\documentclass[a4paper]{article}

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\title{SQRL Features and Usage}
\author{Mike Lee}

\begin{document}
\maketitle
\noindent
Whatever your nominal role, if it comes with access to an organisation's
database, then the occasional explore-pull-summarise task is almost
inevitable.
The aim of SQRL is to turn those around as quickly as possible.

SQRL handles (RODBC) communications in the background; silently opening
connections and applying preferences, without bothering the user.
If you've been handed a multi-statement SQL script, SQRL can run that original
file.
Blocks of R can be added to the same script; in order to parameterise a query,
or transform the resulting data.
SQRL also provides means for living with DBMS limitations, administrator's
restrictions, and unreliable networks.


\section{Quick Start}
Upon loading, SQRL generates a dedicated interface function to each ODBC DSN
found on your system.
Supposing one of those DSNs is called `arx', it could then be queried like so;
\begin{shell}
> library(SQRL)
> arx("use database; select * from table")
\end{shell}

Arguments fed to \cmd{arx(...)} are parsed and interpreted.
In the example above, the single argument looks like a pair of SQL statements.
That being the case, a connection is opened, the first statement is submitted,
then the second, and the result of that query returned.
The connection is left open for reuse.
All communications with the ODBC drivers are made through calls to RODBC
functions.

There is a computational overhead to the parsing, of course, but it is
negligible in comparison to the time it takes to process a query.
In return, less time, effort, and typing is required from the user.
For instance, if `query.sql' should happen to be the path of a readable file,
then
\begin{shell}
> arx("query.sql")
\end{shell}
submits the statements appearing within it.
By and large, SQRL attempts to be flexible; supporting different and minimalist
styles of input, and deducing what you mean it to do.
For the order of precedence, see
\begin{shell}
> help(sqrlUsage)
\end{shell}


% The \newpage is to avoid page breaks within sections. Using this rather than
% the usual \vbox (or similar), because \newpage is proving more consistent
% under a change of paper size between A4 and US Letter.
\newpage
\section{Multi-Statement Scripts}
If this
\begin{file}
\fl\com{/* This script has three statements. */}
\fl\key{use} \sql{database;}
\fl\key{insert into} \sql{table1}
\fl\tab\key{select} \sql{*} \key{from} \sql{table2;}
\fl\key{drop table} \sql{table2;}
\end{file}
is the content of file `sequence.sql', then
\begin{shell}
> arx("sequence.sql")
\end{shell}
submits each statement in turn, and returns the result of the final operation.
This is convenient when you've inherited a script prepared in a SQL IDE.

While the parser does handle basic procedural language (PL), it may sometimes
trip over more esoteric DBMS-specific extensions.
Should that occur, the manual details a robust-parsing fallback mode (requiring
tweaks to the original script), and an unparsed verbatim alternative (for single
statements, only).

{\ }\\
\section{Parameterisaton of Queries}
SQRL's parser supports a hybrid syntax wherein arbitrary blocks of R can be
embedded within SQL script.
Special tags mark the beginning and end of any such R inclusions, in much the
same way as Sweave isolates R within LaTeX.
Here is a listing of one such script;
\begin{file}
\fl\com{-- SQL with R in it.}
\fl\key{select} \sql{*} \key{from} \sql{database.table}
\fl\key{where} \sql{column >} \tag{<R>} \rin{pnorm(x)} \tag{</R>}
\end{file}
If this is the content of a file `query.sqrl', then it could be submitted like
so;
\begin{shell}
> arx("query.sqrl", x = 3)
\end{shell}

While hybrid scripts are parameterised at the user's end, the data source
receives only standard unparameterised SQL; after all R blocks have been
replaced by the results of their evaluation.
This provides an alternative to constructing queries from collections of strings
and variables pasted together within the body of an R script, and allows your
text editor to apply SQL syntax highlighting rules to predominantly SQL scripts.
It is also useful when you don't have the permissions to create parameterised
queries, macros, or functions on the database server itself.

