to transfer $1000 from account-A to account-B, this will involves 2 steps. First withdrawal from account-A, and Second deposit in account-B. Thus, both the steps should be treated as single or atomic unit and at the end account-A should have $1000 & account-B should have $3000 balance. If in case after First step the system fails or any error occurs then first step should also be rolled-back and $1000 withdrawn from account-A should be re-deposited to it, maintaining $2000 back in both the accounts. Thus there should be no intermediate state where account-A has $1000 and account-B still has $2000 balance.
 

–> C: Consistency states that any Transaction happened in a database will take it from one consistent state to another consistent state. The data finally recorded in the database must be valid according to the defined Rules, Constraints, Cascades, Triggers, etc. If in case of any failure to these rules the changes made by any transaction should be rolled-back, this will put the system in earlier consistent state.

For example: If the money deposit process has any Trigger built on top of it. And at the time of money transfer any of the Trigger fails or any database node, the system should automatically Rollback the complete transaction and switch back the system to its previous consistent state before the transaction was started. Or if everything executes successfully then the system is committed to a new consistent state.
 

–> I: Isolation means Transactions performing same functions should run in Isolation and not in parallel to provide more concurrency to the data and avoiding dirty reads & writes. One need to use proper Transaction Isolation levels and locking in order to prevent this.

For example: If two people accessing a joint-account with $5000 balance from 2 terminals to withdraw money. Let’s say at same time John & Marry apply to withdraw $4000 from two different ATMs. If both the Transactions do not run in Isolation and run in parallel then both John & Marry will be able to withdraw $4000 each i.e. $8000 total from their account. To make sure this won’t happen Transactions should be not allowed to run in parallel, by setting Transaction Isolations and/or locking methods on the database objects.
 

–> D: Durability, a transaction should be durable by storing the data permanently and making it available in case of power failure, recovery from system failure, crash, any error, etc. All in all, the data should not get lost in any of the miss-happenings and one should be able to recover data from restore, logging and other methods.