If you’re on the hunt for a tech job, knowledge of SQL is nothing short of essential – and it’s likely that potential employers will be testing your skills. With this article as your guide, you’ll learn all about basic SQL interview questions and answers to get yourself ready for success in any upcoming interviews! From experienced professionals to beginners just starting out with data analysis or software development, these answers provide great insight into how best to ace those tricky SQL queries so you can make sure every employer knows what an amazing asset they’d be getting by hiring YOU!
Table of Contents
Basic SQL Interview Questions
What is SQL and what is it used for?
SQL stands for Structured Query Language and is a programming language used to manage and manipulate databases.
How do you create a table in SQL?
To create a table in SQL, you would use the CREATE TABLE statement followed by the desired column names and data types.
How do you insert records into a table?
To insert records into a table, you would use the INSERT INTO statement followed by the values to be inserted.
How do you update records in a table?
To update records in a table, you would use the UPDATE statement followed by the values to be updated and a WHERE clause to specify which records to update.
How do you delete records from a table?
To delete records from a table, you would use the DELETE FROM statement followed by a WHERE clause to specify which records to delete.
How do you select specific columns in a table?
To select specific columns in a table, you would use the SELECT clause followed by the name of the desired columns.
How do you filter records in a table using a WHERE clause?
To filter records in a table using a WHERE clause, you would include a condition in the WHERE clause that specifies which records to include or exclude.
How do you sort records in a table using an ORDER BY clause?
To sort records in a table using an ORDER BY clause, you would specify the column or columns to sort by and whether the sort should be in ascending or descending order.
How do you create a primary key in a table?
To create a primary key in a table, you would use the PRIMARY KEY constraint when defining the table column.
How do you create a foreign key in a table?
To create a foreign key in a table, you would use the FOREIGN KEY constraint when defining the table column. The foreign key column must reference the primary key of another table.
How do you use GROUP BY and HAVING clauses in a SELECT statement?
The GROUP BY clause is used to group records by a specific column or set of columns, and the HAVING clause is used to filter the grouped records based on a specified condition.
How do you use JOIN clauses to retrieve data from multiple tables?
JOIN clauses are used to retrieve data from multiple tables based on a common column or set of columns. There are various types of JOIN clauses, including INNER JOIN, OUTER JOIN, and CROSS JOIN.
How do you use subqueries to retrieve data from multiple tables?
A subquery is a SELECT statement within another SELECT statement that is used to retrieve data from multiple tables.
Additional Resources
Top 20 SQL Join Interview Questions and Answers
Top 10 SQL Scenario-Based Interview Questions and Answers
Top 15 SQL Query Interview Questions and Answers
How do you use aggregate functions (e.g. COUNT, SUM, AVG, MAX, MIN) in a SELECT statement?
Aggregate functions are used to perform calculations on a set of records and return a single value. Some examples of aggregate functions include COUNT, SUM, AVG, MAX, and MIN.
How do you use CASE statements in a SELECT statement?
The CASE statement is used to evaluate a list of conditions and return a value based on the result of the evaluation.
How do you use UNION and UNION ALL to combine data from multiple tables?
UNION is used to combine the results of two or more SELECT statements into a single result set, while UNION ALL includes duplicate records in the result set.
How do you use views to create virtual tables in a database?
A view is a virtual table that is created from a SELECT statement and can be used to simplify queries and improve security by limiting access to certain data.
How do you use stored procedures to execute repetitive tasks?
A stored procedure is a pre-defined set of SQL statements that can be executed with a single call. Stored procedures are used to automate repetitive tasks and improve performance.
How do you use triggers to automate tasks in a database?
A trigger is a set of SQL statements that are executed automatically in response to a specific event, such as the insertion of a record into a table.
How do you use transactions to ensure the integrity of data in a database?
A transaction is a set of SQL statements that are treated as a single unit of work. Transactions are used to ensure the integrity of data in a database by rolling back any changes if an error occurs.
What are the 5 basic SQL commands?
The 5 basic SQL commands are SELECT, INSERT, UPDATE, DELETE, and CREATE.