An extended discussion of the hybrid syntax, with examples, is provided in the
reference manual and help system;
\begin{shell}
> help(sqrlScript)
\end{shell}


\newpage
\section{Integrated Post-Processing}
For any number of reasons,  SQL-queried data is frequently cleansed, converted,
or otherwise transformed in R, prior to reaching its destination as input to a
model or analysis.
SQRL's parser allows any such processing to be tacked onto the end of a SQL
script;
\begin{file}
\fl\com{/* A SQL query followed by R post-processing. */}
\fl\key{select} \sql{A, B, C} \key{from} \sql{table}
\fl
\fl\com{-- Submit the query, assign its result to 'x', and switch to R.}
\fl\tag{<result -> x>}
\fl
\fl\com{# Manipulate the result in R.}
\fl\rin{x$A <- as.factor(x$A)}
\fl\rin{x$B <- as.Date(x$B, "%d/%m/%Y")}
\fl\rin{x$C <- (x$C - mean(x$C)) / sd(x$C)}
\fl\rin{x} \com{-- Only this final value is returned.}
\end{file}
If this is in file `pull.sqrl', then;
\begin{shell}
> arx("pull.sqrl")
\end{shell}
returns the data already cleaned and formatted.
This helps separate data-preparation script from data-analysis script.
Alternatively, simple analyses can be placed entirely within the post-processing
section.


\section{Set and Forget Communications}
\label{parameters}
Whether opening a connection, submitting a query, or fetching results,
communicating with ODBC drivers involves passing the values of several control
parameters every time.
Since this might be bothersome when one or more of those values is non-default,
SQRL keeps a record of the settings for each data source, and applies them
behind the scenes.
For example,
\begin{shell}
> arx(believeNRows = FALSE, dec = ",")
\end{shell}
sets new values for two of the parameters.
All subsequent queries to that source,
\begin{shell}
> arx("select * from table")
\end{shell}
for instance, inherit those values.
Any other sources use their own values.

Some communications settings cannot be changed while the connection to the data
source is open.
Their values are applied at the time of opening, rather than with each query,
and so can only be altered beforehand.

Some features of SQRL work best when opening a new connection does not involve
manual authentication.
Usernames and passwords can be stored for automatic use on data sources where
the user would otherwise be prompted for input.
Passwords are held with a very modest level of security.

For a complete list of control parameters, see
\begin{shell}
> help(sqrlParams)
\end{shell}


\newpage
\section{Protected Connection Handles}
SQRL interface functions, communications settings, and connection handles are
stored outside of the global environment.
That being the case,
\begin{shell}
> rm(list = ls(all = TRUE))
\end{shell}
is fine; settings are preserved, connections remain open, temp tables persist,
and you shouldn't see any warnings about the closing of unused handles.


\section{Automatic Connection Closure}
By default, connections remain open after use.
To manually close one, use
\begin{shell}
> arx("close")
\end{shell}
This is allowed even when the connection is not open (is already closed).
A new connection will be opened the next time a query is submitted.
Communications settings can be changed in the meanwhile.

To have the connection close automatically after each sequence of operations,
set
\begin{shell}
> arx(autoclose = TRUE)
\end{shell}
With that done,
\begin{shell}
> arx("use database; select * from table")
\end{shell}
opens a connection, submits the first statement, then the query, closes the
connection, and returns the result of the query.
This can be courteous when your administrator has set a short timeout.


\section{Recovery from Lost Connections}
If a query should happen to fail, and that failure appears to have been caused
by an unexpectedly dropped connection, then a single attempt will be made to
reconnect and resubmit.
This occurs automatically, and will usually go unnoticed (unless manual input is
required for authentication, see section \ref{parameters}).
This can be helpful when working over an unreliable network, or when your
administrator has set a short timeout.

