SQL interview questions:Part1

1-  Difference in Delete truncate and Drop

1. Truncate
    
   • Truncate is faster and used by fewer system.
   • Truncate can not be roll back.
   • It is DDL (Data Definition Language) command.
   • You can not use where clause with  Truncate command.
   • Truncate table statement deletes all data from a table without deleting the definition of table.
2. Delete 
    
   •  Delete removes rows once at a time, means you can use delete with and without where clause.
   • Delete can be roll back.
   • It is DML (Data Manipulation Language) command.
   • DELETE does not reset the identity of the table.
3. Drop
    
   • Drop command removes table definition including indexer, trigger, constraints.

2-  Difference between clustered and non-clustered index

1. A clustered index
    
   • A clustered index is a type of index where the table records are physically re-ordered to math the index. A table can have only one clustered index. One of the most important point about clustered index is that "Primary Key has to be Clustered Index".
2. A non-clustered index
    
   •  A non-clustered index is a special type of index in which the logical order of the index does not match physical order of the rows on disk.

3-  Important Data Types in SQL

Data Type	Syntax
integer	integer
numeric	numeric (p,s)
decimal	decimal (p,s)
float	float (p,s)
character	char (x)
date	date
time	time
bit	bit (x)
real	real
smallint	smallint

4-  Create, Drop or Delete, Select and show Database

1. Create a Database
    Syntax
   CREATE DATABASE  databaseName
   
   Example
   
   SQL>CREATE DATABASE test;
    
2. Delete or drop a Database
   
   syntax
   
   DROP DATABASE dataBaseName
   
   Example
   
   SQL>DROP DATABASE TEST;
    
3. Select a Database
   
   syntax
   
   USE DataBaseName;
   
   Example
   
   SQL> USE Test;
    
4. Show all Databases
   Example
   
   For SQL
   sp_databases
   
   And For mysql
   
   mysql>Show DATABASES;