Real interview questions from top companies for SQL. Includes theoretical concepts and coding problems.
What is the difference between INNER JOIN and LEFT JOIN in SQL?
INNER JOIN returns only the rows that have a match in both tables, while LEFT JOIN returns all the rows from the left table and the matched rows from the right table. If there is no match, the result is NULL on the right side.
What is the purpose of the GROUP BY clause in SQL?
The GROUP BY clause is used to group the result-set by one or more columns. It is often used with aggregate functions such as SUM, AVG, MAX, MIN, and COUNT to perform calculations on a group of rows.
What is the difference between HAVING and WHERE clauses in SQL?
The WHERE clause is used to filter rows before grouping, while the HAVING clause is used to filter groups after grouping. The HAVING clause is often used with aggregate functions.
What is the purpose of indexing in SQL?
Indexing in SQL is used to speed up the retrieval of data by providing a quick way to locate data. It is like an index in a book, which helps you to quickly find a specific page.
What is the difference between UNION and UNION ALL in SQL?
UNION returns only distinct rows, while UNION ALL returns all rows including duplicates. If you want to include duplicate rows, use UNION ALL.
What is the purpose of the ORDER BY clause in SQL?
The ORDER BY clause is used to sort the result-set in ascending or descending order. It is often used with the LIMIT clause to retrieve a specific number of rows.
What is the difference between CHAR and VARCHAR in SQL?
CHAR is a fixed-length string, while VARCHAR is a variable-length string. CHAR is used when the length of the string is fixed, while VARCHAR is used when the length of the string is variable.
What is the purpose of the LIMIT clause in SQL?
The LIMIT clause is used to limit the number of rows returned in the result-set. It is often used with the ORDER BY clause to retrieve a specific number of rows.
What is the difference between DELETE and TRUNCATE in SQL?
DELETE is used to delete specific rows, while TRUNCATE is used to delete all rows. TRUNCATE is faster and more efficient, but it cannot be rolled back.
What is the purpose of the EXISTS clause in SQL?
The EXISTS clause is used to check if a subquery returns at least one row. It is often used with the WHERE clause to filter rows based on the existence of related data.
Write a SQL query to find the names of all employees who earn more than the average salary.
SELECT name FROM employees WHERE salary > (SELECTAVG(salary) FROM employees)
Write a SQL query to find the names of all departments that have more than 5 employees.
SELECT department FROM employees GROUPBY department HAVINGCOUNT(*) >5
Write a SQL query to find the names of all employees who work in the sales department and earn more than $50,000.
SELECT name FROM employees WHERE department ='Sales'AND salary >50000
Write a SQL query to find the names of all employees who have a manager who earns more than $100,000.
SELECT e1.name FROM employees e1 JOIN employees e2 ON e1.manager_id = e2.id WHERE e2.salary >100000
Write a SQL query to find the names of all departments that have at least one employee who earns more than $50,000.
SELECTDISTINCT department FROM employees WHERE salary >50000
SHARE ARTICLE
Choose Interviewer type
Please review your order carefully before confirming. Once your purchase is processed, refunds will not be available.
