SQL (Structured Query Language) is a critical skill for many tech roles, especially those involving data management and analysis. Whether you’re preparing for an interview or looking to brush up on your SQL knowledge, this comprehensive guide covers the top 50 most popular SQL interview questions and answers.
Top 50 SQL Interview Questions and Answers
1. What is SQL?
Answer: SQL stands for Structured Query Language. It is used to interconnect with databases to complete tasks such as querying, updating, and managing data.
2. What are the different types of SQL commands?
Answer: SQL commands are categorized into:
- DDL (Data Definition Language): CREATE, ALTER, DROP
- DML (Data Manipulation Language): SELECT, INSERT, UPDATE, DELETE
- DCL (Data Control Language): GRANT, REVOKE
- TCL (Transaction Control Language): COMMIT, ROLLBACK, SAVEPOINT
3. What is a primary key?
Answer: A primary key is a unique identifier for a record in a table. It must comprise exclusive values and cannot contain NULL values.
4. What is a foreign key?
Answer: A foreign key is a field (or collection of fields) in one table that exclusively recognizes a row of another table. It creates a relationship between the two tables.
5. What is a join? Explain its types.
Answer: A join is used to associate rows from two or more tables based on a correlated column. Types of joins include:
- INNER JOIN: Returns records with matching values in both tables.
- LEFT JOIN (LEFT OUTER JOIN): Returns all records from the left table and matched records from the right table.
- RIGHT JOIN (RIGHT OUTER JOIN): Returns all records from the right table and matched records from the left table.
- FULL JOIN (FULL OUTER JOIN): Returns records when there is a match in either left or right table.
6. What is normalization? Explain its types.
Answer: Normalization is the process of organizing data to minimalize redundancy. Types include:
- 1NF (First Normal Form): Ensures each column contains atomic values.
- 2NF (Second Normal Form): Meets all requirements of 1NF and ensures all non-key attributes are fully functional dependent on the primary key.
- 3NF (Third Normal Form): Meets all requirements of 2NF and ensures no transitive dependency.
7. What is denormalization?
Answer: Denormalization is the process of combining normalized tables into larger tables to improve read performance.
8. What is a view in SQL?
Answer: A view is a virtual table based on the result-set of an SQL statement. It can comprise rows and columns from one or more tables.
9. What is an index? Explain its types.
Answer: An index is a database object that expands the speed of data retrieval. Types include:
- Clustered Index: Reorders the physical order of the table.
- Non-Clustered Index: Does not alter the physical order of the table.
10. What is a subquery?
Answer: A subquery is a query nested inside another query. It can be used in SELECT, INSERT, UPDATE, or DELETE statements.
11. What is a stored procedure?
Answer: A stored procedure is a organized SQL code that can be saved and reused. It can accept constraints and perform complex operations.
12. What is a trigger?
Answer: A trigger is a set of SQL statements that automatically execute in response to certain events on a particular table or view.
13. What is a cursor?
Answer: A cursor is a database object used to retrieve, manipulate, and navigate through a result set row by row.
14. What is the difference between DELETE and TRUNCATE?
Answer:
- DELETE: Removes rows one at a time and logs each deletion. It can be rolled back.
- TRUNCATE: Removes all rows from a table without logging individual row deletions. It cannot be rolled back.
15. What is the difference between CHAR and VARCHAR?
Answer:
- CHAR: Fixed-length character data type.
- VARCHAR: Variable-length character data type.
16. What is a UNION operator?
Answer: The UNION operator is used to combine the result sets of two or more SELECT statements, removing duplicates.
17. What is the difference between UNION and UNION ALL?
Answer:
- UNION: Combines result sets and removes duplicates.
- UNION ALL: Combines result sets without removing duplicates.
18. What is a constraint?
Answer: A constraint is a rule enforced on data columns to ensure data integrity. Types include PRIMARY KEY, FOREIGN KEY, UNIQUE, NOT NULL, and CHECK.
19. What is a transaction?
Answer: A transaction is a sequence of one or more SQL operations treated as a single unit of work. It ensures data integrity.
