SQL Interview Questions

==> What is the primary key?

Answer :It is the uniquely identify each record in the table.

==> What is Foreign key?

Answer : Foreign key is key which the reference of the primary key of another table .

==>What is Table ?

Answer: It is the collection of the data in an organized manner in form of rows and columns.

==>What is Field?

Answer: A field refers to the number of columns in the table.

==> What is DELETE command ?

Answer : Delete command is used to delete row in table . Example : DELETE FROM column_name WHERE condition

==> What is Truncate command?

Answer: Truncate command is used to delete all rows from the table.

==>What is DROP command ?

Answer: It is used to delete the table in the database. Example : DROP TABLE table_name

==>What is DDL ?

Answer: DDL is stands for Data Definition Language and it Consists of the command that can be used to define the database.

==> What is DML ?

Answer : DML is stands for Data Manipulating Language and Consists of the command that can be used to Manipulation of data in present in database.

==> What is DCL ?

Answer : DCL stands for Data Control Language and Command which deals with the right, Control of the database system.

==> What is Constrains ?

Answer: It is the used to specify the limit of the data type of the table. It can be specified while creating or altering the table statement.

Ex: NOT NULL,UNIQUE,CHECK,DEFAULT, INDEX.

==>Difference between the CHAR and VARCHAR ?

Answer : -> CHAR has the fixed length and the length size of CHAR is 10.

→ VARCHAR has no fixed length and the length size of VARCHAR is 10,20 or so many. Example : VARCHAR(50)

==> What is join in SQL ?

Answer : Joins clauses is used to combined the rows from two or more tables, based on the related column between them .

==> Types of join ?

Answer : (INNER) JOIN: Returns records that have matching values in both tables

• LEFT (OUTER) JOIN: Returns all records from the left table, and the matched records from the right table
• RIGHT (OUTER) JOIN: Returns all records from the right table, and the matched records from the left table
• FULL (OUTER) JOIN: Returns all records when there is a match in either left or right table

== >What is Unique Key?

Answer: Unique identify a single row in the table. No duplicate values allowed.

==>What is the Data Integrity?

Answer: It defines accuracy of data consistency of data Integrity constrains to enforce business rules on data.

==> What is Clustered index and non-clustered index in SQL?

Answer : Clustered index: Clustered index is used for easy retrieval of data from the database and it is faster. One table have only one clustered index .

Non-clustered index : Non-Clustered index is used for easy retrieval of data from the database and it is slower. One table have many non-clustered index .

==> Write SQL query for display to current date?

Answer : SELECT GETDATE();

== >What do you mean by Denormalization ?

Answer : Increase the performance of entire infrastructure as it introduce redundancy into the table. Adds the redundant data into a table and combine data from various table into a single table.

==>What is the entity ?

Answer: An object or any a person, place or thing in the real world about which data can be stored in database.

==> What is relationship ?

Answer: Relations or links between the entities that have something to do with each other.

==>What is index in SQL.

Answer: Indexes are used to retrieve data from the database more quickly than otherwise. The users cannot see the indexes, they are just used to speed up searches/queries.

Example : CREATE INDEX index_name ON column_name.

==> Explain different types of index in SQL ?

Answer: 1) Clustered Index : Clustered indexes sort and store rows data in a table or view based on their fundamental values.

2) Unique Index: The unique index in SQL guarantees and confirms that the index key does not contain any duplicate values.

3)Non-Clustered Index: Non-clustered index poses a structure, which is separated from data rows.

==>What is the normalization and advantages of it ?

Answer: Normalization is the process to organizing data to avoid duplication redundancy.

Advantages: → Better data organization.

→ Efficiency data access and allow easy to modification.

==> Difference types of normalization ?

Answer : 1 NF : Each table cell should have a single value. so basically all the records must be unique.

2 NF : Database should be 1 NF and should also have single column primary key.

3 NF: Database should be 2 NF and must not have transitive functional dependencies.

BCNF: It has to be in 3rd normal form and every function dependency A → B.

==> What is ACID property in a database ?

