MySQL
MySQL is a language (or set of tools) that interacts with a relational database.
SQL is Structured Query Language
The way it works:
You create a database with N columns (of any type) -- imagine a listing of people in this room, their heights, their ages, and whether they are left- or right-handed
You can then query on the database like this:
%> SELECT name FROM table_name WHERE name LIKE 'trill'
You can do lots of complicated things here. You can update certain values
%> SET handedness = 'left' WHERE name = 'trilling'
MySQL is used a lot for web stuff, for database administration, and for many different kinds of science applications.
Naturally, Perl and Python (and PHP, Java) interact with MySQL with a lot of
canned routines.
Check out Wikipedia
and mysqltutorial.org.
MySQL is good (excellent) at databases. Therefore,
in the era of big data (already here for physics, and
coming soon for astronomy) you will see that
SQL fluency could be critically important.
An example from astronomy is the Sloan Digital
Sky Survey (SDSS). SDSS
has a simple
push-button query interface (actually,
several of them), but sometimes that's not
enough, and for those cases they also have a
SQL query
page.
SDSS has a lot of good tutorials on their web
pages about how to use their SQL query interface
(and about how to use SQL). You should read some
of those tutorials and then do the exercise.