This fixes two problems with MSSQL: * `ALTER TABLE DROP ... IF EXISTS ...` is only supported in SQL Server >16. The `IF EXISTS` here is a belt-and-braces and does not need to be present. Therefore can be dropped. Also stop attempting to drop the indexes as constraints as they're indexes! * System tables like: `sys.indexes` should be lowercase not uppercase because of collation issues. Fix #13615 Signed-off-by: Andrew Thornton <art27@cantab.net> Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>