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Database Tutorial
This section of the manual deals with how Roxen and Pike can
connect to SQL databases, retrieve data and modify the data stored
there. It doesn't aim at teaching SQL or how to design a database,
save for very simple cases, which are not surprisingly the most common
in normal Web-related programming tasks. So you won't find references
to triggers, stored procedures, referential integrity or complex
privileges management here: they CAN be used from Roxen or Pike, but
they're more of an SQL matter, which is out of the scope of this
manual.
Roxen and Pike offer an uniform layer to access all the supported
databases. However such a layer does not cover anything but issuing
queries and retrieving data. SQL is unfortunately another matter: it
is an ANSI standard, but just about every SQL server has its own
dialect, which may be a subset or a superset of the standard. You'll
need to check your server of choice's documentation about its version
of SQL.
This section of the manual tries to be a reference for both Roxen
and Pike programmers. To do so, most examples will be available in two
versions, a Pike snippet of code, and RXML code.
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Note!
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The RXML <sqloutput> and <sqltable> have
been deprecated in favour of the
<emit source="sql">
container. In this manual the 'old' tags are used, but the new tag is
briefly introduced on the The emit and sqlquery Tags page.
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Contents
- Introduction to MySQL
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MySQL by TcX AB is a simple SQL server, very popular among
web-designers. It is a relatively simple and lightweight server,
which aims at being very fast, but is not fully ANSI-SQL compliant,
as it doesn't support features such as triggers or sophisticated
access control.
Since MySQL is so popular among web-developers, it was chosen as the
reference RDBMS for Roxen. This chapter will introduce you to it, and to
some of the pitfalls most easily encountered when using it. The examples
shown are however as cross-platform as they could be: they should work
with any SQL server which claims at least a partial degree of ANSI-SQL
compliance.
- Querying
-
Querying a server is by far the most used DB-related functionality.
Almost everything (in some cases, plain everything) you'll do when
interacting with an SQL server goes through specifying correctly-formed
SQL queries, sending them to a server and then interpreting the results
the server sends back.
RXML 2 offers two different ways to query a server, Pike too. These
are needed to fit all situations; a query may yield results, or it might
not, and the only way to tell the difference is by looking at the SQL
code being executed by the server.
It would seem that programs (or RXML pages) accessing SQL resources are
difficult and cryptic because the results queries can return are
inherently dynamic in number and structure. Fortunately, very few
programs need to handle the full range of possible outcomes from a
query. In fact, most SQL queries are either non-interactive, or are
parametric. This means they have a fixed structure where a few values
(or no value if the query is non-interactive) change on each execution.
This ensures that the results (or lack thereof) can be predicted
accurately; if not in number, at least in structure.
It is best to see SQL statements not as a foreign plug-in into a program's
execution flow, but as an integral part of it. Whenever the data storage
structure changes, the program must be changed according to it (this is why
database design is such an important matter: a bad database design
decision might end requiring an application rewrite almost from scratch).
- Data Extraction
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In this chapter we'll introduce how to perform data-extraction
queries. We'll introduce the SQL syntax for data-extraction, and provide a
few examples, both in RXML and in Pike.
- Data Insertion
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In this chapter we'll introduce how to insert data into a database.
Notice that data insertion and modification are two different operations,
using two different SQL commands.
- Using RXML Features with SQL Databases
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In this chapter we'll examine how to exploit some RXML features when
working with SQL databases.
The examples here contained are geared towards SQL-driven data sources, but
it is not of course the only use for them.
- Database Maintainance
-
Up to this point we have assumed the databases to be already present for
us. But this of course isn't the case in some real-world situations.
Designing a database is a very complex task for nontrivial cases. It is also
a very delicate operation: when dealing with data-storage-related
applications, usually the application is built around the data, and not the
other way around. So a bad data storage design will snowball, leading to a
bad application design, which is very expensive to fix, going as far as a
rewrite from scratch.
So for the umpteenth time we'll remark that if an application uses
non-trivially organized data, the best solution is to hire someone
to design the database.
In this chapter we'll examine how to build and delete a database, how to
set the tables and indices up or remove them. We'll assume that the databse
structure is so simple to be self-evident (which is often the case for
web-related systems), database design won't be taken into account.
Also, the examples will be in pike-only: these activities are meant to be
used only once at database-creation, and are really not suited for
a web-based application.
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