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Technical Round Materials-SQL-Free Download

Technical Round Materials-SQL-Free Download

1. 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.

2. 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.

Example: SQL Server.

3. 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.

Standard SQL Commands are Select.

4. 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.

Example: School Management Database, Bank Management Database.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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.

  • Inner Join.

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

  • Right Join.

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.

  • Left Join.

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.

  • Full Join.

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.

11. What is normalization?

Normalization is the 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.

12. What is Denormalization.

DeNormalization is a 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.

13. What are all the different normalizations?

The normal forms can be divided into 5 forms, and they are explained below -.

  • 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.

  • 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.

  • Third Normal Form (3NF):.

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

  • Fourth Normal Form (3NF):.

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

14. What is a View?

A view is 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.

15. What is an Index?

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.

16. What are all the different types of indexes?

There are three types of indexes -.

  • Unique Index.

This 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.

  • Clustered Index.

This 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.

  • NonClustered Index.

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

17. What is a Cursor?

A database 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.

18. 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.

19. What is a query?

A 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.

20. What is subquery?

A 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.

21. 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.

22. 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.

23. What is a trigger?

A DB 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.

24. 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.

25. 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.

26. What is a constraint?

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.

27. What is data Integrity?

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.

28. 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.

29. 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 nonclustered 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.

30. What is Datawarehouse?

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.

31. What is Self-Join?

Self-join is set to be query used to compare to itself. This is used to compare values in a column with other values in the same column in the same table. ALIAS ES can be used for the same table comparison.

32. What is Cross-Join?

Cross join defines as Cartesian product where number of rows in the first table multiplied by number of rows in the second table. If suppose, WHERE clause is used in cross join then the query will work like an INNER JOIN.

33. What is user defined functions?

User defined functions are the functions written to use that logic whenever required. It is not necessary to write the same logic several times. Instead, function can be called or executed whenever needed.

34. What are all types of user defined functions?

Three types of user defined functions are.

  • Scalar Functions.
  • Inline Table valued functions.
  • Multi statement valued functions.

Scalar returns unit, variant defined the return clause. Other two types return table as a return.

35. What is collation?

Collation is defined as set of rules that determine how character data can be sorted and compared. This can be used to compare A and, other language characters and also depends on the width of the characters.

ASCII value can be used to compare these character data.

36. What are all different types of collation sensitivity?

Following are different types of collation sensitivity -.

  • Case Sensitivity – A and a and B and b.
  • Accent Sensitivity.
  • Kana Sensitivity – Japanese Kana characters.
  • Width Sensitivity – Single byte character and double byte character.

37. Advantages and Disadvantages of Stored Procedure?

Stored procedure can be used as a modular programming – means create once, store and call for several times whenever required. This supports faster execution instead of executing multiple queries. This reduces network traffic and provides better security to the data.

Disadvantage is that it can be executed only in the Database and utilizes more memory in the database server.

38. What is Online Transaction Processing (OLTP)?

Online Transaction Processing (OLTP) manages transaction based applications which can be used for data entry, data retrieval and data processing. OLTP makes data management simple and efficient. Unlike OLAP systems goal of OLTP systems is serving real-time transactions.

Example – Bank Transactions on a daily basis.

39. What is CLAUSE?

SQL clause is defined to limit the result set by providing condition to the query. This usually filters some rows from the whole set of records.

Example – Query that has WHERE condition

Query that has HAVING condition.

40. What is recursive stored procedure?

A stored procedure which calls by itself until it reaches some boundary condition. This recursive function or procedure helps programmers to use the same set of code any number of times.

41. What is Union, minus and Interact commands?

UNION operator is used to combine the results of two tables, and it eliminates duplicate rows from the tables.

MINUS operator is used to return rows from the first query but not from the second query. Matching records of first and second query and other rows from the first query will be displayed as a result set.

INTERSECT operator is used to return rows returned by both the queries.

42. What is an ALIAS command?

ALIAS name can be given to a table or column. This alias name can be referred in WHERE clause to identify the table or column.

Example-.

Select st.StudentID, Ex.Result from student st, Exam as Ex where st.studentID = Ex. StudentID

Here, st refers to alias name for student table and Ex refers to alias name for exam table.

43. What is the difference between TRUNCATE and DROP statements?

TRUNCATE removes all the rows from the table, and it cannot be rolled back. DROP command removes a table from the database and operation cannot be rolled back.

44. What are aggregate and scalar functions?

Aggregate functions are used to evaluate mathematical calculation and return single values. This can be calculated from the columns in a table. Scalar functions return a single value based on the input value.

