backend-mssql: Backend: SQL server

In tidyverse/dbplyr: A 'dplyr' Back End for Databases

Backend: SQL server

Description

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.

Usage

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"))

tidyverse/dbplyr documentation built on April 7, 2024, 1:42 a.m.