#1348 MSSQL adding and dropping multiple columns fix

[nickwhiteley]

This is my first pull request so apologies if something isn't right.
It includes changes to MSSQL for adding and dropping columns. Previously single column changes worked but multiple columns didn't.
#1348

[wolfgang42]

I can confirm that the tests pass for MSSQL.

This isn't specifically related to your PR, but it looks like the MSSQL unit tests need to be checked to make sure they are actually correct T-SQL.

[nickwhiteley]

Indeed. I have had a quick look and there are 10 which don't have valid T-SQL. I don't have time today but will try to investigate further over the next week.

[rhys-vdw]

Thanks @nickwhiteley! 👍