Example -.

Aggregate – max(), count - Calculated with respect to numeric.

Scalar – UCASE(), NOW() – Calculated with respect to strings.

45. How can you create an empty table from an existing table?

Example will be -.

Select * into studentcopy from student where 1=2

Here, we are copying student table to another table with the same structure with no rows copied.

46. How to fetch common records from two tables?

Common records result set can be achieved by -.

Select studentID from student. <strong>INTERSECT </strong> Select StudentID from Exam

47. How to fetch alternate records from a table?

Records can be fetched for both Odd and Even row numbers -.

To display even numbers-.

Select studentId from (Select rowno, studentId from student) where mod(rowno,2)=0

To display odd numbers-.

Select studentId from (Select rowno, studentId from student) where mod(rowno,2)=1

from (Select rowno, studentId from student) where mod(rowno,2)=1.[/sql]

48. How to select unique records from a table?

Select unique records from a table by using DISTINCT keyword.

Select DISTINCT StudentID, StudentName from Student.

49. What is the command used to fetch first 5 characters of the string?

There are many ways to fetch first 5 characters of the string -.

Select SUBSTRING(StudentName,1,5) as studentname from student

Select RIGHT(Studentname,5) as studentname from student

50. Which operator is used in query for pattern matching?

LIKE operator is used for pattern matching, and it can be used as -.

1. % - Matches zero or more characters.

2. _(Underscore) – Matching exactly one character.

Example -.

Select * from Student where studentname like 'a%'

Select * from Student where studentname like 'ami_'

1. What is a Database?
A database is a collection of information in an organized form for faster and better access, storage and manipulation. It can also be defined as a collection of tables, schema, views and other database objects.

2. What is Database Testing?
It is AKA back-end testing or data testing.
Database testing involves in verifying the integrity of data in the front end with the data present in the back end. It validates the schema, database tables, columns, indexes, stored procedures, triggers, data duplication, orphan records, junk records. It involves in updating records in a database and verifying the same on the front end.

3. What is the difference between GUI Testing and Database Testing?

  • GUI Testing is AKA User Interface Testing or Front-end testing
    Database Testing is AKA back-end testing or data testing.
  • GUI Testing deals with all the testable items that are open to the user to interaction such as Menus, Forms etc.
    Database Testing deals with all the testable items that are generally hidden from the user.
  • The tester who is performing GUI Testing doesn’t need to know Structured Query Language
    The tester who is performing Database Testing needs to know Structured Query Language
  • GUI Testing includes invalidating the text boxes, check boxes, buttons, drop-downs, forms etc., majorly the look and feel of the overall application
    Database Testing involves in verifying the integrity of data in the front end with the data present in the back end. It validates the schema, database tables, columns, indexes, stored procedures, triggers, data duplication, orphan records, junk records. It involves in updating records in a database and verifying the same on the front end.

4. What is a Table in a Database?
A table is a database object used to store records in a field in the form of columns and rows that holds data.

5. What is a Field in a Database?
A field in a Database table is a space allocated to store a particular record within a table.

6. What is a Record in a Database?
A record (also called a row of data) is an ordered set of related data in a table.

7. What is a column in a Table?
A column is a vertical entity in a table that contains all information associated with a specific field in a table.

8. What is DBMS?
Database Management System is a collection of programs that enables a user to store, retrieve, update and delete information from a database.

9. What is RDBMS?
RDBMS stands for Relational Database Management System. RDBMS is a database management system (DBMS) that is based on the relational model. Data from relational database can be accessed using Structured Query Language (SQL)

10. What are the popular Database Management Systems in the IT Industry?
Oracle, MySQL, Microsoft SQL Server, PostgreSQL, Sybase, MongoDB, DB2, and Microsoft Access etc.,

11. What is SQL?
SQL Overview: SQL stands for Structured Query Language. It is an American National Standard Institute (ANSI) standard. It is a standard language for accessing and manipulating databases. Using SQL, some of the action we could do are to create databases, tables, stored procedures (SP’s), execute queries, retrieve, insert, update, delete data against a database.

