backend-mssql function

Backend: SQL server

See vignette("translation-function") and vignette("translation-verb") for details of overall translation technology. Key differences for this backend are:

• SELECT uses TOP not LIMIT
• Automatically prefixes # to create temporary tables. Add the prefix yourself to avoid the message.
• String basics: paste(), substr(), nchar()
• Custom types for as.* functions
• Lubridate extraction functions, year(), month(), day() etc
• Semi-automated bit <-> boolean translation (see below)

Use simulate_mssql() with lazy_frame() to see simulated SQL without converting to live access database.

simulate_mssql(version = "15.0")

Arguments

• version: Version of MS SQL to simulate. Currently only, difference is that 15.0 and above will use TRY_CAST() instead of CAST().

Bit vs boolean

SQL server uses two incompatible types to represent TRUE and FALSE

values:

• The BOOLEAN type is the result of logical comparisons (e.g. x \> y) and can be used WHERE but not to create new columns in SELECT. https://docs.microsoft.com/en-us/sql/t-sql/language-elements/comparison-operators-transact-sql
• The BIT type is a special type of numeric column used to store TRUE and FALSE values, but can't be used in WHERE clauses. https://learn.microsoft.com/en-us/sql/t-sql/data-types/bit-transact-sql?view=sql-server-ver15

dbplyr does its best to automatically create the correct type when needed, but can't do it 100% correctly because it does not have a full type inference system. This means that you many need to manually do conversions from time to time.

• To convert from bit to boolean use x == 1
• To convert from boolean to bit use as.logical(if(x, 0, 1))

Examples

library(dplyr, warn.conflicts = FALSE)

lf <- lazy_frame(a = TRUE, b = 1, c = 2, d = "z", con = simulate_mssql())
lf %>% head()
lf %>% transmute(x = paste(b, c, d))

# Can use boolean as is:
lf %>% filter(c > d)
# Need to convert from boolean to bit:
lf %>% transmute(x = c > d)
# Can use boolean as is:
lf %>% transmute(x = ifelse(c > d, "c", "d"))