What are the 4 components of SQL?
The 4 components of SQL are the data definition language (DDL), data manipulation language (DML), data control language (DCL), and data query language (DQL).
What is DDL and DML?
DDL (data definition language) is a set of SQL commands used to create and modify database structures, such as tables and indices. DML (data manipulation language) is a set of SQL commands used to manipulate data within a database, such as inserting, updating, and deleting records.
How to remove tables in SQL?
To remove tables in SQL, you can use the DROP TABLE command followed by the name of the table.
Is SQL a syntax?
SQL is a syntax, or a set of rules and guidelines, for using the SQL language to manipulate and query databases.
What is schema in SQL?
A schema in SQL is the structure or layout of a database, including the tables, indices, and other objects within the database.
What is %s in SQL?
%s in SQL is a placeholder for a string value that will be supplied at runtime.
What is (+) in SQL query?
(+) in a SQL query is used in a join condition to indicate an outer join. An outer join returns all records from one table, even if there are no matching records in the other table.
What does %D in SQL mean?
%D in SQL stands for a date value in the format of “YYYY-MM-DD”.
How to remove duplicates in SQL?
To remove duplicates in SQL, you can use the SELECT DISTINCT statement to only return unique records. You can also use the GROUP BY clause to group duplicates together and use the HAVING clause to filter out the duplicate groups.
How to write a query in SQL?
To write a query in SQL, you can use the SELECT, FROM, and WHERE clauses to specify the data you want to retrieve and the conditions for filtering the data. You can also use other clauses such as GROUP BY, HAVING, and ORDER BY to further refine your query.
What is an index in SQL?
An index in SQL is a data structure that is used to improve the performance of queries by allowing faster access to specific records in a table.
What is the primary key?
The primary key is a column or set of columns in a table that is used to uniquely identify each record. The primary key must be unique and cannot contain null values.
What are matrix and table?
A matrix is a two-dimensional array of data that is often used to represent a table of values. A table in SQL is a collection of related data stored in a structured format within a database.
What is an Alias in SQL?
An alias in SQL is a temporary name that is given to a table or column for the purpose of a specific query. Aliases are used to simplify complex queries, to make the results of a query easier to read, or to avoid naming conflicts when multiple tables or columns have the same name.
To create an alias in SQL, you can use the AS keyword followed by the desired alias name. For example, you can use the following syntax to create an alias for a table:
SELECT * FROM table_name AS alias_name
You can also use the following syntax to create an alias for a column:
SELECT column_name AS alias_name FROM table_name
What is Normalization in SQL?
Normalization in SQL is the process of organizing a database to reduce redundancy and dependency and improve the integrity and efficiency of data.
What are the four 4 types of database normalization?
The four types of database normalization are 1NF (first normal form), 2NF (second normal form), 3NF (third normal form), and BCNF (Boyce-Codd normal form).
What are 1NF 2NF and 3NF?
1NF (first normal form) is a database design rule that requires that all columns in a table must contain atomic values, meaning they cannot be further divided into smaller pieces. 2NF (second normal form) is a database design rule that requires that all columns in a table must depend on the entire primary key. 3NF (third normal form) is a database design rule that requires that all columns in a table must depend only on the primary key and not on any other non-key columns.
How to convert 1NF to 3NF?
To convert a database from 1NF to 3NF, you can follow these steps:
- Identify the primary key of each table.
- Remove any repeating groups from each table, creating a new table for each group and linking it to the original table with a foreign key.
- Remove any non-key columns that are dependent on other non-key columns. These columns should be placed in a new table and linked to the original table with a foreign key.
Can the primary key be NULL?
A primary key cannot contain NULL values, as it is used to uniquely identify each record in a table.
Can a table have two unique keys?
A table can have multiple unique keys, but only one of them can be designated as the primary key.
Can a table have two foreign keys?
A table can have multiple foreign keys, but each foreign key must reference a different primary key in another table.
What are the 5 types of primary keys?
The 5 types of primary keys are
Single a column primary key: a primary key that consists of a single column.
Composite primary key: a primary key that consists of multiple columns.
