SSIS ETL Testing Interview Questions – Real-Time SQL & Data Warehouse Guide

1. What is ETL Testing? (Definition + Example)

ETL Testing is the process of validating whether data is correctly Extracted, Transformed, and Loaded from source systems into a data warehouse according to business rules, mappings, and performance expectations.

Real-Time Example (SSIS Project)

• Source: SQL Server OLTP (Orders, Customers)
  
• Transformation: Deduplication, derived columns, SCD handling
  
• ETL Tool: SQL Server Integration Services (SSIS)
  
• Target: Enterprise Data Warehouse (Fact & Dimension tables)
  
• Reporting: Power BI / SSRS
  

Objective: Ensure reports are generated from accurate, complete, and reconciled data.

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2. Data Warehouse Flow – Source → Staging → Transform → Load → Reporting

DW Layer Responsibilities

Layer	Responsibility
Source	OLTP databases, files, APIs
Staging	Raw extracted data
Transformation	Business rules, joins, SCD logic
Target (DW)	Fact & dimension tables
Reporting	BI dashboards & analytics

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3. SSIS ETL Architecture & Source-to-Target (S2T) Mapping

SSIS ETL Architecture

• Source systems (SQL Server, Oracle, Flat files)
  
• Staging tables
  
• SSIS packages (Control Flow + Data Flow)
  
• Data warehouse
  
• Reporting tools
  

S2T Mapping Validation Covers

• Column-to-column mapping
  
• Data type & length validation
  
• Transformation logic
  
• Default & derived values
  
• Audit fields (load_date, batch_id)
  
• SCD Type 1 / Type 2 rules
  

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4. SSIS ETL Testing Interview Questions (Basic → Advanced)

Below are 80+ real interview-focused SSIS ETL testing interview questions with clear answers.

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A. Basic SSIS ETL Testing Interview Questions (1–20)

1. What is ETL testing?
   Validation of data extraction, transformation, and loading.
   
2. Why is ETL testing important in SSIS projects?
   SSIS feeds DW used for business-critical reporting.
   
3. What is SSIS?
   Microsoft ETL tool for data integration.
   
4. What is a SSIS package?
   Collection of control flow and data flow tasks.
   
5. What is staging table?
   Temporary storage for raw extracted data.
   
6. What is S2T mapping?
   Document defining source-to-target logic.
   
7. Difference between ETL and ELT?
   ETL transforms before load; ELT after load.
   
8. What is data reconciliation?
   Comparing source and target data.
   
9. What is surrogate key?
   System-generated unique key.
   
10. Difference between fact and dimension table?
    Fact stores measures; dimension stores attributes.
    
11. What is full load?
    Load entire dataset.
    
12. What is incremental load?
    Load only new or changed records.
    
13. What are audit fields?
    load_date, batch_id, updated_ts.
    
14. What is data profiling?
    Analyzing source data quality.
    
15. What is truncation testing?
    Ensuring no data loss due to column length.
    
16. What is referential integrity?
    Fact foreign keys must exist in dimension.
    
17. What is CDC?
    Change Data Capture.
    
18. What is reject table?
    Stores invalid records.
    
19. What is lookup transformation in SSIS?
    Retrieves reference data from another table.
    
20. What is data lineage?
    Tracking data from source to report.
    

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B. SQL-Based SSIS ETL Testing Questions (21–45)

Record Count Validation

SELECT COUNT(*) FROM src_orders;

SELECT COUNT(*) FROM tgt_fact_orders;

21. How do you validate record counts?
    Compare counts across source, staging, target.
    
22. How to identify duplicate records?
    

SELECT order_id, COUNT(*)

FROM stg_orders

GROUP BY order_id

HAVING COUNT(*) > 1;

23. How do you validate JOIN logic?
    

SELECT o.order_id, c.customer_name

FROM orders o

JOIN customers c

ON o.customer_id = c.customer_id;

24. How do you validate aggregation logic?
    

SELECT customer_id, SUM(order_amount)

FROM fact_orders

GROUP BY customer_id;

25. How do you detect missing records?
    

SELECT s.id

FROM source_table s

LEFT JOIN target_table t

ON s.id = t.id

WHERE t.id IS NULL;

26. Why is GROUP BY important in ETL testing?
    Validates totals and summaries.
    
27. How to validate null handling?
    

SELECT COUNT(*) FROM dim_customer WHERE email IS NULL;

28. What is Slowly Changing Dimension (SCD)?
    Technique to manage dimension changes.
    
29. Difference between SCD Type 1 and Type 2?
    Type 1 overwrites data; Type 2 keeps history.
    
30. SCD2 validation query
    

SELECT customer_id, COUNT(*)

FROM dim_customer

GROUP BY customer_id

HAVING COUNT(*) > 1;

31. How do you identify current active records?
    Use current_flag = ‘Y’.
    
32. What is hashing in ETL testing?
    Used to detect data changes.
    
33. How do you validate derived columns?
    

SELECT amount * tax_rate AS expected_tax

FROM stg_sales;