Suspect connections are tested with a simple and trusted `ping' query.
This is automatically determined the first time a connection is opened.
It can be manually redefined if that initial value proves unsatisfactory;
\begin{shell}
> arx(ping = "begin null; end;")
\end{shell}

Note that any temp tables will not have survived the initial connection loss.
Even though a connection is reestablished, the query will subsequently fail if
it uses one of those temps.
Automatic recovery attempts can be disabled with
\begin{shell}
> arx(retry = FALSE)
\end{shell}


\newpage
\section{Local Exceptions from Remote Failures}
When a query fails in an unrecoverable way, the corresponding driver and RODBC
messages are raised to an error (instead of being returned as a normal
character vector).
Consequently, exceptions on the remote server immediately halt local execution
in R.
This stops the script at the point of failure, before knock-on effects cause
other, potentially cryptic, problems.

To disable this behaviour, and revert to character notifications, use
\begin{shell}
> arx(errors = FALSE)
\end{shell}


\section{Visible Connection Status}
For a continuous display of connection status, use
\begin{shell}
> arx(visible = TRUE)
\end{shell}
This authorises changes to R's global `prompt' option.
If a connection to arx is open, the prompt should now read
\begin{shell}
a>
\end{shell}
This can be of use when juggling multiple sources.
If we have three, `arx', `box' and `crate', we can make them all visible with
\begin{shell}
> sqrlAll(visible = TRUE)
\end{shell}
If the prompt then reads
\begin{shell}
ac>
\end{shell}
we can see that arx and crate and are open, while box is closed.

On Windows (only), in R.exe, Rterm.exe, or Rgui.exe (MDI or SDI, but not in
RStudio), the window title also changes.
While a connection is open, it might read something like
\begin{shell}
\win{R Console (64-bit) (arx)}
\end{shell}
While a query is running, this becomes
\begin{shell}
\win{R Console (64-bit) (arx)*}
\end{shell}
and while results are being fetched,
\begin{shell}
\win{R Console (64-bit) (arx)+}
\end{shell}
This distinguishes server query time from network data transfer time, and is
potentially helpful in diagnosing a bottleneck.

To change the prompt and title indicators, use
\begin{shell}
> arx(prompt = "A", wintitle = "[ARX]")
\end{shell}


\newpage
\section{Result Retention}
If you've just done this;
\begin{shell}
> arx("my_long_query.sql")
\end{shell}
and forgot to assign the output, fear not.
The result of the last successful job is stored as
\begin{shell}
> arx("result")
\end{shell}

When memory is low, the stored result can be deleted with
\begin{shell}
> arx(result = NULL)
\end{shell}


\section{Automatic Concatenation}
Queries and file paths may be expressed as components.
These are automatically pasted into a single character string.
By default, list elements are collapsed together with empty strings, while
atomic vectors are collapsed with commas.
For example, if we define two atomic vectors;
\begin{shell}
> a <- c("columnA", "columnB")
> b <- c(1, 2, 3)
\end{shell}
then
\begin{shell}
> arx("select ", a, " from table",
+ " where columnC in (", b, ");")
\end{shell}
results in
\begin{shell}
\out{"select columnA,columnB from table where columnC in (1,2,3);"}
\end{shell}
being submitted to the data source.

Note that if \cmd{b} were a character vector, it might still be necessary to put
single quotes about each of its elements;
\begin{shell}
> sq <- function(x) paste0("'", x, "'")
> arx("select * from table where column in (", sq(b), ")")
\end{shell}
This is difficult to automate, because the elements might already be quoted, or
quotes might not be wanted.
The latter occurs when the vector specifies, say, column names (as is the case
with \cmd{a}, above), or when it holds character representations of long
integers.

The collapsing rules can be changed by
{\small\color{rprompt}\begin{verbatim}
> arx(aCollapse = ", ", lCollapse = "\n")
\end{verbatim}}
% The above is not the usual 'shell' (alltt) LaTeX environment, because alltt
% lacks the backslash symbol in pdfLaTeX. \texttt is not used, because the
% vertical environment padding would not be applied.


