sqldf is a library for querying data in a
SQL, optimised for memory consumption. Any query that can be done in
SQL can also be done in the API, but since SQL is widely known, many
developers find it more convenient to use.
To use SQL to query a data frame, the developer uses the
function, using the data frame name (converted to SQL identifier
format) in place of the table name.
sqldf will automatically create
an in-memory SQLite database, copy the contents of the data frame to
it, perform the query, return the results as a new data frame and
delete the database. We have tested this with data frames of 350K
rows and there is no noticeable difference in performance compared to
API based queries.
See the cl-sqlite
documentation for additional functionality provided by the SQLite
library. You can create databases, employ multiple persistent
connections, use prepared statements, etc. with the underlying
sqldf is a thin layer for moving data to/from
sqldf requires the sqlite shared library from the SQLite
project. It may also be available via
your operating systems package manager.
NoteSQLDF relies on CFFI to locate the SQLite shared library. In most cases, this means CFFI will use the system default search paths. If you encounter errors in loading the library, consult the CFFI documentation. For MS Windows, the certain way to successfully load the DLL is to ensure that the library is on the
PATH, regardless of whether you install via MSYS or natively.
(asdf:load-system :sqldf) (use-package 'sqldf) ;access to the symbols
These examples use the R data sets that are loaded using the example ls-init file. If your init file doesn’t do this, go now and load the example datasets in the REPL. Mostly these examples are intended to demonstrate commonly used queries for users who are new to SQL. If you already know SQL, you can skip this section.
NoteAs always when working with lisp-stat, ensure you are in the LS-USER package
Ordering & Limiting
This example shows how to limit the number of rows output by the
query. It also illustrates changing the column name to meet SQL
identifier requirements. In particular, the R CSV file has
sepal.length for a column name, which is converted to
for the data frame, and we query it with
sepal_length for SQL
because ‘-’ is not a valid character in SQL identifers.
First, let’s see how big the
iris data set is:
LS-USER> iris #<DATA-FRAME (150 observations of 6 variables)>
and look at the first few rows:
(head iris) ;; X7 SEPAL-LENGTH SEPAL-WIDTH PETAL-LENGTH PETAL-WIDTH SPECIES ;; 0 1 5.1 3.5 1.4 0.2 setosa ;; 1 2 4.9 3.0 1.4 0.2 setosa ;; 2 3 4.7 3.2 1.3 0.2 setosa ;; 3 4 4.6 3.1 1.5 0.2 setosa ;; 4 5 5.0 3.6 1.4 0.2 setosa ;; 5 6 5.4 3.9 1.7 0.4 setosa
X7 is the row name/number from the data set. Since it was not assigned a
column name in the data set,
lisp-stat gives it a random name upon
import (X1, X2, X3, …).
sqldf for a query:
(pprint (sqldf "select * from iris order by sepal_length desc limit 3")) ;; X7 SEPAL-LENGTH SEPAL-WIDTH PETAL-LENGTH PETAL-WIDTH SPECIES ;; 0 132 7.9 3.8 6.4 2.0 virginica ;; 1 118 7.7 3.8 6.7 2.2 virginica ;; 2 119 7.7 2.6 6.9 2.3 virginica
Averaging & Grouping
Grouping is often useful during the exploratory phase of data
analysis. Here’s how to do it with
(pprint (sqldf "select species, avg(sepal_length) from iris group by species")) ;; SPECIES AVG(SEPAL-LENGTH) ;; 0 setosa 5.0060 ;; 1 versicolor 5.9360 ;; 2 virginica 6.5880
For each species, show the two rows with the largest sepal lengths:
(pprint (sqldf "select * from iris i where x7 in (select x7 from iris where species = i.species order by sepal_length desc limit 2) order by i.species, i.sepal_length desc")) ;; X7 SEPAL-LENGTH SEPAL-WIDTH PETAL-LENGTH PETAL-WIDTH SPECIES ;; 0 15 5.8 4.0 1.2 0.2 setosa ;; 1 16 5.7 4.4 1.5 0.4 setosa ;; 2 51 7.0 3.2 4.7 1.4 versicolor ;; 3 53 6.9 3.1 4.9 1.5 versicolor ;; 4 132 7.9 3.8 6.4 2.0 virginica ;; 5 118 7.7 3.8 6.7 2.2 virginica
Recall the note above about X7 being the row id. This may be different depending on how many other data frames with an unnamed column have been imported in your Lisp-Stat session.
sqldf needs to read and write data frames to the data base, and
these functions are exported for general use.
Write a data frame
write-table can be used to write a data frame
to a database. Each take a connection to a database, which may be file
or memory based, a table name and a data frame. Multiple data frames,
with different table names, may be written to a single SQLite file
Read a data frame
read-table will read a database table into a data frame and update
the column names to be lisp like by converting “.” and “_” to
“-”. Note that the CSV reading tools of SQLite (for example,
DB-Browser for SQLite are much faster
than the lisp libraries, sometimes 15x faster. This means that often
the quickest way to load a data-frame from CSV data is to first read
it into a SQLite database, and then load the database table into a
data frame. In practice, SQLite also turns out to be a convenient
file format for storing data frames.
SQLDF is currently written using an apparently abandoned library,
cl-sqlite. Pull requests
from 2012 have been made with no response from the author, and the
SQLite C API has improved considerably in the 12 years since
cl-sqlite FFI was last updated.
We choose CL-SQLite because, at the time of writing, it was the only SQLite library with a commercially acceptable license. Since then CLSQL has migrated to a BSD license and is a better option for new development. Not only does it support CommonSQL, the de-facto SQL query syntax for Common Lisp, it also supports several additional databases.
Version 2 of SQLDF will use CLSQL, possibly including some of the CSV and other extensions available in SQLite. Benchmarks show that SQLite’s CSV import is about 15x faster than cl-csv, and a FFI wrapper of SQLite’s CSV importer would be a good addition to Lisp-Stat.
Joins on tables are not implemented in SQLDF, though there is no technical reason they could not be. This will be done as part of the CLSQL conversion and involves more advanced SQL parsing. SXQL is worth investigating as a SQL parser.
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