12. What are the different types of SQL commands?
SQL commands are segregated into following types:

  • DDL – Data Definition Language
  • DML – Data Manipulation Language
  • DQL – Data Query Language
  • DCL – Data Control Language
  • TCL – Transaction Control Language

13. What are the different DDL commands in SQL?
DDL commands are used to define or alter the structure of the database.

  • CREATE: To create databases and database objects
  • ALTER: To alter existing database objects
  • DROP: To drop databases and databases objects
  • TRUNCATE: To remove all records from a table but not its database structure
  • RENAME: To rename database objects

14. What are the different DML commands in SQL?
DML commands are used for managing data present in the database.

  • SELECT: To select specific data from a database
  • INSERT: To insert new records into a table
  • UPDATE: To update existing records
  • DELETE: To delete existing records from a table

15. What are the different DCL commands in SQL?
DCL commands are used to create roles, grant permission and control access to the database objects.

  • GRANT: To provide user access
  • DENY: To deny permissions to users
  • REVOKE: To remove user access

16. What are the different TCL commands in SQL?
TCL commands are used to manage the changes made by DML statements.

  • COMMIT: To write and store the changes to the database
  • ROLLBACK: To restore the database since the last commit

17. What is an Index?
An index is used to speed up the performance of queries. It makes faster retrieval of data from the table. The index can be created on one column or a group of columns.

18. What is a View?
A view is like a subset of a table which is stored logically in a database. A view is a virtual table. It contains rows and columns similar to a real table. The fields in the view are fields from one or more real tables. Views do not contain data of their own. They are used to restrict access to the database or to hide data complexity.

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CREATE VIEW view_name AS SELECT column_name1, column_name2 FROM table_name WHERE CONDITION;

19. What are the advantages of Views?
Some of the advantages of Views are

1. Views occupy no space

2. Views are used to simply retrieve the results of complicated queries that need to be executed often.

3. Views are used to restrict access to the database or to hide data complexity.

20. What is a Subquery ?
A Subquery is a SQL query within another query. It is a subset of a Select statement whose return values are used in filtering the conditions of the main query.

21. What is a temp table?
Ans. A temp table is a temporary storage structure to store the data temporarily.

22. How to avoid duplicate records in a query?
The SQL SELECT DISTINCT query is used to return only unique values. It eliminates all the duplicated values.

23. What is the difference between Rename and Alias?
‘Rename’ is a permanent name given to a table or column
‘Alias’ is a temporary name given to a table or column.

24. What is a Join?
Join is a query, which retrieves related columns or rows from multiple tables.

25. What are the different types of joins?
Types of Joins are as follows:

  • INNER JOIN
  • LEFT JOIN
  • RIGHT JOIN
  • OUTER JOIN

26. What is the difference between an inner and outer join?
An inner join returns rows when there is at least some matching data between two (or more) tables that are being compared.
An outer join returns rows from both tables that include the records that are unmatched from one or both the tables.

27. What are SQL constraints?
SQL constraints are the set of rules that enforced some restriction while inserting, deleting or updating of data in the databases.

28. What are the constraints available in SQL?
Some of the constraints in SQL are – Primary Key, Foreign Key, Unique Key, SQL Not Null, Default, Check and Index constraint.

29. What is a Unique constraint?
A unique constraint is used to ensure that there are no duplication values in the field/column.

30. What is a Primary Key?
A PRIMARY KEY constraint uniquely identifies each record in a database table. All columns participating in a primary key constraint must not contain NULL values.

31. Can a table contain multiple PRIMARY KEY’s?

The short answer is no, a table is not allowed to contain multiple primary keys but it allows to have one composite primary key consisting of two or more columns.

32. What is a Composite PRIMARY KEY?
Composite PRIMARY KEY is a primary key created on more than one column (combination of multiple fields) in a table.

33. What is a FOREIGN KEY?
A FOREIGN KEY is a key used to link two tables together. AFOREIGN KEY in a table is linked with the PRIMARY KEY of another table.

34. Can a table contain multiple FOREIGN KEY’s?
A table can have many FOREIGN KEY’s.

35. What is the difference between UNIQUE and PRIMARY KEY constraints?
There should be only one PRIMARY KEY in a table whereas there can be any number of UNIQUE Keys.
PRIMARY KEY doesn’t allow NULL values whereas Unique key allows NULL values.

36. What is a NULL value?
A field with a NULL value is a field with no value. A NULL value is different from a zero value or a field that contains spaces. A field with a NULL value is one that has been left blank during record creation. Assume, there is a field in a table is optional and it is possible to insert a record without adding a value to the optional field then the field will be saved with a NULL value.

37. What is the difference between NULL value, Zero, and Blank space?
As I mentioned earlier, Null value is field with no value which is different from zero value and blank space.
Null value is a field with no value.
Zero is a number
Blank space is the value we provide. The ASCII value of space is CHAR(32).

