UnionAggregate (geometry Data Type)
Applies to: 
SQL Server
Azure SQL Database
Azure SQL Managed Instance
Performs a union operation on a set of geometry objects.
Syntax
UnionAggregate ( geometry_operand )
Arguments
geometry_operand
Is a geometry type table column that holds the set of geometry objects on which to perform a union operation.
Return Types
SQL Server return type: geometry
Exceptions
Throws a FormatException when there are input values that are not valid. See STIsValid (geometry Data Type)
Remarks
Method returns null when the input is empty or the input has different SRIDs. See Spatial Reference Identifiers (SRIDs)
Method ignores null inputs.
Note
Method returns null if all inputted values are null.
Examples
The following example returns the union of a set of geometry objects in a table variable.
-- Setup table variable for UnionAggregate example
DECLARE @Geom TABLE
(
shape geometry,
shapeType nvarchar(50)
);
INSERT INTO @Geom(shape,shapeType)
VALUES('CURVEPOLYGON(CIRCULARSTRING(2 3, 4 1, 6 3, 4 5, 2 3))', 'Circle'),
('POLYGON((1 1, 4 1, 4 5, 1 5, 1 1))', 'Rectangle');
-- Perform UnionAggregate on @Geom.shape column
SELECT geometry::UnionAggregate(shape).ToString()
FROM @Geom;