SQL Interview Question Part 1

SQL Interview Question Part 1

Q 1. How do you select all records from the table?

Answer :- Select * from table_name; 

Q 2. What is a join?

Answer :- Join is a process of retrieve pieces of data from different sets (tables) and returns them to the user or program as one “joined” collection of data. 

Q 3. How do you add record to a table?

Answer :- INSERT into table_name VALUES (‘TEST’, 33 , ‘M’); 

Q 4. How do you add a column to a table?

ALTER TABLE Department ADD (AGE, NUMBER); 

Q 5. How do you change value of the field?

Answer :- UPDATE EMP_table set number = 200 where item_munber = ‘CD’;

update name_table set status = 'enable' where phone = '4161112222';

update SERVICE_table set REQUEST_DATE = to_date ('2006-03-04 09:29', 'yyyy-mm-dd hh24:MM') where phone = '4161112222'; 

Q 6. What does COMMIT do?

Answer :- Saving all changes made by DML statements 

Q 7. What is a primary key?

Answer :- The column (columns) that has completely unique data throughout the table is known as the primary key field. 

Q 8. What are foreign keys?

Answer :- Foreign key field is a field that links one table to another table’s primary or foreign key. 

Q 9. What is the main role of a primary key in a table?

Answer :- The main role of a primary key in a data table is to maintain the internal integrity of a data table. 

Q 10. Can a table have more than one foreign key defined?

A table can have any number of foreign keys defined. It can have only one primary key defined. 

Q 11. List all the possible values that can be stored in a BOOLEAN data field.

There are only two values that can be stored in a BOOLEAN data field: -1(true) and 0(false). 

Q 12. What is the highest value that can be stored in a BYTE data field?

Answer :- The highest value that can be stored in a BYTE field is 255. or from -128 to 127. Byte is a set of Bits that represent a single character. Usually there are 8 Bits in a Byte, sometimes more, depending on how the measurement is being made. Each Char requires one byte of memory and can have a value from 0 to 255 (or 0 to 11111111 in binary). 

Q 13. Describe how NULL work in SQL?

The NULL is how SQL handles missing values. Arithmetic operation with NULL in SQL will return a NULL. 

Q 14. What is Normalization?

Answer :- The process of table design is called normalization. 

Q 15. What is Trigger?

Answer :- Trigger will execute a block of procedural code against the database when a table event occurs. A2. A trigger defines a set of actions that are performed in response to an insert, update, or delete operation on a specified table. When such an SQL operation is executed, in this case the trigger has been activated. 

Q 16. Can one select a random collection of rows from a table?

Answer :- Yes. Using SAMPLE clause. Example:

SELECT * FROM EMPLOYEES SAMPLE(10);

10% of rows selected randomly will be returned. 

Q 17. You issue the following query:

SELECT FirstName FROM StaffList WHERE FirstName LIKE '_A%‘

Which names would be returned by this query? Choose all that apply.

Allen

CLARK

JACKSON

David

Answer :- JACKSON

Q 18. Write a SQL SELECT query that only returns each city only once from Students table? Do you need to order this list with an ORDER BY clause?

Answer :- SELECT DISTINCT City FROM Students; 

Q 19. What is DML and DDL?

Answer :- DML and DDL are subsets of SQL.

DML stands for Data Manipulation Language and DDL – Data Definition Language.

DML consist of INSERT, UPDATE and DELETE

DDL commands

CREATE TABLE, ALTER TABLE, DROP TABLE, RENAME TABLE, CREATE INDEX, ALTER INDEX, DROP INDEX.

CREATE/ALTER/DROP VIEW 

Q 20. Write SQL SELECT query that returns the first and last name of each instructor, the Salary, and gives each of them a number.

Answer :- SELECT FirstName, LastName, Salary, ROWNUM FROM Instructors;