I wanted the angle of the first side of a polygon in SQL Server 2008+.  Our code for entering the shapes evolved over time, specifically which direction drew the shapes.

For example: starting with the NW point drawing clockwise (the anglican way) or drawing counter-clockwise (the way SQL draws Geography).

I have to convert to geography on the fly because the geometry in the DB is combined with other shapes, hence the 2-level CTE.


WITH PolygonGeo (Id, Polygon)
AS (
SELECT
 [MyIDColumn] AS [id]
 ,geometry::STGeomFromText(GeometryText, 4326) as [Polygon]
FROM [MyDatabase].[dbo].[MyTable]
WHERE GeometryText LIKE 'POLYGON%'
),
Points ([Id], Pt0, Pt1)
AS (
SELECT Id
, [Polygon].STPointN(1) AS Pt0
, [Polygon].STPointN(2) AS Pt1
FROM PolygonGeo
)
SELECT [Id]
, ATN2(Pt1.STX - Pt0.STX, Pt1.STY - Pt0.STY) / PI() * 180  AS Angle
FROM Points

Results:

Id_8	Angle
33BAA7F	180
167AAEA	176.395579460831
F6469B3	180
33BAA7F	180
167AAEA	177.137594773874
0D34759	154.490530508003
33BAA7F	165.499833233449
CB398C0	180
CB398C0	143.383248480932
F6469B3	179.283840054506
33BAA7F	-175.114797431858