38. How to Test for NULL Values?
A field with a NULL value is a field with no value. NULL value cannot be compared with other NULL values. Hence, It is not possible to test for NULL values with comparison operators, such as =, <, or <>. For this, we have to use the IS NULL and IS NOT NULL operators.

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SELECT column_names FROM table_name WHERE column_name IS NULL;

1

SELECT column_names FROM table_name WHERE column_name IS NOT NULL;

39. What is SQL NOT NULL constraint?
NOT NULL constraint is used to ensure that the value in the filed cannot be a NULL

40. What is a CHECK constraint?
A CHECK constraint is used to limit the value that is accepted by one or more columns.

E.g. ‘Age’ field should contain only the value greater than 18.

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CREATE TABLE EMP_DETAILS(EmpID int NOT NULL, NAME VARCHAR (30) NOT NULL, Age INT CHECK (AGE &gt; 18), PRIMARY KEY (EmpID));

41. What is a DEFAULT constraint?
DEFAULT constraint is used to include a default value in a column when no value is supplied at the time of inserting a record.

42. What is Normalization?
Normalization is the process of table design to minimize the data redundancy. There are different types of Noramalization forms in SQL.

  • First Normal Form
  • Second Normal Form
  • Third Normal Form
  • Boyce and Codd Normal Form

43. What is Stored procedure?
A Stored Procedure is a collection of SQL statements that have been created and stored in the database to perform a particular task. The stored procedure accepts input parameters and processes them and returns a single value such as a number or text value or a result set (set of rows).

44. What is a Trigger?
A Trigger is a SQL procedure that initiates an action in response to an event (Insert, Delete or Update) occurs. When a new Employee is added to an Employee_Details table, new records will be created in the relevant tables such as Employee_Payroll, Employee_Time_Sheet etc.,

45. Explain SQL Data Types?
In SQL Server, each column in a database table has a name and a data type. We need to decide what type of data to store inside each and every column of a table while creating a SQL table.

46. What are the possible values that can be stored in a BOOLEAN data field?
TRUE and FALSE

47. What is the largest value that can be stored in a BYTE data field?
The largest number that can be represented in a single byte is 11111111 or 255. The number of possible values is 256 (i.e. 255 (the largest possible value) plus 1 (zero), or 28).

48. What are Operators available in SQL?
SQL Operator is a reserved word used primarily in an SQL statement’s WHERE clause to perform operations, such as arithmetic operations and comparisons. These are used to specify conditions in an SQL statement.

There are three types of Operators.

1. Arithmetic Operators

2. Comparison Operators

3. Logical Operators

49. Which TCP/IP port does SQL Server run?
By default it is 1433

50. List out the ACID properties and explain?
Following are the four properties of ACID. These guarantees that the database transactions are processed reliably.

  • Atomicity
  • Consistency
  • Isolation
  • Durability

51. Define the SELECT INTO statement.
The SELECT INTO statement copies data from one table into a new table. The new table will be created with the column-names and types as defined in the old table. You can create new column names using the AS clause.

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SELECT * INTO newtable FROM oldtable WHERE condition;

52. What is the difference between Delete, Truncate and Drop command?
The difference between the Delete, Truncate and Drop command is

  • Delete command is a DML command, it is used to delete rows from a table. It can be rolled back.
  • Truncate is a DDL command, it is used to delete all the rows from the table and free the space containing the table. It cant be rolled back.
  • Drop is a DDL command, it removes the complete data along with the table structure(unlike truncate command that removes only the rows). All the tables’ rows, indexes, and privileges will also be removed.

53. What is the difference between Delete and Truncate?
The difference between the Delete, and Truncate are

DELETE

TRUNCATE

Delete statement is used to delete rows from a table. It can be rolled back.

Truncate statement is used to delete all the rows from the table and free the space containing the table. It cant be rolled back.

We can use WHERE condition in DELETE statement and can delete required rows

We cant use WHERE condition in TRUNCATE statement. So we cant delete required rows alone

We can delete specific rows using DELETE

We can only delete all the rows at a time using TRUNCATE

Delete is a DML command

Truncate is a DDL command

Delete maintains log and performance is slower than Truncate

Truncate maintains minimal log and performance wise faster

We need DELETE permission on Table to use DELETE command

We need at least ALTER permission on the table to use TRUNCATE command

54. What is the difference between Union and Union All command?
This is one of the tricky SQL Interview Questions. Interviewer may ask you this question in another way as what are the advantages of Union All over Union.