34. How do you validate date transformations?
    

SELECT *

FROM fact_orders

WHERE order_date > CURRENT_DATE;

35. What is lookup cache testing?
    Validate cached lookup values.
    
36. What is control table?
    Tracks batch status & counts.
    
37. What is watermark column?
    Used for incremental loads.
    
38. What is late arriving dimension?
    Fact arrives before dimension.
    
39. Difference between truncate and delete?
    Truncate is faster and non-logged.
    
40. How do you validate decimal precision?
    

SELECT CAST(amount AS DECIMAL(10,2))

FROM stg_sales;

41. What is metadata testing?
    Validating schema & data types.
    
42. What is factless fact table?
    Tracks events without measures.
    
43. What is idempotent ETL?
    Same output on multiple runs.
    
44. What is data balancing?
    Totals match across systems.
    
45. What is late arriving fact?
    Fact arrives after reporting cycle.
    

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C. Advanced & Performance SSIS ETL Questions (46–80)

Window Function Example

SELECT customer_id,

ROW_NUMBER() OVER (PARTITION BY customer_id ORDER BY updated_ts DESC) rn

FROM dim_customer;

46. Why use window functions in ETL testing?
    Deduplication, ranking, SCD logic.
    
47. What is ETL performance testing?
    Measuring load time and throughput.
    
48. How do you tune slow SSIS packages?
    Indexing, batch size tuning, parallelism.
    
49. What is SSIS parallel execution?
    Running tasks concurrently.
    
50. What is partitioning?
    Splitting data for faster processing.
    
51. How do you validate data freshness?
    

SELECT MAX(load_date) FROM fact_sales;

52. What is ETL regression testing?
    Ensuring changes don’t break logic.
    
53. How do you test error handling in SSIS?
    Validate error output & logs.
    
54. What is data skew?
    Uneven data distribution.
    
55. What is bulk load in SSIS?
    High-volume data loading.
    
56. How do you validate historical data accuracy?
    Check effective_date ranges.
    
57. What is schema evolution testing?
    Handling source structure changes.
    
58. What is data latency?
    Delay from source to DW.
    
59. How do you test negative scenarios?
    Invalid, null, boundary values.
    
60. What is reconciliation report?
    Summary of counts & totals.
    
61. What is ETL restartability?
    Resume after failure.
    
62. What is data anonymization testing?
    Validate masking of sensitive data.
    
63. How do you validate surrogate key uniqueness?
    

SELECT sk, COUNT(*)

FROM dim_customer

GROUP BY sk

HAVING COUNT(*) > 1;

64. What is audit trail testing?
    Validate batch_id and timestamps.
    
65. What is data archival testing?
    Old data moved correctly.
    
66. What is transformation logic testing?
    Validate business rules.
    
67. What is end-to-end ETL testing?
    Source → report validation.
    
68. Difference between OLTP and OLAP?
    Transactions vs analytics.
    
69. What is data drift?
    Unexpected changes in data patterns.
    
70. What is reject analysis?
    Root cause analysis of rejected records.
    
71. How do you validate currency conversion?
    

SELECT local_amt * rate = usd_amt

FROM stg_sales;

72. What is data mart testing?
    Validating subject-specific DW areas.
    
73. What is checkpoint in SSIS?
    Restart package from failure point.
    
74. What is logging in SSIS?
    Capturing execution details.
    
75. What is most critical SSIS testing skill?
    Strong SQL + package understanding.
    
76. What is precedence constraint testing?
    Validate task execution order.
    
77. What is package deployment testing?
    Validate environment configurations.
    
78. What is connection manager testing?
    Validate source/target connectivity.
    
79. What is environment variable testing?
    Validate dynamic configurations.
    
80. Biggest challenge in SSIS ETL testing?
    Large data volumes with complex transformations.
    

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5. ETL Tools Commonly Used with SSIS

• Informatica
  
• Microsoft SSIS
  
• Ab Initio
  
• Pentaho
  
• Talend
  

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6. ETL Defect Examples + Sample Test Case

Defect: Duplicate records in fact table
Root Cause: Incorrect JOIN condition
Fix: Correct join + hashing logic

Sample Test Case

Field	Value
Scenario	Duplicate detection
SQL	GROUP BY HAVING COUNT > 1
Expected	No duplicates

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7. Quick Revision Sheet (SSIS Interview Ready)

• Validate record counts, sums, duplicates
  
• Understand SCD1 vs SCD2
  
• Practice JOIN, GROUP BY, window functions
  
• Focus on performance tuning & logging
  

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8. FAQs (Featured Snippet Optimised)

Q1. Is SQL mandatory for SSIS ETL testing?
Yes, SQL is the primary validation skill.

Q2. Which SCD type is most asked in interviews?
SCD Type 2.

Q3. What is the key focus in SSIS interviews?
SQL validation + package flow understanding.