Question 1

Write an SQL query to find the second-highest salary from an employee table.

Accepted Answer

**Using subquery with MAX**:
```sql
SELECT MAX(salary) AS second_highest
FROM employee
WHERE salary < (SELECT MAX(salary) FROM employee);
```

**Using LIMIT/OFFSET** (MySQL, PostgreSQL):
```sql
SELECT DISTINCT salary
FROM employee
ORDER BY salary DESC
LIMIT 1 OFFSET 1;
```

**Using DENSE_RANK** (ANSI SQL, most robust):
```sql
SELECT salary
FROM (
  SELECT salary, DENSE_RANK() OVER (ORDER BY salary DESC) AS rk
  FROM employee
) t
WHERE rk = 2;
```

**Architectural Logic & Trade-offs**:
- **Subque...

Question 2

Discuss differences between ROW_NUMBER(), RANK(), and DENSE_RANK(), and provide examples from your projects.

Accepted Answer

**ROW_NUMBER()**: Unique sequential numbers (1, 2, 3...); no ties—deterministic only with ORDER BY uniqueness. **RANK()**: Same rank for ties; skips (1, 2, 2, 4). **DENSE_RANK()**: Same rank for ties; no gaps (1, 2, 2, 3). **Project examples**: ROW_NUMBER() to deduplicate events by (user_id, event_time) keeping first—critical when upstream sends duplicates. DENSE_RANK() for 'top 10 products per category' reports—avoids gaps when filtering....

Question 3

Can you explain the difference between OLTP and OLAP?

Accepted Answer

**OLTP**: Optimized for many small transactions (inserts, updates, deletes). Row-oriented, normalized, high concurrency. Examples: MySQL, PostgreSQL. **OLAP**: Optimized for complex analytical queries and aggregations on large datasets. Column-oriented, denormalized (star/snowflake). Examples: Snowflake, BigQuery, Redshift. **Why the split**: Different access patterns; mixing them degrades both. OLTP needs low latency and ACID; OLAP needs scan throughput....

Question 4

Describe a time when you had to optimize a slow SQL query. What steps did you take?

Accepted Answer

**Situation**: A critical exec report was timing out at 30+ minutes; SLA was 5 minutes. **Task**: Diagnose and fix without changing business logic. **Action**: I ran EXPLAIN (ANALYZE) and found: (1) missing index on join key causing full table scan, (2) cross join that could be inner join with a filter, (3) filter in HAVING that could move to WHERE to reduce rows early, (4) unnecessary ORDER BY on a subquery....

Question 5

Explain the concept of ACID properties in the context of databases.

Accepted Answer

**ACID**: Atomicity (all or nothing), Consistency (valid state transitions), Isolation (concurrent transactions don't interfere), Durability (committed data persists). **Why it matters**: Without ACID, financial and operational data become inconsistent; retries and failures create duplicates or lost updates. **Scalability trade-off**: Strict isolation (e.g., Serializable) limits throughput; most OLTP systems use Read Committed or Repeatable Read....

Question 6

Explain the difference between INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL JOIN.

Accepted Answer

**INNER JOIN**: Only rows with matches in both tables. **LEFT JOIN**: All from left; matches from right; NULLs where no match. **RIGHT JOIN**: All from right; matches from left. **FULL JOIN**: All from both; NULLs where no match. **Why it matters**: Join choice affects result cardinality and semantics. Wrong join = wrong numbers. **Scalability**: Hash joins are common; broadcast for small dimension. FULL OUTER can be expensive—large shuffle....

Question 7

How do you handle NULL values in SQL? Mention functions like COALESCE and NULLIF.

Accepted Answer

**Approaches**: IS NULL / IS NOT NULL for filtering. **COALESCE(val1, val2, ...)**: First non-NULL value; useful for defaults. **NULLIF(val1, val2)**: Returns NULL if equal; e.g., NULLIF(divisor, 0) to avoid divide-by-zero. **Why it matters**: NULL propagates in expressions; aggregate functions ignore NULL (except COUNT(*)). JOIN on NULL yields no match (NULL ≠ NULL). **Scalability**: COALESCE in SELECT is cheap; in WHERE or JOIN it can prevent index use....

Question 8

What is a Common Table Expression (CTE), and when would you use it?

Accepted Answer

**CTE**: A named temporary result set in a WITH clause, referenced in the main query. **Use cases**: Readability—break complex queries into steps. Reusability—reference same CTE multiple times. Recursion—hierarchies (org chart, bills of materials). **Why it matters**: CTEs improve maintainability; deep subqueries are hard to debug. **Scalability**: In some engines (e.g., PostgreSQL), CTEs are optimization fences—materialized once. In others (Snowflake, BigQuery), they're inlined....

Question 9

What is the difference between a primary key and a unique key?

Accepted Answer

**Primary Key**: Unique identifier; NOT NULL; one per table; often clustered. **Unique Key**: Enforces uniqueness; can have NULL (SQL allows one NULL per column in uniqueness); multiple per table. **Why it matters**: PK defines identity and referential integrity; unique constrains alternate keys (e.g., email). **Scalability**: PK is often the clustering key; choice affects physical layout. Unique indexes enable lookups. **Cost**: Each constraint adds index overhead....

Question 10

What is the difference between WHERE and HAVING clauses in SQL?

Accepted Answer

**WHERE**: Filters rows before grouping/aggregation; cannot use aggregate functions. **HAVING**: Filters groups after GROUP BY; used with aggregate functions. **Why it matters**: WHERE reduces rows early (cheaper); HAVING filters on computed values. Misplacing predicates causes wrong results or inefficiency. **Scalability**: Pushing filters to WHERE reduces data before aggregation; HAVING on large grouped result can be expensive....

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Accenture SQL Interview Questions

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