gmExpr: Creating gmExpressions

In gmDatabase: Accessing a Geometallurgical Database with R

Description

gmExpr creates a gmExpression

Usage

gmExpr(expr,...,envir=parent.frame(),EXPR=substitute(expr))

Arguments

expr	an R-like representation of an SQL query.
envir	defines the environment used for evaluation of expr.
EXPR	The quoted version of such an expression.
...	further arguments for later use in other methods.

Details

gmExpr creates an object of class gmExpr, which can be used as argument in gmRead instead of a string. Furthermore it can be extended like any other expression used in gmRead. For use in the geometallurgical database you need to begin every expression with root, since data storage starts from this common root object.

The composition of an expression is done in the following way: for accessing a certain object in a class, use the $-operator, to access its elements use square brackets []. By specifying columnName=operation the user can control the content of a column in the the output. Named columns are always shown in the result. If they need to be dropped set them to operation drop. If you want to group objects by a named column, set its operation to group. This grouping can only be done for elements at least one step down in the hierarchy.

Value

An object of class gmExpr, containing the expression in $EXPR and the envir as attribute.

Author(s)

K. Gerald van den Boogaart, S. Matos Camacho

See Also

gmRead

Examples

expr <- gmExpr(root$project)
## Not run: 
gmConnectServer(MySQL(), ...)
gmRead(expr$series)

gmRead(expr$series[name=gmTitle, name=drop])
## naming the element gmTitle as name and dropping it in the result

gmRead(expr[pID=id]$series[name=gmTitle, pID=group])
## grouping the series by their corresponding project in the result

## End(Not run)

gmDatabase documentation built on May 2, 2019, 9:31 a.m.