absolutelyskilled/analytics-engineering

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Analytics Engineering

A disciplined framework for building trustworthy, well-tested data transformation pipelines using dbt and modern analytics engineering practices. This skill covers dbt model layering, semantic layer design, metrics definitions, dimensional modeling, and self-serve analytics patterns. It is opinionated about dbt Core/Cloud but the modeling principles apply to any SQL-based transformation tool. The goal is to help you build a data warehouse that analysts can trust and navigate without engineering support.

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When to use this skill

Trigger this skill when the user:

• Sets up a new dbt project or restructures an existing one
• Designs the model layer hierarchy (staging, intermediate, marts)
• Writes or reviews dbt models using ref(), source(), or macros
• Defines metrics in YAML (dbt Metrics, MetricFlow, or Cube)
• Builds a semantic layer for self-serve analytics
• Implements slowly changing dimensions (SCD Type 1, 2, 3)
• Writes dbt tests (generic, singular, or custom) and data contracts
• Configures sources, exposures, or freshness checks
• Asks about dimensional modeling (star schema, snowflake schema, OBT)

Do NOT trigger this skill for:

• Data pipeline orchestration (Airflow, Dagster) unrelated to dbt models
• Raw data ingestion or ELT tool configuration (Fivetran, Airbyte connectors)

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Key principles

1. Layer your models deliberately - Use a three-layer architecture: staging (1:1 with source tables, rename and cast only), intermediate (business logic joins and filters), and marts (wide, denormalized tables ready for analysts). Every model lives in exactly one layer. No skipping layers.

2. One source of truth per grain - Each mart model must have a clearly defined grain (one row = one what?). Document it in the YAML schema. If two mart models have the same grain, one of them should not exist.

3. Test everything that matters, nothing that doesn't - Test primary keys with unique and not_null. Test foreign keys with relationships. Test business rules with custom singular tests. Do not write tests that duplicate what the warehouse already enforces.

4. Metrics are code, not queries - Define metrics in version-controlled YAML, not in BI tool calculated fields. This ensures a single definition that every consumer (dashboard, ad-hoc query, API) shares. Disagreements about numbers end when metric definitions are in the repo.

5. Build for self-serve, not for tickets - Every mart should be understandable by a non-engineer. Use clear column names (no abbreviations), add descriptions to every column in the YAML schema, and expose models as documented datasets in the BI tool. If analysts file tickets asking what a column means, the model is incomplete.

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Core concepts

Model layer architecture

| Layer | Prefix | Purpose | Example | |---|---|---|---| | Staging | stg_ | 1:1 with source, rename + cast + basic cleaning | stg_stripe__payments | | Intermediate | int_ | Business logic, joins across staging models | int_orders__pivoted_payments | | Marts | fct_ / dim_ | Analyst-facing, denormalized, documented | fct_orders, dim_customers |

Staging models should be views (no materialization cost). Intermediate models are tables or ephemeral depending on reuse. Marts are always tables (or incremental).

Dimensional modeling

Fact tables (fct_) contain measurable events at a specific grain - orders, payments, page views. They hold foreign keys to dimension tables and numeric measures.

Dimension tables (dim_) contain descriptive attributes - customers, products, dates. They provide the "who, what, where, when" context for facts.

One Big Table (OBT) is a pre-joined wide table combining facts and dimensions. Use OBT for BI tools that perform poorly with joins. It trades storage for query simplicity.

The semantic layer

A semantic layer sits between the data warehouse and consumers (BI tools, notebooks, APIs). It defines metrics, dimensions, and entities in a declarative format so that every consumer gets the same answers. dbt's MetricFlow, Cube, and Looker's LookML are implementations of this pattern. The semantic layer eliminates "which number is right?" debates by making metric logic authoritative and centralized.

Incremental models

For large fact tables, use dbt incremental models to process only new/changed rows instead of rebuilding the entire table. The is_incremental() macro gates the WHERE clause to filter for rows since the last run. Always define a unique_key to handle late-arriving or updated records via merge behavior.

