1. Introduction
Cognizant ETL testing interview questions are commonly asked for ETL QA, Data Warehouse Testing, BI Testing, and Data Validation roles at Cognizant. Cognizant delivers large-scale data platforms for banking, insurance, healthcare, retail, and life sciences, where data accuracy, transformation logic, and SLA compliance are business-critical.
Interviewers at Cognizant typically evaluate:
- Solid ETL & Data Warehouse fundamentals
- Hands-on SQL skills for validation
- Clear understanding of Source-to-Target (S2T) mapping
- Ability to explain real-time ETL defects
- Knowledge of SCD1, SCD2, audit fields, hashing
- Awareness of performance tuning & batch processing
This article is a Cognizant-oriented ETL interview preparation guide, useful for freshers, 2–5 years experienced, and senior ETL testers.
2. What is ETL Testing? (Definition + Cognizant-Style Example)
ETL Testing validates that data is:
- Extracted correctly from source systems
- Transformed accurately using business rules
- Loaded completely into the target data warehouse
Real-Time Cognizant Project Example
- Source: Claims & policy data from OLTP systems
- Transform:
- Deduplication on business keys
- Currency normalization
- Daily/monthly aggregation
- SCD2 handling for customer/policy dimensions
- Deduplication on business keys
- Target: Enterprise Data Warehouse (EDW)
- Reporting: Tableau / Power BI dashboards
At Cognizant, ETL testers validate data correctness + business impact, not just job success.
3. Data Warehouse Flow – Source → Staging → Transform → Load → Reporting
Typical ETL Architecture in Cognizant Projects
- Source Systems – OLTP DBs, flat files, APIs
- Staging Layer – Raw extracted data
- Transformation Layer – Cleansing, enrichment, business logic
- Target Layer (DW/Data Mart) – Fact & Dimension tables
- Reporting Layer – BI tools
Interview Tip: Be ready to explain reconciliation, audit checks, restartability, and failure handling.
4. Cognizant ETL Testing Interview Questions & Answers (Basic → Advanced)
A. Basic ETL Testing Interview Questions
Q1. What is ETL testing?
ETL testing validates extraction, transformation, and loading to ensure accuracy, completeness, and performance.
Q2. Why is ETL testing important in Cognizant projects?
Because enterprise clients rely on accurate data for regulatory reporting and decisions.
Q3. What is a data warehouse?
A centralized repository storing historical and integrated data for analytics.
Q4. What is a staging table?
A temporary table holding raw data before transformations.
B. Data Warehouse & S2T Mapping Questions
Q5. What is Source-to-Target (S2T) mapping?
A document defining how source columns map to target columns, including transformation rules, defaults, and reject logic.
Q6. How do you validate S2T mapping in Cognizant projects?
By comparing source, staging, and target using SQL after applying transformations.
Q7. What challenges do you face during S2T validation?
- Complex joins across multiple sources
- Conditional/derived columns
- Lookup mismatches
- Null/default handling
5. SQL Query Examples (Very Important for Cognizant)
Record Count Validation
SELECT COUNT(*) FROM src_orders;
SELECT COUNT(*) FROM fact_orders;
✔ Ensures no data loss.
Data Validation Using JOIN
SELECT s.order_id,
s.amount AS src_amount,
t.amount AS tgt_amount
FROM src_orders s
JOIN fact_orders t
ON s.order_id = t.order_id
WHERE s.amount <> t.amount;
✔ Detects transformation/load defects.
Finding Missing Records
SELECT s.order_id
FROM src_orders s
LEFT JOIN fact_orders t
ON s.order_id = t.order_id
WHERE t.order_id IS NULL;
✔ Identifies records missing in target.
GROUP BY & Aggregation Validation
SELECT region, SUM(sales_amount)
FROM fact_sales
GROUP BY region;
✔ Validates aggregation logic.
Window Function Example
SELECT customer_id,
SUM(amount) OVER (PARTITION BY customer_id) AS total_spend
FROM fact_orders;
✔ Validates running totals/partition calculations.
Performance Tuning SQL
EXPLAIN ANALYZE
SELECT *
FROM fact_orders
WHERE order_date >= ‘2025-01-01’;
✔ Analyzes slow queries and SLA risk.
6. Slowly Changing Dimension (SCD) Questions
Q8. What is SCD Type 1?
Overwrites old data; no history.
Q9. What is SCD Type 2?
Maintains history using:
- Start date
- End date
- Active flag
SCD2 Validation SQL
SELECT customer_id, start_date, end_date, is_active
FROM dim_customer
WHERE customer_id = 101;
Q10. Common SCD2 defects in Cognizant projects?
- Multiple active records
- Old record not expired
- Incorrect effective dates
7. Scenario-Based ETL Testing Interview Questions (Cognizant Style)
Scenario 1: Record Count Mismatch
Possible Causes:
- Filter mismatch
- Wrong join type
- Duplicate source data
Scenario 2: Null Values in Target
SELECT *
FROM dim_customer
WHERE email IS NULL;
Action: Verify default/reject logic.
Scenario 3: ETL Job Misses SLA
Resolution:
- Analyze execution plan
- Optimize SQL
- Partition large tables
- Tune parallelism
8. ETL Tools Asked in Cognizant Interviews
Cognizant emphasizes concepts + real experience over syntax.
Common ETL tools:
- Informatica
- Microsoft SSIS
- Ab Initio
- Talend
- Pentaho
9. ETL Defect Examples + Test Case Sample
Common ETL Defects in Cognizant Projects
| Defect Type | Example |
| Data loss | Missing records |
| Transformation error | Wrong calculation |
| Duplicate data | Incorrect join |
| SCD defect | Multiple active records |
| Performance issue | Job misses SLA |
Sample ETL Test Case
| Field | Value |
| Test Case ID | CTS_ETL_TC_01 |
| Scenario | Validate SCD Type 2 |
| Source | src_customer |
| Target | dim_customer |
| Expected | One active record |
10. Quick Revision Sheet (Cognizant ETL Interviews)
- ETL = Extract + Transform + Load
- Validate count + data + transformation
- Strong SQL is mandatory (JOIN, GROUP BY, window functions)
- SCD2 is frequently asked
- Performance & SLA awareness is critical
11. FAQs – Cognizant ETL Testing Interview Questions
Q1. Does Cognizant ask tool-specific questions?
Mostly conceptual, based on your project experience.
Q2. Is SQL mandatory for Cognizant ETL interviews?
Yes—strong SQL is non-negotiable.
Q3. Are scenario-based questions common?
Yes, especially for 2+ years experienced roles.
Q4. Is ETL testing manual or automated at Cognizant?
Primarily SQL-driven manual testing with partial automation.