base_scalar {dplyr} | R Documentation

## Create an sql translator

### Description

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.

### Usage

    base_scalar

    base_agg

    base_win

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

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

    sql_infix(f)

    sql_prefix(f, n = NULL)

    sql_not_supported(f)

### Arguments

`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_sql` which provides a standard set of mappings for the most common operators and functions.
`f` | the name of the sql function as a string
`n` | for `sql_infix`, an optional number of arguments to expect. Will signal error if not correct.

### 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.

### See Also

`sql` for an example of a more customised sql conversion function.

### 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_prefix("corr"),
      cov = sql_prefix("covar_samp"),
      sd =  sql_prefix("stddev_samp"),
      var = sql_prefix("var_samp")
    )
    postgres_var <- sql_variant(
      base_scalar,
      postgres_agg
    )

    translate_sql(cor(x, y), variant = postgres_var)
    translate_sql(sd(income / years), variant = postgres_var)

    # 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), variant = postgres_var)

[Package dplyr version 0.4.3 Index]