Schema Mapping

Skill: Semantic Schema Mapping Planning

Follow this structured process and procedures for analyzing source-to-target data relationships and creating a high-fidelity Mapping Plan (also known as a Mapping Manifesto). This process is platform-agnostic and should be used before generating any target-specific pipeline code. When this skill is loaded, you MUST use this plan and replace the existing generic plan.

When to Use

Use this skill when:

• You need to map schemas between a source dataset/database and a target destination database/warehouse.
• You are initiating an ETL, ELT, or data integration task.
• You need to identify schema gaps, data type conflicts, or aggregation requirements.

Required Input Variables

Before creating the plan, you must obtain or request:

1. SOURCE_SCHEMAS: Definitions (schemas, tables, fields, types) of the source data.
2. TARGET_SCHEMAS: Definitions of the desired target/destination schemas.
3. BUSINESS_CONTEXT: Domain details, business rules, or use case description.
4. TARGET_PLATFORM: The database or execution engine (e.g., BigQuery, Snowflake, Postgres, Spark, Beam).

Optional Input Variables

1. KNOWLEDGE_GRAPH: If there's a knowledge graph / property graph available, always inspect the graph for node table definitions, edge table definitions, and foreign key bindings (SOURCE / DESTINATION key references), etc.

The Schema Mapping Planning Procedure

[!IMPORTANT] Execution Strategy: Table-by-Table Iteration

You MUST execute this procedure iteratively, one target table at a time. For each individual table in the TARGET_SCHEMAS, complete Steps 1 through 6 sequentially before moving to the next table. Do not attempt to map or summarize multiple tables in a single batch, as this leads to hallucinations, overlooked constraints, and context window bloat.

Step 1: Semantic & Terminology Translation

Analyze the entity names and attributes in the SOURCE_SCHEMAS against the TARGET_SCHEMAS.

1. Synonym Resolution: Using the BUSINESS_CONTEXT, map matching concepts with different names (e.g., client_id vs customer_num).
2. Identify Domain Standards: Match field values or formats to known standards (e.g., ISO country codes, currency codes, UN/LOCODE, UUIDs) based on the business context.
3. Verify Domain Semantics: Do not rely purely on lexical matching (name similarity). Verify the functional business purpose of the entities in both schemas. Ensure that a target table representing a specific business resource maps to a source table modeling that same resource rather than an unrelated administrative log or generic list table sharing a similar name.

Step 2: Establish the Anchor Table

For each table or collection in the TARGET_SCHEMAS:

1. Identify the primary source table (the "Anchor Table") that holds the core records for this target.
2. Identify contributing/lookup tables in the source that will enrich the target records.
3. Prefer Structured Tables over Generic Key-Value Tables: If the same attribute exists in both a structured column in a domain table and as a generic property in an Entity-Attribute-Value (EAV) key-value/properties table, always anchor on the structured table to ensure schema stability and performant joins.

Step 3: Proactive Data Sampling & Inspection

If a target field mapping is ambiguous or schema types do not tell the whole story (e.g., verifying if a timestamp is ISO-8601, if a string is a JSON array, or checking the distribution of values):

1. Proactive Inspection: If environment access allows, run platform-specific queries (e.g., SELECT ... LIMIT 10, SELECT COUNT(DISTINCT ... )) to sample values.
2. User Inquiry: If direct access is not possible, output sample queries and ask the user to provide the output to confirm assumptions before finalizing the plan.

Step 4: Perform Field-Level Gap Analysis & Cleanliness Design

Evaluate every column in each target table to determine its source mapping. Categorize mappings and plan cleanliness transformations:

• Direct Mapping: A 1-to-1 match.
• Derived Mapping: Requires type casting, string manipulation, date formatting, mathematical derivation, or case statement logic.
• Joined Mapping: Requires looking up values from contributing tables using defined join keys.
• Aggregated Mapping: Requires collapsing 1-to-many relationships (e.g., calculating SUM, COUNT, ARRAY_AGG or string concatenation).
• Gaps (Unmapped fields): Target fields that do not exist in the source.
  • Constraint Checking: Verify whether the target column has a NOT NULL or REQUIRED constraint in the target schema.
  • Handling Nullable Gaps: If the target column is nullable, explicitly flag it as NULL or define a default value.
  • Handling Non-Nullable Gaps: If the target column is NOT NULL, you MUST NOT map it to NULL. (Rationale: Mapping NOT NULL target columns to NULL will cause execution-time database constraint violations and pipeline failures). You must identify a source field to derive it from, default it to a valid non-null placeholder (e.g., 'UNKNOWN', 0, or default dates), or define logic to generate a valid unique reference.

