Extract and Aggregate Metrics with SQL | Dataford Interview Questions

Describe how you would use database querying languages to extract and aggregate performance metrics from a data source.

Using the provided source metadata and performance records, write a query for measurements recorded from 2025-01-01 through 2025-03-31. Include only records whose status is success or failed, while retaining sources with no qualifying records.

Output

One row per source, ordered by source_id ascending.
Columns: source_id, source_name, source_type, completed_measurements, avg_response_ms, peak_response_ms, avg_throughput, and success_rate_pct.
Return NULL metric values for sources without qualifying records.

Schema

sources

Column	Type	Description
source_idPK	INT	Unique identifier for the data source
source_name	VARCHAR(100)	Display name of the data source
source_type	VARCHAR(50)	Classification of the data source

performance_metrics

Column	Type	Description
metric_idPK	INT	Unique identifier for the performance measurement
source_id	INT	Source associated with the measurement
measured_at	DATE	Date on which the measurement was recorded
status	VARCHAR(20)	Outcome of the measurement
response_ms	DECIMAL(10,2)	Response time in milliseconds
throughput	DECIMAL(10,2)	Measured throughput value

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Problem