Add in functions for working out DBI bugs

## Is your feature request related to a problem? Please describe.
Rich has made a couple of functions that I think we can generalise and add into here that will be helpful for querying SQL databases in R.

## Describe the solution you'd like
Functions that can be generalised to most purposes / queries that analysts can use to work around issues more easily.

## Describe alternatives you've considered
None.

## Additional context
Here's the raw functions:

```
# DBI Bugfix
# Error message:
# Error: nanodbc/nanodbc.cpp:3166: 07009: [Microsoft][SQL Server Native Client 11.0]Invalid Descriptor Index
# Warning message:
# In dbClearResult(rs) : Result already cleared
# ========================================================
# Cause:
# nvarchar that has a CHARACTER_MAXIMUM_LENGTH in the schema table of -1 for some entries
tbl_fix_nvarchar <- function(con, schema, table){
column.types <- dbGetQuery(
con,
paste0("SELECT COLUMN_NAME, DATA_TYPE, CHARACTER_MAXIMUM_LENGTH FROM INFORMATION_SCHEMA.COLUMNS ",
"WHERE TABLE_NAME='",table,"'")
)

ct <- column.types %>%
mutate(cml = case_when(
is.na(CHARACTER_MAXIMUM_LENGTH) ~ 10,
CHARACTER_MAXIMUM_LENGTH == -1 ~ 100000,
TRUE ~ as.double(CHARACTER_MAXIMUM_LENGTH)
)
) %>%
arrange(cml) %>%
pull(COLUMN_NAME)

tbl(con, Id(schema=schema, table=table)) %>% select(all_of(ct))

}

#' DBI Bugfix
#' Error message:
#' Error: nanodbc/nanodbc.cpp:3166: 07009: [Microsoft][SQL Server Native Client 11.0]Invalid Descriptor Index
#' Warning message:
#' In dbClearResult(rs) : Result already cleared
#' ========================================================
#' Cause:
#' nvarchar that has a CHARACTER_MAXIMUM_LENGTH in the schema table of -1 for some entries
#' Usage:
#' This applies the fix as a variation on the select function, which works at the end of pipe-chain queries.
#' e.g.
#' tbl(con, Id(schema='dbo', table='mytable')) %>%
#' group_by(x,y) %>% summarise(s=sum(z)) %>%
#' select_fix_nvarchar('mytable', c('x','y','s'))
select_fix_nvarchar <- function( table, table_name, selection){
column.types <- dbGetQuery(
con,
paste0("SELECT COLUMN_NAME, DATA_TYPE, CHARACTER_MAXIMUM_LENGTH FROM INFORMATION_SCHEMA.COLUMNS ",
"WHERE TABLE_NAME='",table_name,"'")
)
ct <- column.types %>%
mutate(cml = case_when(
is.na(CHARACTER_MAXIMUM_LENGTH) ~ 10,
CHARACTER_MAXIMUM_LENGTH == -1 ~ 100000,
TRUE ~ as.double(CHARACTER_MAXIMUM_LENGTH)
)
) %>%
arrange(cml) %>%
pull(COLUMN_NAME)
# I want to add in any new calculated columns in table_name and add them in at the start of the selection
ct_all <- c(setdiff(selection,ct), ct) %>% unique()
ct_intersect <- intersect(ct_all,selection)
table %>% select(all_of(ct_intersect))
}
```