Both Union and Union All concatenate the result of two tables but the way these two queries handle duplicates are different.

Union: It omits duplicate records and returns only distinct result set of two or more select statements.
Union All: It returns all the rows including duplicates in the result set of different select statements.

Performance wise Union All is faster than Union, Since Union All doesn’t remove duplicates. Union query checks the duplicate values which consumes some time to remove the duplicate records.

Assume: Table1 has 10 records, Table2 has 10 records. Last record from both the tables are same.

If you run Union query.

1

2

3

SELECT * FROM Table1

UNION

SELECT * FROM Table2

Output: Total 19 records

If you run Union query.

1

2

3

SELECT * FROM Table1

UNION ALL

SELECT * FROM Table2

Output: Total 20 records

Data type of all the columns in the two tables should be same.

55. What is the difference between Having and Where clause?
Where clause is used to fetch data from a database that specifies particular criteria whereas a Having clause is used along with ‘GROUP BY’ to fetch data that meets particular criteria specified by the Aggregate functions. Where clause cannot be used with Aggregate functions, but the Having clause can.

56. What are aggregate functions in SQL?
SQL aggregate functions return a single value, calculated from values in a column. Some of the aggregate functions in SQL are as follows

  • AVG() – This function returns the average value
  • COUNT() – This function returns the number of rows
  • MAX() – This function returns the largest value
  • MIN() – This function returns the smallest value
  • ROUND() – This function rounds a numeric field to the number of decimals specified
  • SUM() – This function returns the sum

57. What are string functions in SQL?
SQL string functions are used primarily for string manipulation. Some of the widely used SQL string functions are

  • LEN() – It returns the length of the value in a text field
  • LOWER() – It converts character data to lower case
  • UPPER() – It converts character data to upper case
  • SUBSTRING() – It extracts characters from a text field
  • LTRIM() – It is to remove all whitespace from the beginning of the string
  • RTRIM() – It is to remove all whitespace at the end of the string
  • CONCAT() – Concatenate function combines multiple character strings together
  • REPLACE() – To update the content of a string.

Practical SQL Interview Questions:

58. How to add new Employee details in an Employee_Details table with the following details
Employee_Name: John, Salary: 5500, Age: 29?

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INSERT into Employee_Details (Employee_Name, Salary, Age) VALUES (‘John’, 5500 , 29);

59. How to add a column ‘Salary’ to a table Employee_Details?

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ALTER TABLE Employee_Details ADD (Salary);

60. How to change a value of the field ‘Salary’ as 7500 for an Employee_Name ‘John’ in a table Employee_Details?

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UPDATE Employee_Details set Salary = 7500 where Employee_Name = ‘John’;

61. Write an SQL Query to select all records from the table?

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Select * from table_name;

62. How To Get List of All Tables From A DataBase?
To view the tables available on a particular DataBase

1

2

3

4

USE TestDB

GO

SELECT * FROM sys.Tables

GO

63. Define SQL Delete statement.
The SQL Delete statement is used to delete records from a table.

1

DELETE FROM table_name WHERE some_column=some_value;

64. Write the command to remove all Players named Sachin from the Players table.

1

DELETE from Players WHERE Player_Name = ‘Sachin’

65. How to fetch values from TestTable1 that are not in TestTable2 without using NOT keyword?

1

2

3

4

5

6

7

8

--------------

| TestTable1 |

--------------

| 11 |

| 12 |

| 13 |

| 14 |

--------------

1

2

3

4

5

6

--------------

| TestTable2 |

--------------

| 11 |

| 12 |

--------------

By using the except keyword

1

SELECT * FROM TestTable1 EXCEPT SELECT * FROM TestTable2;

66. How to get each name only once from an employee table?
By using the DISTINCT keyword, we could get each name only once.

1

SELECT DISTINCT employee_name FROM employee_table;

67. How to rename a column in the output of SQL query?
By using SQL AS keyword

1

SELECT column_name AS new_name FROM table_name;

68. What is the order of SQL SELECT?
Order of SQL SELECT statement is as follows

SELECT, FROM, WHERE, GROUP BY, HAVING, ORDER BY.

69. How to display the current date in SQL.
By using query

1

SELECT GetDate();

70. Write an SQL Query to find an Employee_Name whose Salary is equal or greater than 5000 from the below table Employee_Details.

