Does MySQL compress the stored data? When I export a database in MySQL, it gives me a .sql file. Let's say this .sql file is 80 MB.

Are they compressed in order to reduce disk space usage? 

Is that 80MB real physical-storage size?
It depends on how you export your database:

if you use mysqldump, then the result is a flat text file containing the sql commands.

you can compress it manually or in one commandline like this:

mysqldump &lt; mysqldump options&gt; | gzip &gt; outputfile.sql.gz


see: http://www.ducea.com/2006/10/28/compressing-mysqldump-output/



if your question is, if the tables are stored compressed in the working directory, there are some possibilities:

if you have MyISAM tables that would work if they are read-only, you can Generate Compressed, Read-Only MyISAM Tables with myisampack

And by setting InnoDB configuration options, you can create tables where the data is stored in compressed form.

see: dev.mysql.com/doc/refman/5.5/en/innodb-compression.html

just alter the table to add the compressed option:

ALTER TABLE `tablename` ENGINE = InnoDB ROW_FORMAT = COMPRESSED KEY_BLOCK_SIZE =4