SQL

• Join 2 or more tables and show it as one object
• View data without storing the data into the object
• Restrict the view/visibility of the able
• Restrict access to the table
• Views do not store the data, only view defination occupies space so view does not occupy any physical space in the DB
• Can perform DML on views but the underlying tables are affected

• When the underlying table(s) is dropped the view becomes inactive
• As views just represent data in the underlying tables we cannot change the data of a viewing without changing the data of the tables.

• Truncate is faster than Delete. (Why)
• Truncate applies to whole table but Delete can be executed for a filtered set of records
• Data cannot be recoved in Truncate, but we can recover data in Delete. (Why)
• Truncate is DDL and Delete is DML

• Truncate will not delete the structure of the table.
• Drop will delete everything including the structure.

Yes but not always. (Why)

NULL is a marker that represents missing, unknown, or inapplicable data. Null is untyped in SQL, meaning that it is not designated as a NUMBER, CHAR, or any other specific data type. Do not use NULL to represent a value of zero, because they are not equivalent.

So it does not have a datatype as it is untyped?

Oracle allows up to 255 levels of subqueries in the WHERE clause.

To kill session using SQL command. Just check serial no and sid (system identifier) from v$session view as follows:

select sid,serial# from v$session where machine='GPTWORKGROUP';

alter system kill session '9,171' immediate;

immediate is used for forced kill

• Though the HAVING clause specifies a condition that is similar to the purpose of a WHERE clause, the two clauses are not interchangeable. Listed below are some differences to help distinguish between the two:
• The WHERE clause specifies the criteria which individual records must meet to be selcted by a query. It can be used without the GROUP BY clause. The HAVING clause cannot be used without the GROUP BY clause.
• The WHERE clause selects rows before grouping. The HAVING clause selects rows after grouping.
• The WHERE clause cannot contain aggregate functions. The HAVING clause can contain aggregate functions.

• IN:Returns true if a specified value matches any value in a subquery or a list
• Exists:Returns true if a subquery contains any rows.