20. What is ACID property?
Answer: ACID stands for Atomicity, Consistency, Isolation, Durability. These properties ensure reliable processing of database transactions.
21. What is a self-join?
Answer: A self-join is a regular join but the table is joined with itself.
22. What is a cross join?
Answer: A cross join returns the Cartesian product of two tables, combining all rows from both tables.
23. What is a composite key?
Answer: A composite key is a primary key composed of multiple columns.
24. What is a schema?
Answer: A schema is a logical container for database objects like tables, views, and procedures.
25. What is a sequence?
Answer: A sequence is a database object that generates a sequence of unique numbers.
26. What is a synonym?
Answer: A synonym is an alias for a database object, providing an alternative name.
27. What is a materialized view?
Answer: A materialized view is a database object that contains the results of a query and can be refreshed periodically.
28. What is a pivot?
Answer: A pivot is a SQL operation that transforms rows into columns.
29. What is a dynamic SQL?
Answer: Dynamic SQL is SQL code generated and executed at runtime.
30. What is a correlated subquery?
Answer: A correlated subquery is a subquery that references columns from the outer query.
31. What is a temporary table?
Answer: A temporary table is a table that exists temporarily during a session or transaction.
32. What is a common table expression (CTE)?
Answer: A CTE is a temporary result set defined within the execution scope of a single SELECT, INSERT, UPDATE, or DELETE statement.
33. What is a window function?
Answer: A window function performs calculations across a set of table rows related to the current row.
34. What is a recursive query?
Answer: A recursive query is a query that refers to itself to produce a result set.
35. What is a partitioned table?
Answer: A partitioned table is a table whose data is divided into smaller, more manageable pieces called partitions.
36. What is a bitmap index?
Answer: A bitmap index is a type of index that uses bitmaps and is efficient for columns with a low cardinality.
37. What is a hash join?
Answer: A hash join is a join operation that uses a hash table to find matching rows.
38. What is a clustered index scan?
Answer: A clustered index scan is a full scan of the clustered index.
39. What is a covering index?
Answer: A covering index is an index that contains all the columns needed to satisfy a query.
40. What is a deadlock?
Answer: A deadlock occurs when two or more transactions are waiting for each other to release resources, causing a cycle of dependencies.
41. What is a lock escalation?
Answer: Lock escalation is the process of converting many fine-grained locks into fewer coarse-grained locks.
42. What is a phantom read?
Answer: A phantom read occurs when a transaction reads a set of rows that satisfy a condition, and another transaction inserts or deletes rows that satisfy the same condition.
43. What is a dirty read?
Answer: A dirty read occurs when a transaction reads data that has not yet been committed.
44. What is a repeatable read?
Answer: A repeatable read ensures that if a transaction reads a row, it will read the same row again even if other transactions modify the data.
45. What is a snapshot isolation?
Answer: Snapshot isolation provides a consistent view of the database at the start of a transaction and ensures that the transaction sees a consistent snapshot of the data.
46. What is a read committed isolation level?
Answer: Read committed isolation level ensures that any data read is committed at the moment it is read.
47. What is a read uncommitted isolation level?
Answer: Read uncommitted isolation level allows a transaction to read data that has not yet been committed.
48. What is a serializable isolation level?
Answer: Serializable isolation level ensures that transactions are completely isolated from each other.
49. What is a serializable isolation level?
Answer: Serializable isolation level ensures that transactions are completely isolated from each other. It is the strictest isolation level, preventing dirty reads, non-repeatable reads, and phantom reads.
50. What is a database trigger?
Answer: A database trigger is a set of SQL statements that automatically execute in response to certain events on a particular table or view, such as INSERT, UPDATE, or DELETE operations.
Conclusion
Mastering SQL is essential for many roles in the tech industry, especially those involving data management and analysis. By familiarizing yourself with these common interview questions and answers, you’ll be better prepared to demonstrate your SQL knowledge and skills during job interviews.
Good luck with your interview preparation! If you have any more questions or need further assistance, feel free to ask.
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