Surrogate primary key: a primary key that is artificially generated and has no inherent meaning.
Natural primary key: a primary key that is based on a column or set of columns that have inherent meaning in the business context.
Candidate key: a column or set of columns that could potentially be used as the primary key, but is not necessarily the primary key.
What is a tuple in SQL?
A tuple in SQL is a row of data in a table.
What is an entity in SQL?
An entity in SQL is an object or concept about which data is stored in a database.
What is a domain in SQL?
A domain in SQL is the set of possible values that can be assigned to an attribute (column) in a table.
What is a list in SQL?
A list in SQL is a set of values that are separated by commas. Lists are often used in the IN and NOT IN clauses of a SELECT statement to filter records based on a specific set of values.
What is an attribute in SQL?
An attribute in SQL is a column in a table that represents a specific characteristic or property of an entity.
What is a relationship in SQL?
A relationship in SQL is a connection between two or more entities in a database.
What is the super key in SQL?
A super key in SQL is a set of columns in a table that uniquely identifies each record. The primary key is a special type of super key that has additional constraints, such as being non-null and unique.
How do you define schema?
A schema in SQL is the structure or layout of a database, including the tables, indices, and other objects within the database.
How do you create a database in SQL?
To create a database in SQL, you can use the CREATE DATABASE statement followed by the desired database name.
How do you create a user and grant permissions in SQL?
To create a user and grant permissions in SQL, you can use the CREATE USER and GRANT statements. The GRANT statement is used to give specific permissions to a user, such as the ability to SELECT, INSERT, UPDATE, DELETE, and other operations on a database.
How do you use data types in SQL to define the type and size of a column?
Data types in SQL are used to define the type and size of a column in a table. Some common data types include CHAR, VARCHAR, INT, FLOAT, and DATE.
How do you use constraints (e.g. NOT NULL, UNIQUE, CHECK) to enforce data integrity in a table?
Constraints in SQL are used to enforce data integrity in a table by setting rules for the values that can be entered into a column. Some examples of constraints include NOT NULL (which requires a value to be entered), UNIQUE (which requires unique values), and CHECK (which allows you to specify a condition that values must meet).
How do you use transactions to ensure the integrity of data in a database?
Transactions in SQL are used to ensure the integrity of data in a database by rolling back any changes if an error occurs. To use transactions, you can use the BEGIN TRANSACTION and COMMIT TRANSACTION statements to enclose the SQL statements that you want to execute as a single unit of work.
How do you use triggers to automate tasks in a database?
Triggers in SQL are used to automate tasks in a database by executing a set of SQL statements in response to a specific event, such as the insertion of a record into a table. To create a trigger, you can use the CREATE TRIGGER statement followed by the event and actions to be taken.
How do you use stored procedures to execute repetitive tasks?
Stored procedures in SQL are used to execute repetitive tasks by pre-defining a set of SQL statements that can be called with a single call. To create a stored procedure, you can use the CREATE PROCEDURE statement followed by the desired parameters and SQL statements.
How do you use views to create virtual tables in a database?
Views in SQL are used to create virtual tables in a database by storing the results of a SELECT statement. To create a view, you can use the CREATE VIEW statement followed by the SELECT statement that defines the view.
How do you use subqueries to retrieve data from multiple tables?
Subqueries in SQL are SELECT statements that are nested within another SELECT statement and are used to retrieve data from multiple tables. To use a subquery, you can include it within the WHERE or FROM clauses of a SELECT statement.
How do you use pivot tables to summarize and analyze data in a database?
Pivot tables in SQL are used to summarize and analyze data by rearranging the rows and columns of a table to create a new table with different dimensions. To create a pivot table, you can use the PIVOT and UNPIVOT operators in a SELECT statement, followed by the desired grouping and aggregating functions.
Conclusion
In conclusion, understanding basic SQL is an essential skill for many roles in the tech industry. From creating tables and inserting data to filtering and sorting records, the knowledge and understanding of these basic SQL commands can be crucial in a job interview. By reviewing the common SQL interview questions and answers provided in this article, you can feel more prepared and confident in your next SQL interview.