SQL Server Journey with SQL Authority

Follow up questions are the most fun part of writing a blog post. Earlier I wrote about SQL SERVER – Validating Spatial Object with IsValidDetailed Function and today I received a follow up question on the same subject. The question was mainly about how NULL is handled by spatial functions. Well, NULL is NULL. It is very easy to work with NULL. There are two different ways to validate if the passed in the value is NULL or not.

1) Using IsNULL Function

IsNULL function validates if the object is null or not, if object is not null it will return you value 0 and if object is NULL it will return you the value NULL.

DECLARE @p GEOMETRY = 'Polygon((2 2, 3 3, 4 4, 5 5, 6 6, 2 2))'
SELECT @p.ISNULL ObjIsNull
GO
DECLARE @p GEOMETRY = NULL
SELECT @p.ISNULL ObjIsNull
GO

2) Using IsValidDetailed Function

IsValidateDetails function validates if the object is valid or not. If the object is valid it will return 24400: Valid but if the object is not valid it will give message with the error number. In case object is NULL it will return the value as NULL.

DECLARE @p GEOMETRY = 'Polygon((2 2, 3 3, 4 4, 5 5, 6 6, 2 2))'
SELECT @p.IsValidDetailed() IsValid
GO
DECLARE @p GEOMETRY = NULL
SELECT @p.IsValidDetailed() IsValid
GO

When to use what?

Now you can see that there are two different ways to validate the NULL values. I personally have no preference about using one over another. However, there is one clear difference between them. In case of the IsValidDetailed Function the return value is nvarchar(max) and it is not always possible to compare the value with nvarchar(max). Whereas the ISNULL function returns the bit value of 0 when the object is null and it is easy to determine if the object is null or not in the case of ISNULL function. Additionally, ISNULL function does not check if the object is valid or not and will return the value 0 if the object is not NULL.

Now you know even though either of the function can be used in place of each other both have very specific use case. Use the one which fits your business case.

Reference: Pinal Dave (http://blog.sqlauthority.com)

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Recently I was reading excellent blog post by Lenni Lobel on Spatial Database. He has written very interesting function ShortestLineTo in Spatial Data Classes. I really loved this new feature of the finding shortest distance between two shapes in SQL Server. Following is the example which is same as Lenni talk on his blog article .

DECLARE @Shape1 geometry = 'POLYGON ((-20 -30, -3 -26, 14 -28, 20 -40, -20 -30))'
DECLARE @Shape2 geometry = 'POLYGON ((-18 -20, 0 -10, 4 -12, 10 -20, 2 -22, -18 -20))'
SELECT @Shape1
UNION ALL
SELECT @Shape2
UNION ALL
SELECT @Shape1.ShortestLineTo(@Shape2).STBuffer(.25)
GO

When you run this script SQL Server finds out the shortest distance between two shapes and draws the line. We are using STBuffer so we can see the connecting line clearly.

Now let us modify one of the object and then we see how the connecting shortest line works.

DECLARE @Shape1 geometry = 'POLYGON ((-20 -30, -3 -30, 14 -28, 20 -40, -20 -30))'
DECLARE @Shape2 geometry = 'POLYGON ((-18 -20, 0 -10, 4 -12, 10 -20, 2 -22, -18 -20))'
SELECT @Shape1
UNION ALL
SELECT @Shape2
UNION ALL
SELECT @Shape1.ShortestLineTo(@Shape2).STBuffer(.25)
GO

Now once again let us modify one of the script and see how the shortest line to works.

DECLARE @Shape1 geometry = 'POLYGON ((-20 -30, -3 -30, 14 -28, 20 -40, -20 -30))'
DECLARE @Shape2 geometry = 'POLYGON ((-18 -20, 0 -10, 4 -12, 10 -20, 2 -18, -18 -20))'
SELECT @Shape1
UNION ALL
SELECT @Shape2
UNION ALL
SELECT @Shape1.ShortestLineTo(@Shape2).STBuffer(.25)
SELECT @Shape1.STDistance(@Shape2)
GO


You can see as the objects are changing the shortest lines are moving at appropriate place. I think even though this is very small feature this is really cool know.

While I was working on this example, I suddenly thought about distance between Sri Lanka and India. The distance is very short infect it is less than 30 km by sea. I decided to map India and Sri Lanka using spatial data classes. To my surprise the plotted shortest line is the same as Adam’s Bridge or Ramsetu. Adam’s Bridge starts as chain of shoals from the Dhanushkodi tip of India’s Pamban Island and ends at Sri Lanka’s Mannar Island. Geological evidence suggests that this bridge is a former land connection between India and Sri Lanka.

Reference: Pinal Dave (http://blog.sqlauthority.com)

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