UnionAggregate (geography Data Type)
Applies to: 
SQL Server
Azure SQL Database
Azure SQL Managed Instance
SQL database in Microsoft Fabric
Performs a union operation on a set of geography objects.
UnionAggregate ( geography_operand )
geography_operand
Is a geography type table column that holds the set of geography objects on which to perform a union operation.
SQL Server return type: geography
Method returns null if the input has different SRIDs. See Spatial Reference Identifiers (SRIDs).
Method ignores null inputs.
Note
Method returns null if all inputted values are null.
The following example performs a UnionAggregate on a set of geography location points within a city.
USE AdventureWorks2022
GO
SELECT City,
geography::UnionAggregate(SpatialLocation) AS SpatialLocation
FROM Person.Address
WHERE PostalCode LIKE('981%')
GROUP BY City;