Structured Query Language (SQL) is the standard programming language used by database administrators and data analysts to query databases. It is widely used in the programming languages of websites, apps, and other platforms.

It is used to access and manipulate data in a database, including creating and modifying tables and columns and querying the data using SQL commands.

This post will undoubtedly give you some insights if you’re looking for questions and answers to use while you prepare for a SQL interview. In a job interview for a data engineer, data analyst, database administrator, etc., the following questions should be expected.

What are the basic SQL commands?

Some of the most important SQL Commands are:

  • SELECT – extracts data from a database.
  • CREATE TABLE – creates a new table.
  • DELETE – deletes data from a database.
  • INSERT INTO – inserts new data into a database.
  • ALTER DATABASE – modifies a database.
  • CREATE DATABASE – creates a new database.
  • UPDATE – updates data in a database.

What are the four important SQL statements?

Main SQL statements are divided into these categories:

  • Data Definition Language (DDL) Statements
  • Data Manipulation Language (DML) Statements
  • Data Control Language (DCL) Statements
  • Transaction Control Language (TCL) Statements

What is the meaning of the primary key in SQL?

A primary key is a column (or set of columns) that allows each row in a database to be uniquely identified. SQL databases place a lot of importance on primary keys. They provide each row in a database table with a unique ID. A primary key can consist of one or more fields, and there can only be one primary key per table.

What are SQL data types?

A data type is an attribute that describes the kind of data an object can store, such as binary strings, numeric data, character data, financial data, date and time data, and so on.

In SQL, data types are categorized into the following groups:

  • Exact numerics
  • Approximate numerics
  • Date and time
  • Character strings
  • Unicode character strings
  • Binary strings

Where are user names and passwords stored in SQL Server?

User names and passwords are stored in the SQL table’s sys.server principals and sys.sql logins, respectively. Passwords are not stored in normal text.

What is SQL injection?

SQL injection attacks are one of the most common types of cyber-attacks today. They allow malicious attackers to access data by manipulating an application’s database. This can result in a wide range of negative consequences, from financial losses to the loss of sensitive data. The best way to protect against SQL injection attacks is to prevent them from happening in the first place.

What is a trigger in SQL and its types?

A trigger is a special type of stored procedure that automatically runs when an event occurs in the database server. Triggers are used to evaluate data before or after data modification using DDL and DML statements.

There are three types of triggers – LOGON, DDL, and DML.

  • LOGON triggers: These triggers are fired when a user initiates a Logon event.
  • DDL triggers are triggered whenever a DDL command such as CREATE, ALTER, or DROP is issued.
  • DML Triggers: These are triggered whenever a DML command modifies data. Comparable to INSERT, UPDATE, and DELETE

How would you distinguish between single-row and multiple-row functions?

A single row in a table can be affected by single row functions at once. They execute a row and then just give back one result. It is well known that length and case conversions are single-row functions.

A table’s rows can be affected by many row functions simultaneously. They are also known as group functions, executing multiple rows before returning a single output.

What is Database Normalization, and what are its main four types in SQL?

Database Normalization is a process through which data is organized for faster accessibility and to reduce data redundancy. A database’s columns and tables are organized during normalization to make sure that any dependencies are correctly upheld by database integrity constraints.

The four types of database normalization processes are as follows:

  • First Normal Form (1 NF)
  • Second Normal Form (2 NF)
  • Third Normal Form (3 NF)
  • Boyce Codd Normal Form or Fourth Normal Form (BCNF or 4 NF)

What are indexes and constraints in SQL?

Indexes are one of the most important concepts in SQL. They allow us to quickly find specific rows of data in large databases. They also help us enforce constraints, which are rules that tell the database how to behave when certain conditions are met.

Indexes are used to improve the performance of queries by speeding up the searching of data in tables. They are also used to improve the readability of data in tables.

Constraints are used to limit the type of data that can go into a table. This ensures the accuracy and reliability of the data in the table. If there is any violation between the constraint and the data action, the action is aborted.

What are the commonly used SQL constraints?

Commonly used SQL constraints are:

  • CREATE INDEX: This command ensures that indexes are created for tables so that data retrieval is made simpler.
  • FOREIGN KEY: A table’s foreign key must connect tables with similar attributes.
  • DEFAULT: If no value is supplied for the fields of a column, it supplies a default value for Those fields.
  • UNIQUE: This specifies that each value in a column must be distinct.
  • PRIMARY KEY: A table’s primary key must identify each row.
  • NOT NULL:  This condition ensures that NULL values are not accepted by columns.
  • CHECK: It ensures that every column field complies with a predetermined requirement.

