use **try_cast** instead of **cast** with Microsoft SQL (2012 and later)

[DavidPatShuiFong]

A failure to cast (e.g. as.numeric or as.double, when the string contains a non-numeric character) results in an error:

> library(magrittr)
> x <- mtcars %>% dplyr::mutate(wt = as.character(wt))
> x[1,6] <- "<2.62" # adds a non-numeric character '<' to an element
> DBI::dbWriteTable(con, "##mtcars", x) # where 'con' is an already defined connection to a MSSQL database
> con %>% dplyr::tbl("##mtcars") %>% dplyr::mutate(wt = as.numeric(wt))
Error: <SQL> 'SELECT TOP(11) "mpg", "cyl", "disp", "hp", "drat", CAST("wt" AS NUMERIC) AS "wt", "qsec", "vs", "am", "gear", "carb"
FROM "##mtcars"'
  nanodbc/nanodbc.cpp:1587: 42000: [Microsoft][ODBC SQL Server Driver][SQL Server]Error converting data type varchar to numeric. 

For Microsoft SQL (2012 and later) try_cast allows a more graceful return of NA. In dbplyr 1.4.2, this can be done by changing lines 225-232 in translate-sql-helpers.R (but this will probably only work for Microsoft SQL 2012 and later):

#' @rdname sql_variant
#' @export
sql_cast <- function(type) {
  type <- sql(type)
  function(x) {
    sql_expr(try_cast(!!x %as% !!type))
  }
}

...which results in a more graceful failure (returns NA):

> con %>% dplyr::tbl("##mtcars") %>% dplyr::mutate(wt = as.numeric(wt))
# Source:   lazy query [?? x 11]
# Database: Microsoft SQL Server 12.00.2000[bpsrawdata@DAVIDLENOVOPC\BPSINSTANCE/BPSSAMPLES]
     mpg   cyl  disp    hp  drat    wt  qsec    vs    am  gear  carb
   <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
 1  21       6  160    110  3.9     NA  16.5     0     1     4     4
 2  21       6  160    110  3.9      3  17.0     0     1     4     4
 3  22.8     4  108     93  3.85     2  18.6     1     1     4     1
 4  21.4     6  258    110  3.08     3  19.4     1     0     3     1
 5  18.7     8  360    175  3.15     3  17.0     0     0     3     2
 6  18.1     6  225    105  2.76     3  20.2     1     0     3     1
 7  14.3     8  360    245  3.21     4  15.8     0     0     3     4
 8  24.4     4  147.    62  3.69     3  20       1     0     4     2
 9  22.8     4  141.    95  3.92     3  22.9     1     0     4     2
10  19.2     6  168.   123  3.92     3  18.3     1     0     4     4
# ... with more rows

Is it possible to re-define sql_cast for Microsoft SQL backends only?

[DavidPatShuiFong]

I've added commits DavidPatShuiFong@30d079c, DavidPatShuiFong@34936b1 and DavidPatShuiFong@4933037 which adds sql_try_cast to R/translate-sql-helpers.R. Then, within R/backend-mssql.R, calls are made to sql_try_cast instead of sql_cast.

In DavidPatShuiFong@34936b1, sql_try_cast is used only if MSSQL is version 11 (equivalent to MSSQL 2012) or greater.

In DavidPatShuiFong@4933037 I've changed the casting of as.integer from INT/INTEGER to NUMERIC. Casting to NUMERIC also returns an integer and is more flexible because if a single row contains a string which can't be converted, try_casting to NUMERIC returns a NA for just that row, whereas INT/INTEGER returns NA for the entire column!

Following is some additional test code.

With this test code trying to use as.character and as.POSIXct to convert the string version of POSIXct datetimes causes the con to 'hang' and become unuseable with the current master branch of dbplyr 1.4.2. The commits I have added don't change that behaviour.

library(magrittr)
library(lubridate)
library(tidyr)
library(dplyr)

df <- data.frame(China = c(5.72, 3.86, 2.6, 2.65, 2.31, 1.66, 1.60, 1.61, 1.63),
                Taiwan = c(3.83, 2.99, 2.48, 1.96, 1.74, 1.74, 1.5, 1.16, 1.05),
                Vietnam = c(6.46, 5.97, 5.05, 4.20, 3.55, 2.71, 2.01, 1.89, 1.94),
                ROK = c(4.34, 3.46, 2.54, 1.84, 1.58, 1.62, 1.35, 1.17, 1.21),
                Year = as.POSIXct(c(1970, 1975, 1980, 1985, 1990, 1995, 2000, 2005, 2010), origin = "1970-01-01"),
                row.names = c("1970", "1975", "1980", "1985", "1990", "1995", "2000", "2005", "2010"))

df <- df %>%
 gather(key = "Country", value = "NchildPerWoman", -Year) %>%
 rename(n = NchildPerWoman) %>%
 mutate(n = as.character(n), Year = as.character(Year)) # convert two columns to character

df[1,3] <- "5.72x" # creates invalid numeric value
df[2,1] <- "xyz" # create invalid date

# assumes 'con' is a connection to a MSSQL database

DBI::dbWriteTable(con, "##nchild", df)
con %>% dplyr::tbl("##nchild") %>% dplyr::mutate(n = as.character(n))
con %>% dplyr::tbl("##nchild") %>% dplyr::mutate(n = as.integer64(n))
con %>% dplyr::tbl("##nchild") %>% dplyr::mutate(n = as.integer(n))
con %>% dplyr::tbl("##nchild") %>% dplyr::mutate(n = as.numeric(n))
con %>% dplyr::tbl("##nchild") %>% dplyr::mutate(n = as.double(n))
con %>% dplyr::tbl("##nchild") %>% dplyr::mutate(n = as.logical(n))


con %>% dplyr::tbl("##nchild") %>% dplyr::mutate(Year = as.Date(Year))
con %>% dplyr::tbl("##nchild") %>% dplyr::mutate(Year = as_date(Year))
con %>% dplyr::tbl("##nchild") %>% dplyr::mutate(Year = as_datetime(Year))

con %>% dplyr::tbl("##nchild") %>% dplyr::mutate(Year = as.character(Year)) # this causes hang on master
con %>% dplyr::tbl("##nchild") %>% dplyr::mutate(Year = as.POSIXct(Year)) # this causes hang on master

[hadley]

Do you want to turn this into a PR?

[DavidPatShuiFong]

Do you want to turn this into a PR?

Yes, thanks. will change into a PR over the next few eays.