Document FunctionsBuilder::stringAgg()

Adds documentation for the new portable stringAgg() method, covering
per-driver translation and supported ordering.

Author: dereuromark

docs/en/appendices/5-4-migration-guide.md

@@ -98,6 +98,9 @@ version is reported as `unknown`), the header is omitted.
   See [Query Builder](../orm/query-builder#advanced-conditions).
 - Added `inOrNull()` and `notInOrNull()` methods for combining `IN` conditions with `IS NULL`.
 - Added `isDistinctFrom()` and `isNotDistinctFrom()` methods for null-safe comparisons.
+- Added `FunctionsBuilder::stringAgg()` for portable string aggregation.
+  Translates to `STRING_AGG` or `GROUP_CONCAT` per driver.
+  See [Query Builder](../orm/query-builder#string-aggregation).
 - Added PostgreSQL index access method reflection. Non-btree indexes (`gin`,
   `gist`, `spgist`, `brin`, `hash`) are now reflected with an `accessMethod`
   field and regenerated with the correct `USING` clause. The `Index` class

docs/en/orm/query-builder.md

@@ -360,6 +360,11 @@ Calculate the max of a column. `Assumes arguments are literal values.`
 `count()`
 Calculate the count. `Assumes arguments are literal values.`
 
+`stringAgg()`
+Aggregate string values using a separator. Translates to `STRING_AGG()`,
+`GROUP_CONCAT()`, or `LISTAGG()` depending on the database driver.
+`Assumes the first argument is a literal value.`
+
 `cast()`
 Convert a field or expression from one data type to another.
 
@@ -471,6 +476,38 @@ FROM articles;
 > [!NOTE]
 > Use `func()` to pass untrusted user data to any SQL function.
 
+#### String Aggregation
+
+The `stringAgg()` method provides a portable way to aggregate string values
+using a separator. It translates to the appropriate native SQL function for
+each driver (`STRING_AGG()` on PostgreSQL and SQL Server, `GROUP_CONCAT()` on
+MySQL, and `STRING_AGG()` or `GROUP_CONCAT()` on MariaDB/SQLite depending on
+version):
+
+```php
+$query = $articles->find();
+$query->select([
+    'category_id',
+    'titles' => $query->func()->stringAgg('title', ', '),
+])
+->groupBy('category_id');
+```
+
+You can optionally specify an ordering for the aggregated values via the
+third argument:
+
+```php
+$query->func()->stringAgg('title', ', ', ['title' => 'ASC']);
+```
+
+`STRING_AGG` with aggregate-local ordering is supported on PostgreSQL,
+SQL Server, MariaDB 10.5+ and SQLite 3.44+. MySQL translates the call to
+`GROUP_CONCAT` in all cases.
+
+::: info Added in version 5.4.0
+`FunctionsBuilder::stringAgg()` was added.
+:::
+
 ### Ordering Results
 
 To apply ordering, you can use the `orderBy()` method: