SQL or Structured Query Language is one of the most popular terms in the Database Management System. And with Data becoming the modern age fuel, SQL remains a central part of Data Analytics. From an organization’s perspective, SQL professionals are in demand from small, medium, or large, corporate and MNCs. The interview and selection process varies from one organization to another.
However, the SQL fundamental remains the same. Mainly, the following SQL interview questions for freshers can help them be ready for their careers ahead. We have also put together a dedicated section at the end – SQL interview questions for experienced professionals. You will also find some tricky questions on SQL server interview questions for practical learning.Â
Here is a list of top SQL interview questions for freshers and experienced professionals that one should know about to succeed in 2021.Â
A Database is a collection of organized information or data in a digital form. It is administered by the Database Manage System (DMBS) to control and manage the information. This makes it easy for the information to be accessible, modified, updated, or deleted. SQL is one of the most widely used database languages for managing information and querying data.Â
As two of the essential parts of the indexes in SQL, there are several differences between the clustered and non-clustered indexes.Â
The TRUNCATE command in SQL, once executed, can remove all rows from a given table. And on completion, the process cannot be rolled back.Â
Whereas the DROP command in SQL removes the table entirely from a given database, and the operation cannot be rolled back either.
The main difference in the results of Natural Join and Cross Join. Natural Join uses all columns by the same data types and name from both the given tables, while Cross Join gets the Cross product or Cartesian product from two tables.Â
BETWEEN operator fetches the rows from a given range of rows while IN operator plays a crucial role in checking the specific values in a given row. Here is a short code example of the two.
Example of IN Operator:
SELECT * FROM students where ROLL_NO IN (7,13,23);
Example of BETWEEN Operator:
SELECT * FROM Students where ROLL_NO BETWEEN 20 AND 58;
Both WHERE and HAVING clauses are used for restricting the return of rows in a given SELECT query.Â
WHERE clause is applied to filter rows before the grouping process is done, while the HAVING clause takes place after the grouping process takes place.Â
A NULL value has a specific role with major differences from zero and blank space. The NULL value represents a value as unassigned, unknown, unavailable, or not applicable. While Zero represents a particular number, and a blank space is valued as a character only.
There are two ways you deal with NULL values in a column.Â
SQL queries that include SET operations are known as Compound Queries. Union, Intersect, and Minus operators are the example of SET operators in SQL.Â
You can create a new empty table from an existing table by using the following query:
Select * into peoplecopy from people where 1=2
Here, you can copy people’s tables from an existing table using the same structure that’s not using or copying any rows.Â
There are two authentication modes with Windows Mode and Mixed Mode. Here are the steps to make a change in the authentication of the SQL Server:
In SQL, we have a built-in function with a particular as GetDate() that on execution fetches the current date/timestamp, respectively.Â
Here, you can use the SELECT command to get all records from SQL.
SELECT * FROM [Customers]
Number of total records = 4
In SQL, LIKE Operator is the main operator for pattern making using % and _ characters.Â
% (Percentage – that matches zero or another character)
_ (Underscore that matches single or particular character)
Example includes:
Select * from People where peoplename like ‘a%’
Select * from People where peoplename like ‘mit_’
SQL scenario-based interview questions
Unique records in a table can help you find the values that exist only once in a particular table.Â
Select DISTINCT peopleID from People
Here, you can see that DISTINCT commands help the user find unique values or IDs from the table People.Â
Alternate records are both even and odd row numbers present in the table. You can use the following query or command to get the specific alternate records from the database, respectively:Â
Select peopleId from (Select rowno, peopleId from people) where mod(rowno,2)=1
Select peopleId from (Select rowno, peopleId from student) where mod(rowno,2)=0
There are many ways to count the given number of records for a particular table in SQL. Here are the commands:
SELECT * FROM table2
SELECT COUNT(*) FROM table2
SELECT rows FROM sysindexes WHERE id = OBJECT_ID(table2) AND indid < 3
Most often, we have to search for specific people from a long list of employees. You can use the following query to get the right result:
SELECT * FROM Table_name WHERE EmpName like ‘B%’
Here are the two ways to get the desired first 4 characters of the given string:
Select SUBSTRING(PeopleName,1,4) as peoplename from people
Select LEFT(PeopleName,5) as peoplename from people
Here is a SQL query to create SQL table:
create table employee(empid char(4), name varchar(10), salary float(15), department.field varchar(15));
+—————–+————–+——+—–+———+——-+
| Field                | Type        | Null | Key | Default | Extra |
| DEPARTMENT_ID Â Â Â Â | decimal(5,0) | NO Â | PRIÂ | 0 Â Â Â Â Â | Â Â Â |
| DEPARTMENT_NAME | varchar(25) Â | NO Â | Â Â Â Â | NULLÂ Â | Â Â Â |
| MANAGER_ID Â Â Â Â Â Â Â | decimal(4,0) | NO Â | PRIÂ | 0 Â Â Â Â Â | Â Â Â |
| LOCATION_IDÂ Â Â Â Â Â Â Â | decimal(3,0) | YESÂ | Â Â Â Â | NULLÂ Â | Â Â Â |
Now create a table with the following things:
– employee_id, first_name, last_name, email, phone_number hire_date, job_id, salary, commission, manager_id and department_id
– No duplicates exist in employee_id
– Department and Manager ID follow key insertion with unique combinations.