\newpage
\section{A Longer Script, using Feedback}
This script;
\begin{file}
\fl\com{-- Use these communications settings.}
\fl\tag{<with>}
\fl\tab\rin{as.is = TRUE}
\fl\tag{</with>}
\fl
\fl\com{-- Define an R function, for later use.}
\fl\tag{<R>}
\fl\tab\rin{asdate <- function(d) \{format(d, "'%Y-%m-%d'")\}}
\fl\tag{<do>}
\fl
\fl\com{-- A parameterised SQL query (template).}
\fl\key{select} \sql{columnA} \key{from} \sql{table1}
\fl\tab\key{where} \sql{columnB = }\tag{<R>} \rin{asdate(date)} \tag{</R>}
\fl
\fl\com{-- Submit the above query, assign its result to 'x',}
\fl\com{-- and transform that data in R.}
\fl\tag{<result -> x>}
\fl\tab\rin{x <- unique(x$columnA %/% 10000)}
\fl\tag{<do>}
\fl
\fl\com{-- Submit another query, incorporating the}
\fl\com{-- (transformed) intermediate results.}
\fl\key{select} \sql{*} \key{from} \sql{table2}
\fl\tab\key{where} \sql{columnC in (}\tag{<R>} \rin{x} \tag{</R>}\sql{)}
\end{file}
in file `script.sqrl', could be run with
\begin{shell}
> arx("script.sqrl", date = Sys.Date() - 1)
\end{shell}

The first section, lines 1 through 4, forces the script to run with the
specified communications settings.
Original settings will be restored afterwards.
The second section, lines 6 through 9, evaluates R script into a temporary
working environment.
The global environment is unaffected.
The third section, lines 11 through 19, constructs a query from an argument,
\cmd{date}, assigns the result of the query to \cmd{x} (in the temporary
environment), and manipulates that result.
The final section, lines 21 through 24, constructs a second query from the
(transformed) intermediate result, and returns the result of that query.

This example is simple enough it could have been done with a join, but
circumstances might be that the intermediate data manipulation is too complex
for SQL, or involves more data from R, or from another data source, or you
don't have enough server memory quota, or permissions for a temp table, or can't
nest queries, or can't upload data, or want to reduce load on the server (at the
expense of increased network traffic), or are fighting an unfamiliar DBMS, or
some other administrative restriction.
More advanced features of the hybrid syntax allow for conditional submissions,
loops, early returns, and stored procedures.
\begin{shell}
> help(sqrlScript)
\end{shell}


\newpage
\section{Verbose Mode}
For debugging, verbose mode is enabled by
\begin{shell}
> arx(verbose = TRUE)
\end{shell}


\section{Data Sources}
To see what sources are available, call
\begin{shell}
> sqrlSources()
\end{shell}

New sources can be defined by specifying a DSN, or connection string, or from a
configuration file containing the string and communications settings;
\begin{shell}
> sqrlSource(foo = "config.txt")
\end{shell}
Detailed examples are provided in the manual;
\begin{shell}
> help(sqrlSource)
> help(sqrlConfig)
\end{shell}


\section{Help}
Being user and system dependent, the names of the interface functions, like
\texttt{arx}, are unknown at package build time.
Consequently, they do not appear within the SQRL reference manual or the
standard R help system.
For examples of general usage, try
\begin{shell}
> help(sqrlUsage)
\end{shell}
Alternatively,
\begin{shell}
> arx("help")
\end{shell}
provides a smaller set of examples, along with configuration details specific to
interface \texttt{arx}.

For a list of all communications settings, use
\begin{shell}
> arx("config")
\end{shell}
and for one specific setting,
\begin{shell}
> arx("believeNRows")
\end{shell}
(for instance).

For the current connection status (open or closed), try
\begin{shell}
> arx("is open")
\end{shell}
This may ping the source for confirmation.
The space is optional.


\newpage
\section{Ending Sessions}
To close the connection to data source `arx', use
\begin{shell}
> arx("close")
\end{shell}
To close the connections to all SQRL data sources, use
\begin{shell}
> sqrlAll("close")
\end{shell}

To close the connections to all SQRL data sources, remove their interface
functions, and unload the SQRL library, use
\begin{shell}
> sqrlOff()
\end{shell}
After this, no further communication is possible with any data source (via SQRL,
until it is reloaded).


\end{document}
% EOF
