STWithin (geometry Data Type)
Applies to: 
SQL Server
Azure SQL Database
Azure SQL Managed Instance
Returns 1 if a geometry instance is completely within another geometry instance; otherwise, returns 0. The STWithin command is case-sensitive.
Syntax
.STWithin ( other_geometry )
Arguments
other_geometry
Is another geometry instance to compare against the instance on which STWithin() is invoked.
Return Types
SQL Server return type: bit
CLR return type: SqlBoolean
Remarks
This method always returns null if the spatial reference IDs (SRIDs) of the geometry instances do not match.
Examples
The following example uses STWithin() to test two geometry instances to see if the first instance is completely within the second instance.
DECLARE @g geometry;
DECLARE @h geometry;
SET @g = geometry::STGeomFromText('POLYGON((0 0, 2 0, 2 2, 0 2, 0 0))', 0);
SET @h = geometry::STGeomFromText('POLYGON((1 1, 3 1, 3 3, 1 3, 1 1))', 0);
SELECT @g.STWithin(@h);