Answer: Atomicity: Execute transition either full transition or none transition ,

Consistency: Measurement transition of correctness,

Isolation: It transition which should not transitions of another transitions,

Durability: Update the transition value permanent.

==> What do you mean by triggered ?

Answer : It defines to execute the automatically in place or after the modifications.

==> What are different types of operator in SQL?

Answer : 1) Arithmetic Operator

2) Bitwise operator

3) Comparison operator

4) Compound operator

5) Logical operator

==> What is cross join and natural join ?

Answer : Cross join : produces the cross product or Cartesian product of two value.

Natural join : Based on all the column having the same name and data types in both the tables.

==> What is subquery in SQL ?

Answer : A subquery is a query inside the another query where a query is defined to retrieve data or information back from the database.

Example : SELECT column_name FROM table_name WHERE column_name IN(SELECT column_name FROM table_name WHERE condition).

==> Can you list the ways to get the count of records in a table?

Answer: To count the number of records in a table.

Example : SELECT COUNT(column_name or *) FROM table_name;

==> Write a query to find the names of employees that begin with the “A”?

Answer: SELECT * FROM employee WHERE Empname LIKE ‘A%’;

==> Write the SQL query to get the third highest salary of an employee from employee table ?

Answer : SELECT TOP 1 salary FROM(SELECT TOP 3 salary FROM employee_table ORDER BY salary DESC) AS emp ORDER BY salary DESC.

==> What are the different types of Relationship ?

Answer : 1) Many to many relationship

2) One to One Relationship

3) One to Many Relationship

4) Many to One Relationship.

==> What is the main difference between “BETWEEN’ and “IN”?

Answer : BETWEEN : Used to display rows based on range of values in a row.

Example: SELECT * FROM Students WHERE ROLL_NUM BETWEEN 15 AND 50.

IN : used to check for values in a specific set of values.

Example : SELECT * FROM students WHERE ROLL_NUM IN (8,15,25).

==> What is MERGE in SQL ?

Answer: it allows conditional update or insertion of data into the table. It performs an update if a row exists, or an insert if the row does not exists.

==> What is the recursive stored procedure ?

Answer : Recursive stored procedure refers to a stored procedure which calls by itself until it reaches the some boundary conditions.

==>What is clause in SQL?

Answer : SQL clause helps to limit the result set by providing the condition to the query. Example : HAVING and WHERE clause.

==>What is the difference between “HAVING” and “WHERE” clause ?

Answer: HAVING : can be used only with SELECT statement. It is usually used in GROUP BY statement.

Example : SELECT column_name(s) FROM table_name WHERE condition GROUP BY column_name(s) HAVING condition ORDER BY column_name(s);

WHERE : It is used to extract only those records that fulfill a specified condition.

Example : SELECT column1, column2, … FROM table_name WHERE condition;

==> How can you fetch common records from two tables?

Answer :SELECT column_name FROM table_name INTERSECT SELECT column_name FROM table_name.

==> What is an ALIAS command ?

Answer: Alias are used to rename a table name or column name.

Example : SELECT column_name FROM table_name AS alias_name;

==> What is aggregations ?

Answer: Used to evaluate the mathematical calculations and returns a single values. Example : MAX(), COUNT().

==> How to fetch the alternate records from a table ?

Answer: To display odd number →

SELECT StudentID FROM(SELECT rowno, StudentId FROM Student) WHERE mod(rowno,2)=0;

To display even number →

SELECT StudentID FROM(SELECT rowno, StudentId FROM Student) WHERE mod(rowno,2)=1;

==> How can you select unique records from a table ?

Answer : SELECT DISTINCT emply_id FROM employee;

==> How can use first 5 character of the string?

Answer: SELECT SUBSTRING(Studentname,1,5) as Studentname FROM Student

==> What is the advantages and disadvantages of Stored Procedure ?

Answer: Advantages: It can be used as a modular programming which means create once, store and call for several times whenever it is required.

Disadvantages: It can be executed only in database.

==>What are the local and global variable?

Answer: Local Variable : It can be used and exist only inside the function

Global variable : It can used throughout the function. It can not be created whenever that function is called.