Daily Coding Quiz: Tuesday, 3 November 2026

All 10 coding questions with answers and explanations. 0 players took part.

1. How many edges does a tree with n nodes have?

   DSA · easy

   1. n
   2. n − 1
   3. n + 1
   4. 2n

   Answer: B. n − 1

   Explanation: A tree is connected and acyclic: every node except the root has exactly one edge to its parent, giving n − 1 edges.

2. Which statement removes all rows from a table but keeps the table itself?

   SQL · easy

   1. DROP TABLE
   2. DELETE TABLE
   3. TRUNCATE TABLE
   4. REMOVE TABLE

   Answer: C. TRUNCATE TABLE

   Explanation: TRUNCATE empties the table quickly (and in MySQL resets AUTO_INCREMENT). DROP deletes the table definition too. DELETE without WHERE also removes all rows, row by row.

3. What does this return?

   SQL · easy

   -- Table employees(id, name, dept, salary):
   -- (1, Asha,  Eng,   90)
   -- (2, Ravi,  Eng,   70)
   -- (3, Meera, Sales, 60)
   -- (4, Kabir, Sales, 60)
   -- (5, Neha,  HR,    NULL)
   SELECT COUNT(*), COUNT(salary) FROM employees;

   1. 5, 5
   2. 5, 4
   3. 4, 4
   4. 4, 5

   Answer: B. 5, 4

   Explanation: COUNT(*) counts rows (5). COUNT(column) skips NULLs, and Neha’s salary is NULL, so 4.

4. What does this return for nums = [2, 7, 11, 15], target = 9?

   DSA · medium

   def solve(nums, target):
       seen = {}
       for i, x in enumerate(nums):
           if target - x in seen:
               return [seen[target - x], i]
           seen[x] = i
       return []

   1. [0, 1]
   2. [1, 0]
   3. [2, 7]
   4. []

   Answer: A. [0, 1]

   Explanation: At i=1 (x=7), 9−7=2 is already in seen at index 0, so it returns [0, 1]. This is the O(n) hash-map solution to Two Sum.

5. Which query finds the second-highest distinct salary?

   SQL · medium

   1. SELECT MAX(salary) FROM employees WHERE salary < (SELECT MAX(salary) FROM employees)
   2. SELECT salary FROM employees ORDER BY salary LIMIT 2
   3. SELECT MAX(salary) - 1 FROM employees
   4. SELECT salary FROM employees WHERE salary = 2

   Answer: A. SELECT MAX(salary) FROM employees WHERE salary < (SELECT MAX(salary) FROM employees)

   Explanation: Take the maximum among salaries below the overall maximum. ORDER BY salary DESC LIMIT 1 OFFSET 1 also works only if there are no duplicates at the top; DISTINCT fixes that.

6. What does this return?

   SQL · medium

   SELECT COALESCE(NULL, NULL, 'x', 'y'), NULLIF(5, 5), IFNULL(NULL, 0);  -- MySQL

   1. x, NULL, 0
   2. NULL, 5, 0
   3. y, 0, NULL
   4. x, 5, 0

   Answer: A. x, NULL, 0

   Explanation: COALESCE returns the first non-NULL argument. NULLIF(a, b) is NULL when a = b. IFNULL(a, b) is MySQL’s two-argument COALESCE.

7. A composite index exists on (last_name, first_name). Which WHERE clause can use it best?

   SQL · medium

   1. WHERE first_name = 'Ravi'
   2. WHERE last_name = 'Shah'
   3. WHERE UPPER(last_name) = 'SHAH'
   4. WHERE first_name LIKE '%vi'

   Answer: B. WHERE last_name = 'Shah'

   Explanation: A B-tree index on (a, b) is sorted by a first, so it helps filters on its leftmost column. Wrapping the column in a function, or filtering only on b, generally prevents its use.

8. You need the k largest of n numbers arriving as a stream, using little memory. The best approach:

   DSA · hard

   1. Sort everything at the end: O(n log n) time, O(n) memory
   2. A min-heap of size k: O(n log k) time, O(k) memory
   3. A max-heap of all n elements
   4. A hash set of size k

   Answer: B. A min-heap of size k: O(n log k) time, O(k) memory

   Explanation: Keep the k largest so far in a min-heap: if a new number beats the heap’s minimum, replace it. Only k items are ever stored.

9. Which query returns the highest-paid employee in every department (ties included)?

   SQL · hard

   1. SELECT dept, name, MAX(salary) FROM employees GROUP BY dept
   2. SELECT dept, name, salary FROM (SELECT *, RANK() OVER (PARTITION BY dept ORDER BY salary DESC) r FROM employees) t WHERE r = 1
   3. SELECT dept, name, salary FROM employees ORDER BY salary DESC LIMIT 1
   4. SELECT DISTINCT dept, MAX(name) FROM employees

   Answer: B. SELECT dept, name, salary FROM (SELECT *, RANK() OVER (PARTITION BY dept ORDER BY salary DESC) r FROM employees) t WHERE r = 1

   Explanation: Ranking within each department and keeping rank 1 returns the right name with ties. The GROUP BY version selects a name not tied to the MAX (an error under ONLY_FULL_GROUP_BY, an arbitrary name otherwise).

10. Which isolation level prevents dirty reads, non-repeatable reads and phantom reads (per the SQL standard)?

    SQL · hard

    1. READ UNCOMMITTED
    2. READ COMMITTED
    3. REPEATABLE READ
    4. SERIALIZABLE

    Answer: D. SERIALIZABLE

    Explanation: Only SERIALIZABLE forbids all three in the standard. (MySQL InnoDB’s default REPEATABLE READ also blocks most phantoms using next-key locks.)

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