def upgrade():
    ### commands auto generated by Alembic - please adjust! ###
    op.add_column('table', sa.Column('col3', sa.Boolean(name='ck_col3'), nullable=True))
    ### end Alembic commands ###
def downgrade():
    ### commands auto generated by Alembic - please adjust! ###
    op.drop_column('table', 'col3', mssql_drop_check=True)
    ### end Alembic commands ###

declare @const_name varchar(256)
select @const_name = [name] from sys.check_constraints
where parent_object_id = object_id('table')
and col_name(parent_object_id, parent_column_id) = 'col3'
-- expected
exec('alter table [table] drop constraint [' + @const_name + ']')
-- actually emitted
exec('alter table table drop constraint ' + @const_name)