Universal Data Cleanliness Rules to Incorporate in Mappings:

1. Null Standardization: Map source strings like "NULL", "None", "N/A", or empty spaces to true database NULL values.
2. Trim & Casing: Plan to trim leading/trailing whitespaces. Convert standardized codes (e.g., ISO codes, status strings) to uppercase.
3. Temporal Consistency: Plan to parse all source timestamps into standard ISO-8601 format (YYYY-MM-DDTHH:MM:SSZ) or standard destination TIMESTAMP format. Plan checks to ensure logical temporal progression (e.g., start_time <= end_time).
4. Defensive Checks: Plan checks for strict destination types (e.g., checking if string is a valid number before casting to DECIMAL).

Step 5: Map Relationships & Joins

Specify the logical join path to connect the Anchor Table with all contributing source tables:

1. Define the join condition/keys (e.g., source_order.customer_id = source_customer.id).
2. Identify join scale properties:
   • Large-to-Large: Joining two high-volume transaction tables.
   • Large-to-Small: Joining a transaction table to a static lookup table (ideal for Map-side/Broadcast joins to optimize speed/cost).
3. Document potential join challenges:
   • Many-to-many risks or potential duplicate generation.
   • Type mismatches on join keys (e.g., joining an INT column to a STRING column).
4. Graph Validation: If a graph was identified in input, cross-reference the proposed join conditions with the graph's edge table definitions to validate foreign key relationships.

Step 6: Draft the "Mapping Manifesto" (Output Format & Example)

Analyze the schemas and reference the ONE_SHOT_EXAMPLE below to structure your output.

ONE_SHOT_EXAMPLE (How to Structure the Mapping Manifesto)

Example Source Schemas

dataset: my_music_librarystyle: {id: int, style: string}band: {name: string, biography: string, style: int}cd: {name: string, year: int, artist: string, numbers: array<string>}track: {id: string, number: int, name: string}

Example Target Schemas

dataset: music_standardartist: {name: string, albums: array<string>}album: {name: string, year: int, genre: string, tracks: array<string>}

The Mapping Manifesto (Expected Output Format)

For each target table, document your column-to-column reasoning:

# Mapping Plan: `album`* **Anchor Source Table**: `cd`* **Join Paths & Optimization**:    * `cd` JOIN `band` ON `cd.artist = band.name`    * `band` JOIN `style` ON `band.style = style.id`    * `cd` JOIN `track` ON `track.id IN UNNEST(cd.numbers)`
### Field Mappings
| Target Field | Source Field / Logic | Mapping Type | Rationale / Transformation Details || :--- | :--- | :--- | :--- || `album.name` | `cd.name` | Direct | A CD is a physical medium representing an album; direct semantic match || `album.year` | `cd.year` | Direct | Direct semantic match for release year || `album.genre` | `style.style` | Joined | Resolved via `cd.artist` -> `band.name` -> `band.style` (ID) -> `style.id` -> `style.style` (String) || `album.tracks` | `ARRAY_AGG(track.name)` | Aggregated | Aggregation Point: Collapses 1-to-many track IDs in `cd.numbers` into an array of track names |

Critical Operational Rules

• Plan Before Execution: You MUST NOT generate any ETL code, target-specific pipeline configurations, or migration scripts until the Mapping Manifesto has been presented to and approved by the user. (Rationale: Establishing clear mapping logic first prevents coding errors, avoids circular dependencies, and ensures user alignment on semantic mappings before wasting resources on implementation).
• Strict Schema Grounding: Every source table and field name referenced in the mapping plan MUST exactly match the names and data types present in the provided SOURCE_SCHEMAS. You MUST NOT reference non-existent columns, guess field names, or make assumptions about source schemas without explicitly confirming them in the schema definitions. (Rationale: Proposing guesses leads to compilation errors and invalid mapping specifications).
• Target Schema Constraint Integrity: You must never propose a mapping that writes NULL to a column defined as NOT NULL or REQUIRED in the TARGET_SCHEMAS.
• Exhaustive Field Search: Before declaring a target field as an "Unmapped / Gap", search all available source schemas to verify the data is not in a less-obvious table. (Rationale: Lazy mappings that default target fields to NULL lead to downstream data loss and incomplete pipelines).
• Defensive Type Mapping: Explicitly plan the transformation rules for strict destination types (e.g. TIMESTAMP, BOOLEAN, DECIMAL) to avoid load failures. (Rationale: Different databases and platforms handle type validation strictly; pre-planning casts prevents execution-time runtime errors).
• Data Integrity Check: Identify join conditions that could cause Cartesian product expansion or data duplication, and note prevention strategies in the plan. (Rationale: Unvalidated joins can distort aggregated metrics or exhaust processing memory on large datasets).