1

2

3

4

5

6

7

| Employee_Name | Salary|

-----------------------------

| John | 2500 |

| Emma | 3500 |

| Mark | 5500 |

| Anne | 6500 |

-----------------------------

Syntax:

1

SELECT Employee_Name FROM Employee_Details WHERE Salary>=5000;

Output:

1

2

3

4

5

| Employee_Name | Salary|

-----------------------------

| Mark | 5500 |

| Anne | 6500 |

-----------------------------

71. Write an SQL Query to find list of Employee_Name start with ‘E’ from the below table

1

2

3

4

5

6

7

| Employee_Name | Salary|

-----------------------------

| John | 2500 |

| Emma | 3500 |

| Mark | 5500 |

| Anne | 6500 |

-----------------------------

Syntax:

1

SELECT * FROM Employee_Details WHERE Employee_Name like 'E%';

Output:

1

2

3

4

| Employee_Name | Salary|

-----------------------------

| Emma | 3500 |

-----------------------------

72. Write SQL SELECT query that returns the FirstName and LastName from Employee_Details table.

1

SELECT FirstName, LastName FROM Employee_Details;

73. How to rename a Table?

1

SP_RENAME TABLE 'SCOREBOARD', 'OVERALLSCORE'

To rename Table Name & Column Name

1

2

sp_rename OldTableName,NewTableName

sp_rename 'TableName.OldColumnName', 'NewColumnName'

74. How to select all the even number records from a table?
To select all the even number records from a table:

1

2

Select * from table where id % 2 = 0

75. How to select all the odd number records from a table?
To select all the odd number records from a table:

1

Select * from table where id % 2 != 0

76. What is the SQL CASE statement?
SQL Case statement allows embedding an if-else like clause in the SELECT statement.

77. Can you display the result from the below table TestTable based on the criteria M,m as M and F, f asF and Null as N and g, k, I as U

1

SELECT Gender from TestTable

1

2

3

4

5

6

7

8

9

10

11

| Gender |

------------

| M |

| F |

| NULL |

| m |

| f |

| g |

| H |

| i |

------------

By using the below syntax we could achieve the output as required.

1

2

3

4

5

6

7

8

SELECT Gender,

case

when Gender='i' then 'U'

when Gender='g' then 'U'

when Gender='H' then 'U'

when Gender='NULL' then 'N'

else upper(Gender)

end as newgender from TestTable GROUP BY Gender

78. What will be the result of the query below?

1

select case when null = null then 'True' else 'False' end as Result;

This query returns “False”. In the above question, we could see null = null is not the proper way to compare a null value. To compare a value with null, we use IS operator in SQL.

So the correct way is as follows

1

select case when null is null then 'True' else 'False' end as Result;

79. What will be the result of the query below?

1

select case when null is null then 'Queries In SQL Server' else 'Queries In MySQL' end as Result;

This query will returns “Queries In SQL Server”.

80. How do you update F as M and M as F from the below table TestTable?

1

2

3

4

5

6

7

| Name | Gender |

------------------------

| John | M |

| Emma | F |

| Mark | M |

| Anne | F |

------------------------

By using the below syntax we could achieve the output as required.

1

UPDATE TestTable SET Gender = CASE Gender WHEN 'F' THEN 'M' ELSE 'F' END

81. Describe SQL comments?
Single Line Comments: Single line comments start with two consecutive hyphens (–) and ended by the end of the line
Multi-Line Comments: Multi-line comments start with /* and end with */. Any text between /* and */ will be ignored.

82. How to get unique records from a table?
By using DISTINCT keyword.

1

SELECT DISTINCT Col1, Col2 from Table1

83. What is the difference between NVL function, IFNULL function, and ISNULL function?
These three functions work in the same way. These functions are used to replace NULL value with another value. Oracle developers use NVL function, MySQL developers use IFNULL function and SQL Server developers use ISNULL function.
Assume, some of the values in a column are NULL.
If you run below statement, you will get result as NULL

1

SELECT col1 * (col2 + col3) FROM Table1

Suppose any of the value in col3 is NULL then as I said your result will be NULL.

To overcome this we use NVL() function, IFNULL() function, ISNULL() Function.

ORACLE:

1

SELECT col1 * (col2 + NVL(col3,0)) FROM Table1

MySQL:

1

SELECT col1 * (col2 + IFNULL(col3,0)) FROM Table1

Also, you can use the COALESCE() function

1

SELECT col1 * (col2 + COALESCE(col3,0)) FROM Table1

SQL Server:

1

SELECT col1 * (col2 + ISNULL(col3,0)) FROM Table1

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