Programming the Perl DBI

Simple Queries

The simplest SQL query is to ask for certain columns in all rows of a table. The SELECT syntax for this form of query can be expressed as simply as:

SELECT column, column, ..., column
FROM table

or:

SELECT *
FROM table

which will query and fetch back all the columns within the specified table.

Therefore, to select some of the rows from some columns in the megaliths table, the following SQL statement can be used:

SELECT name, location, mapref
FROM megaliths

which would return the following information:

+---------------------------------------------------------------+
| name         | location                          | mapref     |
+---------------------------------------------------------------+
| Callanish I  | Callanish, Isle of Lewis          | NB 213 330 |
| Lundin Links | Lundin Links, Fife, Scotland      | NO 404 027 |
| Stonehenge   | Near Amesbury, Wiltshire, England | SU 123 400 |
| Avebury      | Avebury, Wiltshire, England       | SU 103 700 |
| Sunhoney     | Near Insch, Aberdeenshire         | NJ 716 058 |
+---------------------------------------------------------------+

So even with the simplest SQL imaginable, the inherent flexibility of the syntax allows us to easily specify exactly which information we want from the database without having to write lots of excruciating lines of code to get it.

Another aspect of the relational database methodology is now visible, in that even though the database contains information on all the columns within a particular table, only a subset of the available columns needs to be retrieved. Therefore, ...