30 Basic to Intermediate SQL Interview Questions & Practice Tests

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In "30 Basic to Intermediate SQL Interview Questions & Practice Tests" video:
👉 Most competitive questions asked in the interview
👉 You can check your score at the end of the video
👉 Frequently asked SQL Questions as a practice set

Here are the Questions answers with their explanations:

Q1. Which SQL clause is executed first in a SELECT query?
✅ Answer: D) FROM
👉 The FROM clause identifies the source tables before filtering or selecting.

Q2. What will COUNT(column_name) return?
✅ Answer: B) Total non-NULL values
👉 COUNT(column_name) ignores NULLs and counts only valid entries.

Q3. Which JOIN returns records that have matching values in both tables?
✅ Answer: D) INNER JOIN
👉 INNER JOIN keeps only rows with matching keys in both tables.

Q4. Which keyword is used to remove duplicate records?
✅ Answer: B) DISTINCT
👉 DISTINCT ensures unique results by eliminating duplicates.

Q5. Which constraint ensures uniqueness but allows NULL?
✅ Answer: C) UNIQUE
👉 UNIQUE enforces non-duplicate values but permits NULL entries.

Q6. What is the result of TRUNCATE TABLE?
✅ Answer: B) Deletes rows without rollback
👉 TRUNCATE quickly clears all rows but cannot be rolled back.

Q7. Which function returns current date and time?
✅ Answer: D) All of the above
👉 NOW(), GETDATE(), and SYSDATE() all fetch system date-time.

Q8. Which subquery returns only one value?
✅ Answer: C) Scalar subquery
👉 A scalar subquery outputs a single value, not multiple rows.

Q9. What does ACID stand for?
✅ Answer: A) Atomicity, Consistency, Isolation, Durability
👉 ACID defines reliable transaction properties in databases.

Q10. Which index improves performance of SELECT queries?
✅ Answer: C) Both A & B
👉 Both clustered and non-clustered indexes speed up lookups.

Q11. Which command is used to change table structure?
✅ Answer: C) ALTER
👉 ALTER modifies schema by adding, dropping, or changing columns.

Q12. Which normal form removes transitive dependency?
✅ Answer: C) 3NF
👉 Third Normal Form eliminates indirect dependencies for cleaner design.

Q13. Which SQL statement is used to create a view?
✅ Answer: A) CREATE VIEW
👉 CREATE VIEW saves query results as a virtual table.

Q14. What is a correlated subquery?
✅ Answer: C) Query that depends on outer query
👉 A correlated subquery runs row-by-row linked to the outer query.

Q15. Which clause filters groups?
✅ Answer: C) HAVING
👉 HAVING applies conditions after grouping results.

Q16. Which command removes all rows and table structure?
✅ Answer: B) DROP
👉 DROP deletes the entire table definition and data.

Q17. What is the default JOIN in SQL?
✅ Answer: C) INNER JOIN
👉 By default, SQL uses INNER JOIN when no type is specified.

Q18. Which operator is used for pattern matching?
✅ Answer: C) LIKE
👉 LIKE matches strings using wildcards like % and _.

Q19. Which key uniquely identifies a row?
✅ Answer: C) Primary key
👉 A primary key ensures each row is uniquely identifiable.

Q20. Which function converts NULL into a value?
✅ Answer: D) All of the above
👉 IFNULL(), NVL(), and COALESCE() replace NULLs with given values.

Q21. Which SQL command is DCL?
✅ Answer: B) GRANT
👉 GRANT provides user permissions, part of Data Control Language.

Q22. Which transaction command saves changes permanently?
✅ Answer: C) COMMIT
👉 COMMIT finalizes all changes in a transaction.

Q23. What is a composite key?
✅ Answer: B) Combination of columns as primary key
👉 Composite keys use multiple columns together for uniqueness.

Q24. Which SQL command is used to remove a column?
✅ Answer: C) ALTER
👉 ALTER TABLE DROP COLUMN removes a column from a table.

Q25. Which JOIN returns all records when there is a match in either table?
✅ Answer: D) FULL JOIN
👉 FULL JOIN combines results from both tables, including unmatched rows.

Q26. Which constraint checks condition?
✅ Answer: B) CHECK
👉 CHECK enforces rules on column values.

Q27. Which clause sorts the result?
✅ Answer: B) ORDER BY
👉 ORDER BY arranges query results in ascending or descending order.

Q28. Which SQL object stores query result?
✅ Answer: B) View
👉 A view is a saved query result acting like a virtual table.

Q29. Which statement is true about DELETE?
✅ Answer: C) Can rollback
👉 DELETE removes rows but supports rollback within transactions.

Q30. Which keyword is used to check existence of records?
✅ Answer: B) EXISTS
👉 EXISTS tests if a subquery returns any rows.

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