SQL Server ALTER datetime to datetime2 does not work - r

Question

I am trying to convert a "datetime" variable to "datetime2" format.
# Load libraries
library(DBI)
library(tidyverse)
# Create dataframe
df <- data.frame("myid" = stringi::stri_rand_strings(5, 5),
"mydate" = c(Sys.time(), Sys.time()-1, Sys.time()-2, Sys.time()-3, Sys.time()-4) )
# Create SQL table sschema.ttable
DBI::dbWriteTable(conn = connection,
name = DBI::Id(schema = "sschema", table = "ttable"),
value = df,
overwrite = TRUE,
append = FALSE)
# Query for variable type in the SQL table
query <- paste0("exec sp_columns ", "ttable")
query <- DBI::dbSendQuery(connection, query)
res <- NULL
res <- DBI::dbFetch(query)
DBI::dbClearResult(query)
view(res)
# Alter mydate to datetime2
query <- DBI::dbSendStatement(conn = connection,
statement = paste0("ALTER TABLE sschema.ttable ALTER COLUMN mydate datetime2"))
DBI::dbFetch(query)
DBI::dbClearResult(query)
but this leads to the error
Error: nanodbc/nanodbc.cpp:1617: 00000: [Microsoft][ODBC Driver 17 for SQL Server][SQL Server]The UPDATE permission was denied on the object 'ttable', database 'dbo', schema 'sschema'.
'ALTER TABLE sschema.ttablename ALTER COLUMN mydate datetime2'
However, converting another VARCHAR(10) variable in the same table to VARCHAR(100) works fine. Any idea what is the problem? How to get this working?
I am working with Microsoft SQL Azure version 12, by operating on an RStudio-server and the DBI library.

Answer 1

To change the data type of a column you must have both the ALTER permission and UPDATE permission on the table.
From the docs:
Adding a column that updates the rows of the table requires UPDATE permission on the table.
ALTER TABLE - permissions
This goes for ALTERing an existing column too, as you can verify like this:
use tempdb
go
revert
go
if exists(select * from sys.database_principals where name = 'fred')
drop user fred
go
drop table if exists tablename
go
create user fred without login
create table tablename(id int, variablename varchar(20))
go
grant select on tablename to fred
--grant update on tablename to fred --uncomment to clear error
grant alter on schema::dbo to fred
execute as user='fred'
ALTER TABLE dbo.tablename ALTER COLUMN variablename datetime2
revert