Common SQL Interview Questions

Common SQL Interview Questions
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What is an inner join in SQL?

This is the default type of join if no join is specified. It returns all rows in which there is at least one match in both tables.

SELECT * FROM A x JOIN B y ON y.aId = x.Id

What is a left join in SQL?

A left join returns all rows from the left table, and the matched rows from the right table. Rows in the left table will be returned even if there was no match in the right table. The rows from the left table with no match in the right table will have null for right table values.

SELECT * FROM A x LEFT JOIN B y ON y.aId = x.Id

What is a right join in SQL?

A right join returns all rows from the right table, and the matched rows from the left table. Opposite of a left join, this will return all rows from the right table even where there is no match in the left table. Rows in the right table that have no match in the left table will have null values for left table columns.

SELECT * FROM A x RIGHT JOIN B y ON y.aId = x.Id

What is a full join in SQL?

A full join returns all rows for which there is a match in either of the tables. So if there are rows in the left table that do not have matches in the right table, those will be included. As well as if there are rows in the right table that do not have matches in the left table, those will be included.

SELECT Customers.CustomerName, Orders.OrderID
FROM Customers
FULL OUTER JOIN Orders
ON Customers.CustomerID=Orders.CustomerID
ORDER BY Customers.CustomerName

What is the result of the following command?

  DROP VIEW view_name

Here it’ll be an error because we can’t perform a DML operation on a view.

Can we perform a rollback after using ALTER command?

No, because ALTER is a DDL command and Oracle server performs an automatic COMMIT when the DDL statements are executed.

Which is the only constraint that enforces rules at column level?

NOT NULL is the only constraint that works at the column level.

What are the pseudocolumns in SQL? Give some examples?

A pseudocolumn is a function which returns a system generated value. The reason it is known as so because a pseudocolumn is an Oracle assigned value used in the same context as an Oracle database column but not stored on disk.

    ROWNUM, ROWID, USER, CURRVAL, NEXTVAL etc.

Create a user my723acct with password kmd26pt. Use the user data and temporary data tablespaces provided by PO8 and provide to this user 10M of storage space in user data and 5M of storage space in temporary_data.

    CREATE USER my723acct IDENTIFIED BY kmd26pt
    DEFAULT TABLESPACE user_data
    TEMPORARY TABLESPACE temporary_data
    QUOTA 10M on user_data QUOTA 5M on temporary_data

Create the role role tables and_views.

    CREATE ROLE role_tables_and_views

Grant to the role of the previous question the privileges to connect to the database and the privileges to create tables and views.

The privilege to connect to the database is CREATE SESSION The privilege to create table is CREATE TABLE The privilege to create view is CREATE VIEW

    GRANT Create session, create table, create view TO role_tables_and_views

Grant the previous role in the question to the users anny and rita

    GRANT role_tables_and_views TO anny, rita

Create a user my723acct with password kmd26pt. Use the user data and temporary data tablespaces provided by PO8 and provide to this user 10M of storage space in user data and 5M of storage space in temporary_data.

    CREATE USER my723acct IDENTIFIED BY kmd26pt
    DEFAULT TABLESPACE user_data
    TEMPORARY TABLESPACE temporary_data
    QUOTA 10M on user_data QUOTA 5M on temporary_data

Create the role role tables and_views.

    CREATE ROLE role_tables_and_views

Grant to the role of the previous question the privileges to connect to the database and the privileges to create tables and views.

The privilege to connect to the database is CREATE SESSION The privilege to create table is CREATE TABLE The privilege to create view is CREATE VIEW

    GRANT Create session, create table, create view TO role_tables_and_views

Grant the previous role in the question to the users anny and rita

    GRANT role_tables_and_views TO anny, rita

Write a command to change the password of the user rita from abcd to dfgh

    ALTER USER rita IDENTIFIED BY dfgh

The users rita and anny do not have SELECT privileges on the table INVENTORY that was created by SCOTT. Write a command to allow SCOTT to grant the users SELECT priviliges on these tables.

    GRANT select ON inventory TO rita, anny

User rita has been transferred and no longer needs the privilege that was granted to her through the role role tables and_views. Write a command to remove her from her previous given priviliges except that she still could connect to the database.

    REVOKE select ON scott.inventory FROM rita
    REVOKE create table, create view FROM rita

The user rita who was transferred is now moving to another company. Since the objects that she created is of no longer use, write a commmand to remove this user and all her objects.

Here CASCADE option is necessary to remove all the objects of the user in the database.

   DROP USER rita CASCADE

### User rita has been transferred and no longer needs the privilege that was granted to her through the role role_tables_and_views. Write a command to remove her from her previous given priviliges except that she still could connect to the database.
``` sql    
    REVOKE select ON scott.inventory FROM rita
    REVOKE create table, create view FROM rita

The user rita who was transferred is now moving to another company. Since the objects that she created is of no longer use, write a commmand to remove this user and all her objects.

Here CASCADE option is necessary to remove all the objects of the user in the database.

   DROP USER rita CASCADE

Write SQL query to find the nth highest salary from table.

   SELECT TOP 1 Salary
   FROM (
      SELECT DISTINCT TOP N Salary
      FROM Employee
      ORDER BY Salary DESC
      )
    ORDER BY Salary ASC