sql_variant function

Create an sql translator

When creating a package that maps to a new SQL based src, you'll often want to provide some additional mappings from common R commands to the commands that your tbl provides. These three functions make that easy. data

sql_substr(f = "SUBSTR")

sql_str_sub(subset_f = "SUBSTR", length_f = "LENGTH", optional_length = TRUE)

sql_paste(default_sep, f = "CONCAT_WS")

sql_paste_infix(default_sep, op, cast)

sql_variant(
  scalar = sql_translator(),
  aggregate = sql_translator(),
  window = sql_translator()
)

sql_translator(..., .funs = list(), .parent = new.env(parent = emptyenv()))

sql_infix(f, pad = TRUE)

sql_prefix(f, n = NULL)

sql_aggregate(f, f_r = f)

sql_aggregate_2(f)

sql_aggregate_n(f, f_r = f)

sql_not_supported(f)

sql_cast(type)

sql_try_cast(type)

sql_log()

sql_cot()

sql_runif(rand_expr, n = n(), min = 0, max = 1)

base_scalar

base_agg

base_win

base_no_win

base_odbc_scalar

base_odbc_agg

base_odbc_win

Arguments

• f: the name of the sql function as a string
• scalar, aggregate, window: The three families of functions than an SQL variant can supply.
• ..., .funs: named functions, used to add custom converters from standard R functions to sql functions. Specify individually in ..., or provide a list of .funs
• .parent: the sql variant that this variant should inherit from. Defaults to base_agg which provides a standard set of mappings for the most common operators and functions.
• pad: If TRUE, the default, pad the infix operator with spaces.
• n: for sql_infix(), an optional number of arguments to expect. Will signal error if not correct.
• f_r: the name of the r function being translated as a string

Helper functions

sql_infix() and sql_prefix() create default SQL infix and prefix functions given the name of the SQL function. They don't perform any input checking, but do correctly escape their input, and are useful for quickly providing default wrappers for a new SQL variant.

Examples

# An example of adding some mappings for the statistical functions that
# postgresql provides: http://bit.ly/K5EdTn

postgres_agg <- sql_translator(.parent = base_agg,
  cor = sql_aggregate_2("CORR"),
  cov = sql_aggregate_2("COVAR_SAMP"),
  sd =  sql_aggregate("STDDEV_SAMP", "sd"),
  var = sql_aggregate("VAR_SAMP", "var")
)

# Next we have to simulate a connection that uses this variant
con <- simulate_dbi("TestCon")
sql_translation.TestCon <- function(x) {
  sql_variant(
    base_scalar,
    postgres_agg,
    base_no_win
  )
}

translate_sql(cor(x, y), con = con, window = FALSE)
translate_sql(sd(income / years), con = con, window = FALSE)

# Any functions not explicitly listed in the converter will be translated
# to sql as is, so you don't need to convert all functions.
translate_sql(regr_intercept(y, x), con = con)

See Also

win_over() for helper functions for window functions.

sql() for an example of a more customised sql conversion function.