SQL is used in a wide variety of programming jobs. It's important to be familiar with SQL if you are going to be interviewing soon for a software position. This is especially true if you are going to interview at a top tech company such as Amazon, Apple, or Google.
This guide will cover basic SQL syntax as a refresher and then list some common SQL interview questions. The answers for all questions are given and you can use this information to study for your programming interview.
Basic SQL Syntax Example
SQL is an international standard (ISO), but you will find some differences between implementations. This guide uses MySQL as an example because it's the most popular implementation of SQL.
How to use a specific database
Here is the SQL command used to select the database containing the tables for your SQL statements:
SELECT and FROM clauses
Use SELECT to determine which columns of the data you want to show in the results. There are also options you can use to show data that is not a table column.
The following example shows two columns selected from the “student” table, and two calculated columns. The first of the calculated columns is a meaningless number, and the other is the system date.
SELECT studentID, FullName, 3+2 AS five, now() AS currentDate FROM student;
The WHERE clause specifies a condition while getting data. The WHERE clause is used to limit the number of rows returned. It's often used in a SELECT statement but can also be used in other statements such as UPDATE and DELETE.
Here is the basic syntax of the WHERE clause:
SELECT column1, column2 FROM table_name WHERE [condition]
The condition in a WHERE clause can include logical operators like >, <, =, LIKE, NOT, AND, OR.
Here is an example of a SQL statment using the WHERE clause. It specifies that if any of the students have certain SAT scores (1000, 1400), they will not be presented:
SELECT studentID, FullName, sat_score, recordUpdated FROM student WHERE (studentID BETWEEN 1 AND 5 OR studentID = 8 OR FullName LIKE '%Maximo%') AND sat_score NOT IN (1000, 1400);
ORDER BY (ASC, DESC)
ORDER BY gives us a way to sort the result set by one or more of the items in the SELECT section.
Here is the same list as above, but sorted by the student's Full Name. The default sort order is ascending (ASC), but to sort in the opposite order (descending) you use DESC, as in the example below:
SELECT studentID, FullName, sat_score FROM student WHERE (studentID BETWEEN 1 AND 5 OR studentID = 8 OR FullName LIKE '%Maximo%') AND sat_score NOT IN (1000, 1400) ORDER BY FullName DESC;
GROUP BY and HAVING
GROUP BY gives us a way to combine rows and aggregate data. The HAVING clause is like the above WHERE clause, except that it acts on the grouped data.
The SQL statement below answers the question: “Which candidates received the largest number of contributions (ordered by count (*)) in 2016, but only those who had more than 80 contributions?”
Ordering this data set in a descending (DESC) order places the candidates with the largest number of contributions at the top of the list.
SELECT Candidate, Election_year, SUM(Total_$), COUNT(*) FROM combined_party_data WHERE Election_year = 2016 GROUP BY Candidate, Election_year HAVING count(*) > 80 ORDER BY count(*) DESC;
Common SQL Interview Questions
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 or full outer join in SQL?
A full outer join and a full join are the same thing. The full outer join or full join returns all rows from both tables, matching up the rows wherever a match can be made and placing NULLs in the places where no matching row exists.
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
This will result in an error because you can’t perform a DML operation on a view. A DML operation is any operation that manipulates the data such as DROP, INSERT, UPDATE, and DELETE.
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. DDL statements define data structures such as
CREATE table and
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 behaves like a column, but is not actually stored in the table because it is all generated values. The values of a pseudocolumn can be selected but they cannot be inserted, updated, or deleted.
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
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 privileges on theses 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 previously given privileges. She should still be able to 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 she created are no longer used, write a command to remove this user and all her objects.
The CASCADE option is necessary to remove all the objects of the user in the database.
DROP USER rita CASCADE
Write an SQL query to find the nth highest "Salary" from the "Employee" table.
SELECT TOP 1 Salary FROM ( SELECT DISTINCT TOP N Salary FROM Employee ORDER BY Salary DESC ) ORDER BY Salary ASC
If you think you can answer all these questions, you may be ready for your interview. Good luck!