Here is a SQL query to fetch the above table:
CREATE TABLE IF NOT EXISTS employees (Â
EMPLOYEE_ID decimal(5,0) NOT NULL PRIMARY KEY,Â
FIRST_NAME varchar(15) DEFAULT NULL,Â
LAST_NAME varchar(20) NOT NULL,Â
EMAIL varchar(30) NOT NULL,Â
PHONE_NUMBER varchar(30) DEFAULT NULL,Â
HIRE_DATE date NOT NULL,Â
JOB_ID varchar(15) NOT NULL,Â
SALARY decimal(10,4) DEFAULT NULL,Â
COMMISSION_PCT decimal(2,2) DEFAULT NULL,Â
MANAGER_ID decimal(4,0) DEFAULT NULL,Â
DEPARTMENT_ID decimal(5,0) DEFAULT NULL,Â
FOREIGN KEY(DEPARTMENT_ID,MANAGER_ID)Â
REFERENCESÂ departments(DEPARTMENT_ID,MANAGER_ID)
)ENGINE=InnoDB;
These SQL important interview questions provide in-depth learning of this language. As an individual, you must understand that both theory and practical learning go hand-in-hand. This list starts with basic SQL interview questions, then goes on to advanced SQL interview questions, covering preparation for all levels of professionals. Â
SQL remains one of the essential parts of modern-day Data Science, and these questions are paramount in your preparations for SQL interview questions for experienced professionals for securing top positions in MNCs.
You should read this list of SQL interview questions for testers, developers, data analyst professionals, software engineers, software architects, cloud-native professionals, and database experts of all kinds.Â
Jigsaw Academy is one of the top online learning platforms for modern professionals to prepare for their careers smartly. Interested applicants can apply for their Integrated Program in Business Analytics and enhance their career opportunities. This course is also available online and certified under the collaboration of the Indian Institute of Management, Indore, with Jigsaw Academy.Â
Database Management System (DBMS) presents a method for building, managing, and maintaining databases, so that information or data is saved in tables (rows and columns) inside the database. DBMS acts as an interface for giving database access to the user. Overall, DBMS allows the swift movement of data access and data retrieval easier.Â
Relational Database Management System (RDBMS) also uses the same tables and common fields to save data. But in addition, it provides relational operators to make changes and get desired results.Â
Structured Query Language (SQL) is an industry-standard query language for managing data inside relational databases. You can create, modify, update, and delete databases using SQL queries.Â
Although the initial version of SQL was developed by Donald D. Chamberlin and Raymond F. Boyce at IBM based on Edgar F. Codd models. It became the ISO standards (International for Standardization) in 1986 and American National Standards Institute (ANSI) subsequently in 1987. Since then, newer SQL versions have been coming in, the most recent one in 2016. Â
The following are the main uses of SQL in general:
In SQL, information is procured or saved in tables and fields by rows and columns. In the database, a table has a limited number of columns but can have any number of rows. A column represents the vertical positions, and rows represent the horizontal data put in to match with the column value.Â
Keys present a specific set of attributes that define the relations between a given row and table. Thus, it allows users to build a relation between two tables. Keys provide a way to combine values from one table to another and get more desired results. Generally, there are eight types of keys present in DBMS based on different attributes. These are Super Key, Primary Key, Candidate Key, Alternate Key, Foreign Key, Composite Key, Compound Key, and Surrogate Key.
In SQL, a Primary Key defines a collection or combination of fields that come from unique rows. Primary Keys present a special case scenario for a Unique Key with a NOT NULL constraint, i.e., values inside the Primary Keys can’t be NULL.
