Relational expressions — new_relexpr • duckplyr

These functions provide a backend-agnostic way to construct expression trees built of column references, constants, and functions. All subexpressions in an expression tree can have an alias.

new_relexpr() constructs an object of class "relational_relexpr". It is used by the higher-level constructors, users should rarely need to call it directly.

relexpr_reference() constructs a reference to a column.

relexpr_constant() wraps a constant value.

relexpr_function() applies a function. The arguments to this function are a list of other expression objects.

relexpr_window() applies a function over a window, similarly to the SQL OVER clause.

relexpr_set_alias() assigns an alias to an expression.

Usage

new_relexpr(x, class = NULL)

relexpr_reference(name, rel = NULL, alias = NULL)

relexpr_constant(val, alias = NULL)

relexpr_function(name, args, alias = NULL)

relexpr_window(
  expr,
  partitions,
  order_bys = list(),
  offset_expr = NULL,
  default_expr = NULL,
  alias = NULL
)

relexpr_set_alias(expr, alias = NULL)

Arguments

x: An object.
class: Classes added in front of the "relational_relexpr" base class.
name: The name of the column or function to reference.
rel: The name of the relation to reference.
alias: An alias for the new expression.
val: The value to use in the constant expression.
args: Function arguments, a list of expr objects.
expr: An expr object.
partitions: Partitions, a list of expr objects.
order_bys: which variables to order results by (list).
offset_expr: offset relational expression.
default_expr: default relational expression.

Value

an object of class "relational_relexpr"

Examples

relexpr_set_alias(
  alias = "my_predicate",
  relexpr_function(
    "<",
    list(
      relexpr_reference("my_number"),
      relexpr_constant(42)
    )
  )
)
#> List of 3
#>  $ name : chr "<"
#>  $ args :List of 2
#>   ..$ :List of 3
#>   .. ..$ name : chr "my_number"
#>   .. ..$ rel  : NULL
#>   .. ..$ alias: NULL
#>   .. ..- attr(*, "class")= chr [1:2] "relational_relexpr_reference" "relational_relexpr"
#>   ..$ :List of 2
#>   .. ..$ val  : num 42
#>   .. ..$ alias: NULL
#>   .. ..- attr(*, "class")= chr [1:2] "relational_relexpr_constant" "relational_relexpr"
#>  $ alias: chr "my_predicate"
#>  - attr(*, "class")= chr [1:2] "relational_relexpr_function" "relational_relexpr"