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Common tasks

Set up dbt project structure

my_project/
  dbt_project.yml
  models/
    staging/
      stripe/
        _stripe__models.yml    # source + model definitions
        _stripe__sources.yml   # source freshness config
        stg_stripe__payments.sql
        stg_stripe__customers.sql
      shopify/
        _shopify__models.yml
        _shopify__sources.yml
        stg_shopify__orders.sql
    intermediate/
      int_orders__pivoted_payments.sql
    marts/
      finance/
        _finance__models.yml
        fct_orders.sql
        dim_customers.sql
      marketing/
        _marketing__models.yml
        fct_ad_spend.sql
  tests/
    singular/
      assert_order_total_positive.sql
  macros/
    cents_to_dollars.sql

Use underscores for filenames, double underscores to separate source system from entity (e.g. stg_stripe__payments). Group staging models by source system, marts by business domain.

Write a staging model

Staging models rename, cast, and apply minimal cleaning. No joins, no business logic.

-- models/staging/stripe/stg_stripe__payments.sql
with source as (
    select * from {{ source('stripe', 'payments') }}
),

renamed as (
    select
        id as payment_id,
        order_id,
        cast(amount as integer) as amount_cents,
        cast(created as timestamp) as created_at,
        status,
        lower(currency) as currency
    from source
)

select * from renamed

Build a mart fact table

-- models/marts/finance/fct_orders.sql
{{
    config(
        materialized='incremental',
        unique_key='order_id',
        on_schema_change='sync_all_columns'
    )
}}

with orders as (
    select * from {{ ref('stg_shopify__orders') }}
),

payments as (
    select * from {{ ref('int_orders__pivoted_payments') }}
),

final as (
    select
        orders.order_id,
        orders.customer_id,
        orders.order_date,
        orders.status,
        payments.total_amount_cents,
        payments.payment_method,
        payments.total_amount_cents / 100.0 as total_amount_dollars
    from orders
    left join payments on orders.order_id = payments.order_id
    {% if is_incremental() %}
    where orders.updated_at > (select max(updated_at) from {{ this }})
    {% endif %}
)

select * from final

Define metrics in YAML (MetricFlow)

# models/marts/finance/_finance__models.yml
semantic_models:
  - name: orders
    defaults:
      agg_time_dimension: order_date
    model: ref('fct_orders')
    entities:
      - name: order_id
        type: primary
      - name: customer_id
        type: foreign
    dimensions:
      - name: order_date
        type: time
        type_params:
          time_granularity: day
      - name: status
        type: categorical
    measures:
      - name: order_count
        agg: count
        expr: order_id
      - name: total_revenue_cents
        agg: sum
        expr: total_amount_cents
      - name: average_order_value_cents
        agg: average
        expr: total_amount_cents

metrics:
  - name: revenue
    type: derived
    label: "Total Revenue"
    description: "Sum of all order payments in dollars"
    type_params:
      expr: total_revenue_cents / 100
      metrics:
        - name: total_revenue_cents
  - name: order_count
    type: simple
    label: "Order Count"
    type_params:
      measure: order_count

Write dbt tests and data contracts

# models/marts/finance/_finance__models.yml
models:
  - name: fct_orders
    description: "One row per order. Grain: order_id."
    config:
      contract:
        enforced: true
    columns:
      - name: order_id
        data_type: varchar
        description: "Primary key - unique order identifier"
        tests:
          - unique
          - not_null
      - name: customer_id
        description: "FK to dim_customers"
        tests:
          - not_null
          - relationships:
              to: ref('dim_customers')
              field: customer_id
      - name: total_amount_cents
        data_type: integer
        description: "Total order value in cents"
        tests:
          - not_null
          - dbt_utils.accepted_range:
              min_value: 0

-- tests/singular/assert_order_total_positive.sql
-- Returns rows that violate the rule (should return 0 rows to pass)
select order_id, total_amount_cents
from {{ ref('fct_orders') }}
where total_amount_cents < 0

Configure source freshness

# models/staging/stripe/_stripe__sources.yml
sources:
  - name: stripe
    database: raw
    schema: stripe
    loaded_at_field: _loaded_at
    freshness:
      warn_after: { count: 12, period: hour }
      error_after: { count: 24, period: hour }
    tables:
      - name: payments
        description: "Raw Stripe payment events"
        columns:
          - name: id
            tests:
              - unique
              - not_null

Run dbt source freshness in CI to catch stale source data before it propagates into marts.