Is NULL equal to 0 in SQL?

A NULL value in SQL denotes a value that is unavailable or assigned. A space (‘ ‘) or zero (0) are not equivalent to the value NULL. You cannot compare the NULL value to any other value using comparison operators like “=” or”>” since it cannot be equal to or unequal to any other value.

How to prevent SQL injection attacks?

Along with constant scanning and penetration testing, security methods like input validation, sanitization, prepared statements, and parameterized SQL queries are crucial for preventing SQL injection attacks. Additionally, quick defenses like a firewall would help to safeguard the SQL database.

What is Dynamic SQL, and when can you use it?

Dynamic SQL is a feature you can use to modify your queries at run time based on one or more criteria, including the current date, the current time, or any other criteria you choose.

You can use dynamic SQL to change your queries in meaningful ways and can help you achieve your goals or simply enhance your programming skills. Whether you are creating a new app, improving a database, or adding data warehouse functionality to your production system, you can use Dynamic SQL to do it all.

What are the different types of Keys in SQL?

Primary Key: Each row or record in a database table is uniquely identified by a field called Primary Key. The unique value must be in the primary key. Primary key fields cannot contain NULL values. There can be only one primary key per table, consisting of one or more fields.

Foreign Key:  The field or group of fields in a table that relates to the primary key of another table is called a foreign key. Tables with primary keys are called parent tables, while tables with foreign keys are called child tables.

Super key: A super key is a key or set of keys that help identify entries in a table. Although not all attributes are required to identify a record, a super key can have one or more of them.

Candidate key: A Candidate key is a subset of super keys that can be used to identify records in a database based on one or more attributes. Unlike Superkey, all the features of a candidate key must be useful for identifying records.

Composite key: A composite key is a combination of two or more columns in a table that is used to identify rows in a table. A composite key is a primary key with some other attribute or column.

What are the types of indexes in SQL?

The following are types of indexes in SQL

  • Hash index
  • memory-optimized Non-clustered
  • Clustered index
  • Non-clustered index
  • Unique index
  • Column Store index
  • Index with included columns
  • Index on computed columns
  • Filtered index
  • Spatial index
  • XML index
  • Full-Text index

What do you mean by buffer pool and mention its benefits?

In SQL, a buffer pool is also referred to as a buffer cache. A buffer pool can be used by all resources to store their cached data pages. When setting up a SQL Server instance, the buffer pool’s size can be specified. The size of a buffer pool determines how many pages it can hold.

The following are the benefits of a buffer pool:

  • Improvements in I/O performance
  • Transaction throughput growth
  • I/O latency reduction
  • Improvement reading performance

What do you mean by dependency and mention the different dependencies?

When one object is referenced by name in a SQL statement kept in another object, a dependency between the two objects is established. The term “referred entity” refers to an object that appears in a SQL expression, whereas the term “referencing entity” refers to an object that contains a SQL expression.

The following are the different types of dependencies in SQL.

  • Functional dependency
  • Fully-functional dependency
  • Multivalued dependency
  • Transitive dependency
  • Partial dependency

Joins are a fundamental part of data management in SQL. They allow two or more tables to be joined together to create a single table that can be used to analyze data. They also provide a way to filter results based on criteria in a single table, which can be useful when trying to find specific rows or columns in a large table.

There are four main types of JOINs in SQL: INNER JOIN, OUTER JOIN, CROSS JOIN, and SELF JOIN.

What are the set operators in SQL?

Data from one or more tables of the same type can be combined using the set operator. The SQL set and SQL join operators are similar, but there are some key differences. SQL set operators collect records from different queries, whereas SQL joins combine columns from different tables. Compound SQL queries are those that include set operations.

Conclusion

I hope these SQL interview questions offer you a good picture of what to expect from an interview perspective. More exposure to the SQL environment is necessary to crack the SQL interview; this can be attained by studying SQL in-depth. In order to evaluate key SQL principles, you must also practice SQL query interview questions and keep reading.

If you are looking for SQL query clarification, please visit the SQL cheat sheet to get more insights about SQL statements.

To gain hands-on experience by executing SQL queries, check out these SQL practice platforms.

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