Individual skill

schema-mapper

Document column-level mappings between source and target schemas. Use when integrating data from multiple systems, designing ETL transformations, or documenting how raw fields become analytical assets.

02-documentation-knowledge/schema-mapper/SKILL.md

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# Schema Mapper

# When to use
- Integrating a new data source and need to map its fields to the existing data model
- Designing an ETL or dbt transformation and need to document the logic
- Auditing what happened to a field during a migration
- Onboarding a new analyst who needs to understand where columns come from
- Preparing a data catalog entry that requires lineage at the column level

# Process
1. **Collect the source schema** — list every column name, data type, nullable flag, and a brief description. Pull from `INFORMATION_SCHEMA`, a data dictionary, or the source API documentation.
2. **Collect the target schema** — same structure for the destination table or model. If the target doesn't exist yet, draft it based on the analytical requirements.
3. **Map source columns to target columns** — for each target column, identify the source column(s) that feed it. Record direct mappings (rename only) and derived mappings (calculation, type cast, lookup join). Use `scripts/schema_compare.py` to automate direct-name matches.
4. **Document transformation rules** — for each derived mapping, write the exact transformation (e.g., `CAST(amount_cents AS FLOAT) / 100.0`, `COALESCE(first_name, email)`).