Build a self-serve dimension table

-- models/marts/finance/dim_customers.sql
with customers as (
    select * from {{ ref('stg_shopify__customers') }}
),

orders as (
    select * from {{ ref('fct_orders') }}
),

customer_metrics as (
    select
        customer_id,
        count(*) as lifetime_order_count,
        sum(total_amount_cents) as lifetime_value_cents,
        min(order_date) as first_order_date,
        max(order_date) as most_recent_order_date
    from orders
    group by customer_id
),

final as (
    select
        customers.customer_id,
        customers.full_name,
        customers.email,
        customers.created_at as customer_since,
        coalesce(customer_metrics.lifetime_order_count, 0)
            as lifetime_order_count,
        coalesce(customer_metrics.lifetime_value_cents, 0)
            as lifetime_value_cents,
        customer_metrics.first_order_date,
        customer_metrics.most_recent_order_date,
        case
            when customer_metrics.lifetime_order_count >= 5
                then 'high_value'
            when customer_metrics.lifetime_order_count >= 2
                then 'returning'
            else 'new'
        end as customer_segment
    from customers
    left join customer_metrics
        on customers.customer_id = customer_metrics.customer_id
)

select * from final

Every column has a clear, human-readable name. Analysts should never need to ask what lv_cents means - call it lifetime_value_cents.

---

Anti-patterns / common mistakes

| Mistake | Why it's wrong | What to do instead | |---|---|---| | Business logic in staging models | Staging should be a clean 1:1 mirror; mixing logic here makes debugging impossible | Move all joins, filters, and calculations to intermediate or mart layers | | Metrics defined in BI tool only | Multiple dashboards will define "revenue" differently, causing trust erosion | Define metrics in YAML (MetricFlow/Cube) and expose through the semantic layer | | No grain documentation | Without a stated grain, analysts build incorrect aggregations (double-counting) | Add "Grain: one row per X" to every mart model's YAML description | | Skipping the intermediate layer | Mart models become 300+ line monsters with 8 CTEs and nested joins | Extract reusable transformations into int_ models that marts can ref() | | Using SELECT * in models | Schema changes upstream silently add/remove columns, breaking downstream | Explicitly list every column in staging models | | Hardcoded filter values | WHERE status != 'test' in 12 models; when the value changes, half get missed | Create a macro or a staging-layer filter applied once at the source boundary | | No incremental strategy for large tables | Full table rebuilds take hours and spike warehouse costs | Use incremental models with a reliable updated_at or event timestamp |

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Gotchas

1. Incremental models with a broken unique_key silently duplicate rows - If the unique_key doesn't match how the source system generates IDs (e.g., composite keys, NULL-able columns), the merge strategy falls back to appending and your fact table will have duplicate rows. Always test with a unique dbt test on the mart's primary key after the first incremental run.

2. ref() creates a compile-time dependency but not a runtime guarantee - dbt's ref() ensures build order, but if a staging model's source table is empty or missing, the downstream mart builds with zero rows and no error. Add not_null and row count tests to staging models so silent empty builds surface in CI.

3. Metrics defined in both the semantic layer and the BI tool diverge - If analysts can also create calculated fields in Looker/Tableau/Power BI, they will. Within months there will be two definitions of "revenue" and no one knows which is correct. Enforce a semantic-layer-first policy and audit BI tool custom fields quarterly.

4. SELECT * in staging models breaks on upstream schema changes - When a source table adds or removes a column, SELECT * staging models silently change shape, potentially breaking downstream marts. Explicitly list every column in staging models so schema changes cause a compile error rather than silent breakage.

5. Hardcoded dates in incremental WHERE clauses don't survive full refreshes - An incremental model that filters with where created_at > '2024-01-01' will drop historical data on a --full-refresh. Use {{ this }} to reference the current max timestamp, and document what happens on a forced full refresh.

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References

For detailed patterns and implementation guidance, load the relevant file from references/:

• references/dbt-patterns.md - Advanced dbt patterns including macros, packages, hooks, custom materializations, and CI/CD integration
• references/semantic-layer.md - Deep dive into MetricFlow configuration, Cube setup, dimension/measure types, and BI tool integration
• references/self-serve-analytics.md - Patterns for building analyst-friendly data platforms, documentation strategies, and data catalog integration

Only load a references file if the current task requires it - they are long and will consume context.

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