Querying frameworks


The Query.jl package provides advanced data manipulation capabilities for DataTables (and many other data structures). This section provides a short introduction to the package, the Query.jl documentation has a more comprehensive documentation of the package.

To get started, install the Query.jl package:


A query is started with the @from macro and consists of a series of query commands. Query.jl provides commands that can filter, project, join, group, flatten and group data from a DataTable. A query can return an iterator, or one can materialize the results of a query into a variety of data structures, including a new DataTable.

A simple example of a query looks like this:

using DataTables, Query

dt = DataTable(name=["John", "Sally", "Roger"], age=[54., 34., 79.], children=[0, 2, 4])

q1 = @from i in dt begin
     @where i.age > 40
     @select {number_of_children=i.children, i.name}
     @collect DataTable

The query starts with the @from macro. The first argument i is the name of the range variable that will be used to refer to an individual row in later query commands. The next argument dt is the data source that one wants to query. The @where command in this query will filter the source data by applying the filter condition i.age > 40. This filters out any rows in which the age column is not larger than 40. The @select command then projects the columns of the source data onto a new column structure. The example here applies three specific modifications: 1) it only keeps a subset of the columns in the source DataTable, i.e. the age column will not be part of the transformed data; 2) it changes the order of the two columns that are selected; and 3) it renames one of the columns that is selected from children to number_of_children. The example query uses the {} syntax to achieve this. A {} in a Query.jl expression instantiates a new NamedTuple, i.e. it is a shortcut for writing @NT(number_of_children=>i.children, name=>i.name). The @collect statement determines the data structure that the query returns. In this example the results are returned as a DataTable.

A query without a @collect statement returns a standard julia iterator that can be used with any normal julia language construct that can deal with iterators. The following code returns a julia iterator for the query results:

q2 = @from i in dt begin
     @where i.age > 40
     @select {number_of_children=i.children, i.name}
nothing # hide

One can loop over the results using a standard julia for statement:

total_children = 0
for i in q2
    total_children += i.number_of_children

println("Total number of children: $(get(total_children))")

Or one can use a comprehension to extract the name of a subset of rows:

y = [i.name for i in q2 if i.number_of_children > 0]

The last example (extracting only the name and applying a second filter) could of course be completely expressed as a query expression:

q3 = @from i in dt begin
     @where i.age > 40 && i.children > 0
     @select i.name

A query that ends with a @collect statement without a specific type will materialize the query results into an array. Note also the difference in the @select statement: The previous queries all used the {} syntax in the @select statement to project results into a tabular format. The last query instead just selects a single value from each row in the @select statement.

These examples only scratch the surface of what one can do with Query.jl, and the interested reader is referred to the Query.jl documentation for more information.