There are three different subsets present in SQL; they are defined as:
SQL Joins Interview Questions
SQL Join provides a way to connect information from two or more tables. So, users can merge or retrieve data from multiple table fields to get preferred results. In Join, the use of keys plays a crucial role in fetching precise information from the database.Â
There are four types of SQL Join in general.Â
Joins offer a way to get information from two tables and bring collective results for further processing. There are different types and combinations of Joins that you can use to get any desired result.Â
In SQL, there is no specific faster Join. But in some cases, one Join may perform and fetch results quicker than another Join. And Joins will always fetch results better than subquery in general.Â
Self Join is a unique scenario and offers a highly useful clause that allows a table to compare with itself. So, you can compare one value of a column to the second column within a table. Self Joins are used to change a hierarchical structure into a flat structure.Â
In SQL, Views represent a virtual table showing subsets of data present in the table. As Views don’t take a particular position, they take even less space to store. A View can have values or data from one or two tables based on a specific relationship. Views represent a searchable object and are easily findable by using a query to search.Â
An Index is a performance-enhancing method that allows quick fetching of information from the database or a table. The Index has a unique value that cannot be duplicated. So, these Index values are easier to fetch.Â
There are three types of Indexes in general.
A Query represents a set of instructions written in a way to get specific results. You can use single or multiple queries to get precise information from the database in a table.Â
Entities represent a physical person, object, or a particular thing that can be seamlessly stored in an electronic database. For instance, a database with a list of customers and their specific information for age, salary, duration, etc.Â
Relationships/Relations define the links between entities that are connected. For instance, a customer name has a unique identity as customer ID, contact information, etc.Â
A SubQuery represents a query defined inside the main query. The outer layer is termed as the main query, while the inner query is termed as SubQuery. In SQL, SubQuery is executed first, and the result is then carried forward to the original query. Â
There are two general types of SubQueries defined in SQL:
A database trigger is a specific program or code that executes an automatic response to a particular event in a view or a table. These triggers play an essential role in maintaining database integrity. When a new customer record is added to a given database, simultaneously related columns, such as the ID, name, phone number, notes, etc., are created.Â
Constraints in SQL define the specific rules and regulations for limiting the data type on a column inside the table. Constraints help you add relevant data to the table and stop users from adding irrelevant information to the database. There are two broad levels of Constraints in SQL that are represented as:Â
Some of these Constraints are UNIQUE, Primary Key, Foreign Key, Default, Not NULL, CHECK, etc.Â
Data Integrity presents a method or way to keep the data accurate and consistent inside the database. You must define the integrity constraints to get business data accurate while submitting data or information in the database.Â
Local variables are the ones that exist or are used within the functions only. These local variables don’t work or can’t be called in other functions, as they are not known. However, variables work when that specific function is called upon.
Global variables are the ones that exist or can be used across the program. A global variable, once created, can never be used in any function.
Auto Increment field defines a unique number automatically generated whenever a new record is inserted in the database or the table. Most often, this represents a Primary Key field that is created when each new record is inserted in the table.
Data Warehouse is one large set of information or data and represents a central repository that collects data from all sources of information of any kind. In Data Warehouse, data is consolidated, manipulated, transformed, and processed further for processing and mining. Here, you may find data subsets termed as Data Marts. Â
User-defined functions represent the functions that allow specific logic to be used whenever and wherever required. So, you don’t need to write that same logic again and again.
There are three types of user-defined functions in SQL.
A Collation in SQL represents the specific set of rules and regulations for sorting and comparing character data. For instance, character width, case sensitivity, etc.
The four types of Collation sensitivity are:
Stored procedures are the collection of functions containing multiple SQL statements for accessing data in a specific form. SQL has several stored procedures that you can write once and store to run them whenever required repeatedly.
Stored procedure advantage lies in its modular nature of programming, i.e., you can create these once, save, and call them as per your preference. This results in quicker execution, lowering network traffic, and keeping a check on the security aspects.
One of the main disadvantages of stored procedures is they run or execute in the database by consuming a lot of memory in the database server.
SQL CLAUSE provides a way to limit the query by adding conditions and getting more specific results. Thus, acting as a filter to the whole result. For instance, WHERE, HAVING, etc.
Normalization is a process of lowering dependencies and redundancies by organizing tables and fields in a database. Here, the Normalization is done with the main focus to add, modify, or delete fields to collect them into one table.
Denormalization is a process for accessing data from higher to lower forms in a database. Denormalization introduces redundancy in the table by adding values from the related tables in the database.
Normalization comes in multiple types, termed normal forms, with each having a dependency on the other.Â
A cursor provides a way to control the return on rows in SQL statements, such as updating records in a particular row in singleton. A cursor is quite useful for traversing functions, for instance, addition, removal, and retrieval of information or values from the database.
ACID stands for Atomicity, Consistency, Isolation, and Durability. Every database has ACID properties to make it more reliable. Here each logical operation within data is termed a transaction.
Online Transaction Processing (OLTP) is responsible for all transactions being carried out with respect to entering data, retrieving data, and then processing data. Overall, OLTP keeps the data procedure efficient and easy to manage. For instance, financial transactions in a banking institution.
ALIAS command is used to give a secondary name to identify any column or the table. The below code refers to Ex as Alias name for exam table and st as student table, respectively.Â
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