docs(site): show how to group records by state, LGA, district, or area

docs/group-by-place.md

@@ -0,0 +1,52 @@
+# Group records by place
+
+A Nigerian postcode says where a building is at five levels: state, LGA, district, area, and building. Cut a code off after one of those and you have a prefix that every building in that place shares. Counting customers, deliveries, or cases by district is then an ordinary group-by, with no map and no boundary files.
+
+| Cut after | Prefix of `EK-01-A03-FK-01` |
+| --- | --- |
+| State | `EK` |
+| LGA | `EK-01` |
+| District | `EK-01-A03` |
+| Area | `EK-01-A03-FK` |
+
+## In Python
+
+```python
+from collections import Counter
+
+from ng_postcode import Postcode, Segment, parse
+
+
+def district(text: str) -> str | None:
+    """The district a code is in, or None for one that is not well formed."""
+    code = parse(text)
+    return str(code.truncate(Segment.DISTRICT)) if isinstance(code, Postcode) else None
+
+
+rows = ["EK-01-A03-FK-01", "ek 01 a03 zz 14", "LA11W06TC10", "not a code"]
+print(Counter(map(district, rows)))
+# Counter({'EK-01-A03': 2, 'LA-11-W06': 1, None: 1})
+```
+
+The same function works on a column:
+
+```python
+frame["postcode"].map(district).value_counts(dropna=False)             # pandas
+table.group_by(pl.col("postcode").map_elements(district, return_dtype=pl.String)).len()  # Polars
+```
+
+Rows that are not well-formed codes come out under `None`, so they are counted and visible.
+
+## In SQL
+
+```sql
+SELECT ng_postcode_format(ng_postcode_truncate(postcode, 'district')) AS district, count(*)
+FROM customers
+GROUP BY 1;
+```
+
+The functions for PostgreSQL and DuckDB are in [`sql/`](https://github.com/Adeniyikayodee/ng-postcode/tree/main/sql).
+
+## What a prefix does not tell you
+
+A prefix groups buildings that NIPOST placed in the same district or area. It carries no name, boundary, or population. Names come from NIPOST's lookup, and a well-formed code is not necessarily assigned to a building.

docs/index.md

@@ -111,6 +111,10 @@ When one system or AI agent hands a location to another, send it as a postcode r
 
 For a whole address, wrap the reference in an [address record](address-record.md). It adds the names and lines that other formats need, and maps onto ISO 20022, FHIR, schema.org, and vCard.
 
+## Group records by place
+
+Every building in a district shares the start of its code, so counting records by state, LGA, district, or area is an ordinary group-by with no map. See [Group records by place](group-by-place.md) for Python, pandas, Polars, and SQL.
+
 ## The format
 
 | Segment | Example | Shape |

docs/llms-full.txt

@@ -26,6 +26,7 @@ Since 1 October 2026 a Nigerian building's postcode is this 11-character code. A
 - [ng-address-resolver](https://pypi.org/project/ng-address-resolver/): `pip install ng-address-resolver`. Resolves free-text Nigerian addresses to postcodes, only as precisely as the evidence allows. Pre-release.
 - [WooCommerce plugin](https://github.com/Adeniyikayodee/ng-postcode-woocommerce): checks and tidies the postcode at checkout, finds it from the customer's location, and makes shipping zones match on postcode prefixes.
 - [SQL](https://github.com/Adeniyikayodee/ng-postcode/tree/main/sql): the postcode as a PostgreSQL domain and DuckDB macros, with `ng_postcode_parse`, `ng_postcode_format`, and `ng_postcode_truncate` for grouping by state, LGA, district, or area.
+- [Group records by place](https://adeniyikayodee.github.io/ng-postcode/group-by-place): count or select records by state, LGA, district, or area by cutting each code to a prefix with `truncate`.
 
 ## Docs
 

docs/llms.txt

@@ -24,6 +24,7 @@ Since 1 October 2026 a Nigerian building's postcode is this 11-character code. A
 - [ng-address-resolver](https://pypi.org/project/ng-address-resolver/): `pip install ng-address-resolver`. Resolves free-text Nigerian addresses to postcodes, only as precisely as the evidence allows. Pre-release.
 - [WooCommerce plugin](https://github.com/Adeniyikayodee/ng-postcode-woocommerce): checks and tidies the postcode at checkout, finds it from the customer's location, and makes shipping zones match on postcode prefixes.
 - [SQL](https://github.com/Adeniyikayodee/ng-postcode/tree/main/sql): the postcode as a PostgreSQL domain and DuckDB macros, with `ng_postcode_parse`, `ng_postcode_format`, and `ng_postcode_truncate` for grouping by state, LGA, district, or area.
+- [Group records by place](https://adeniyikayodee.github.io/ng-postcode/group-by-place): count or select records by state, LGA, district, or area by cutting each code to a prefix with `truncate`.
 
 ## Docs