SQL interview- http://career.guru99.com/top-50-sql-question-answers/

What is a join?

This is a keyword used to query data from more tables based on the relationship between the fields of the tables. Keys play a major role when JOINs are used.

Third Normal Form (3NF):

This should meet all requirements of 2NF. Removing the columns which are not dependent on primary key constraints.

First Normal Form (1NF):

This should remove all the duplicate columns from the table. Creation of tables for the related data and identification of unique columns

What is a foreign key?

A foreign key is one table which can be related to the primary key of another table. Relationship needs to be created between two tables by referencing foreign key with the primary key of another table.

What is a primary key?

A primary key is a combination of fields which uniquely specify a row. This is a special kind of unique key, and it has implicit NOT NULL constraint. It means, Primary key values cannot be NULL.

What are tables and Fields?

A table is a set of data that are organized in a model with Columns and Rows. Columns can be categorized as vertical, and Rows are horizontal. A table has specified number of column called fields but can have any number of rows which is called record. Example:.Table: Employee. Field: Emp ID, Emp Name, Date of Birth. Data: 201456, David, 11/15/1960.

What is Auto Increment?

Auto increment keyword allows the user to create a unique number to be generated when a new record is inserted into the table. AUTO INCREMENT keyword can be used in Oracle and IDENTITY keyword can be used in SQL SERVER. Mostly this keyword can be used whenever PRIMARY KEY is used.

What is the difference between Cluster and Non-Cluster Index?

Clustered index is used for easy retrieval of data from the database by altering the way that the records are stored. Database sorts out rows by the column which is set to be clustered index. A non-clustered index does not alter the way it was stored but creates a complete separate object within the table. It point back to the original table rows after searching.

What is a query?

DB query is a code written in order to get the information back from the database. Query can be designed in such a way that it matched with our expectation of the result set. Simply, a question to the Database.

What is the difference between DELETE and TRUNCATE commands?

DELETE command is used to remove rows from the table, and WHERE clause can be used for conditional set of parameters. Commit and Rollback can be performed after delete statement. TRUNCATE removes all rows from the table. Truncate operation cannot be rolled back.

What is a relationship and what are they?

Database Relationship is defined as the connection between the tables in a database. There are various data basing relationships, and they are as follows:. One to One Relationship. One to Many Relationship. Many to One Relationship. Self-Referencing Relationship.

What is a Database?

Database is nothing but an organized form of data for easy access, storing, retrieval and managing of data. This is also known as structured form of data which can be accessed in many ways.

What is RDBMS?

RDBMS stands for Relational Database Management System. RDBMS store the data into the collection of tables, which is related by common fields between the columns of the table. It also provides relational operators to manipulate the data stored into the tables.

What is Denormalization.

technique used to access the data from higher to lower normal forms of database. It is also process of introducing redundancy into a table by incorporating data from the related tables

Clustered Index.

type of index reorders the physical order of the table and search based on the key values. Each table can have only one clustered index.

What is DBMS?

A Database Management System (DBMS) is a program that controls creation, maintenance and use of a database. DBMS can be termed as File Manager that manages data in a database rather than saving it in file systems.

What is a unique key?

A Unique key constraint uniquely identified each record in the database. This provides uniqueness for the column or set of columns. A Primary key constraint has automatic unique constraint defined on it. But not, in the case of Unique Key. There can be many unique constraint defined per table, but only one Primary key constraint defined per table.

Full Join.

return rows when there are matching rows in any one of the tables. This means, it returns all the rows from the left hand side table and all the rows from the right hand side table.

Inner join.

return rows when there is at least one match of rows between the tables.

What are local and global variables and their differences?

Local variables are the variables which can be used or exist inside the function. They are not known to the other functions and those variables cannot be referred or used. Variables can be created whenever that function is called. Global variables are the variables which can be used or exist throughout the program. Same variable declared in global cannot be used in functions. Global variables cannot be created whenever that function is called.

Second Normal Form (2NF):

Meeting all requirements of the first normal form. Placing the subsets of data in separate tables and Creation of relationships between the tables using primary keys

Fourth Normal Form (3NF):

Meeting all the requirements of third normal form and it should not have multi- valued dependencies.

What is SQL?

SQL stands for Structured Query Language , and it is used to communicate with the Database. This is a standard language used to perform tasks such as retrieval, updation, insertion and deletion of data from a database.

What is a stored procedure?

Stored Procedure is a function consists of many SQL statement to access the database system. Several SQL statements are consolidated into a stored procedure and execute them whenever and wherever required.

Left Join.

return rows which are common between the tables and all rows of Left hand side table. Simply, it returns all the rows from Left hand side table even though there are no matches in the Right hand side table.

What are all the different types of indexes?

There are three types of indexes. Unique Index. Clustered Index. NonClustered Index

What are the types of subquery?

There are two types of subquery - Correlated and Non-Correlated. A correlated subquery cannot be considered as independent query, but it can refer the column in a table listed in the FROM the list of the main query. A Non-Correlated sub query can be considered as independent query and the output of subquery are substituted in the main query.

What are the types of join and explain each?

There are various types of join which can be used to retrieve data and it depends on the relationship between tables

What is a View?

a virtual table which consists of a subset of data contained in a table. Views are not virtually present, and it takes less space to store. View can have data of one or more tables combined, and it is depending on the relationship

What is a constraint?

can be used to specify the limit on the data type of table. Constraint can be specified while creating or altering the table statement. Sample of constraint are. NOT NULL. CHECK. DEFAULT. UNIQUE. PRIMARY KEY. FOREIGN KEY.

What is data Integrity?

defines the accuracy and consistency of data stored in a database. It can also define integrity constraints to enforce business rules on the data when it is entered into the application or database.

NonClustered Index

does not alter the physical order of the table and maintains logical order of data. Each table can have 999 nonclustered indexes.

Right Join.

return rows which are common between the tables and all rows of Right hand side table. Simply, it returns all the rows from the right hand side table even though there are no matches in the left hand side table.

Unique Index.

indexing does not allow the field to have duplicate values if the column is unique indexed. Unique index can be applied automatically when primary key is defined.

What is Datawarehouse

is a central repository of data from multiple sources of information. Those data are consolidated, transformed and made available for the mining and online processing. Warehouse data have a subset of data called Data Marts.

What is a trigger?

is a code or programs that automatically execute with response to some event on a table or view in a database. Mainly, trigger helps to maintain the integrity of the database. Example: When a new student is added to the student database, new records should be created in the related tables like Exam, Score and Attendance tables.

What is a Cursor?

is a control which enables traversal over the rows or records in the table. This can be viewed as a pointer to one row in a set of rows. Cursor is very much useful for traversing such as retrieval, addition and removal of database records.

What is subquery?

is a query within another query. The outer query is called as main query, and inner query is called subquery. SubQuery is always executed first, and the result of subquery is passed on to the main query.

What is an Index?

is performance tuning method of allowing faster retrieval of records from the table. An index creates an entry for each value and it will be faster to retrieve data.

What are all the different normalizations

normal forms can be divided into 5 form

What is normalization?

process of minimizing redundancy and dependency by organizing fields and table of a database. The main aim of Normalization is to add, delete or modify field that can be made in a single table.