All things you Want to Know about SQL & Oracle

• What is SQL?
• SQL stands for structured query language and it is the standard language for dealing withRelational databases. SQL was originally developed at IBM in early 1970s or a prototype called system R.It was initially spelt and pronounced as ‘SEQUEL’ but now is popularly called SQL only. It has an ANSI as well as ISO standard.
•  Which is the different version of SQL?
• Sql got its first ANSI standard in 1986. After the next standardization was in 1989 and the one in 1992 became very popular. It was also called SQL2. The latest version is SQL3. Which is being implemented by oracle 8 onwards? It supports some object-oriented features.
• Which is the different version of Sql?
• SQL can be broadly classified as-
• Data definition language (DDL)
• Data manipulation Language (DML)
• Data control language (DCL)
• Transaction control
• Data retrieval (queries)
• Sometimes the DCL commands are also considered to be part of DDL commands
• What do you understand by DDL?
• The DDL commands stands for Data definition Language and it related to the structured of an object.
• E.g.: Create, Alter, drop, rename, truncate
• What do you understand by DML?
• The DML commands are those related to the content of the table. They deal with insertion, updating and deletion of rows in a table.
• E.g. insert, update, delete
• What do you understand by DCL?
•  The DCL commands are required to give or take back access rights on object.
• E.g. GRANT, REVOKE
• What is transaction control?
• These commands are used to handle the unit of work. A transaction executes either as a whole or none of its statements execute.
• What is data retrieval?
• The main purpose of data retrieval is to display data (raw, column) in the required format.It is mainly used for querying and reporting purpose.
• What is the difference between char and varchar2?
• The CHAR and VARCHAR2 both are used to store data.
• The CHAR type is used to store fixed-length character data. The default and minimum size is 1 and the maximum size is 2000 characters.
• The Char data types uses all the space assigned to it as per the size mentioned and hence has more storage efficiency because of which it processes data faster than VARCHAR2
• .What is the concept of DUAL table?
• DUAL is the work table of oracle, which has only one raw and column.
• The column name is DUMMY with data type CHAR (1). 
• When you want to perform some temporary calculation using only literals (no variables) then this table is of great use. The actual dummy column is irrelevant.
• You can do all your temporary work on this table.
• What is the difference between the Where clause and the HAVING clause?
• The where clause is used to restrict rows. 
• It checks for the condition for each and every row of the table. The having clause is used to restrict groups.
• It is used immediately after GROUP BY clause and it checks for the conditions considering each group as a whole. 
• In syntax as well as during execution the WHERE clause is always evaluated before the HAVING clause.

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