SQL SERVER
I may be on the wrong frequency with my response but I think this is the answer, if not it's a good tutorial.
For a server based relational database system to support distributed requests from multiple HTML web clients it must have a server program running on the server system to support concurrent data access from each web program. It works like this. A client program(HTML web page) passes requested data-keys to a cgi server program(example: perl) on the server-site, which puts together a SQL request to retrieve some data. Since many users may use the same web site to process the same file & maybe the same record(like a seat on an airplane). Other web pages may also access the same airplane's seating record. If each of these requests(multiple concurrent executing server programs) were allowed to read & update the records at the same time with out co-ordinating their requests then all kinds of integrity problems could arise. To remove this problem, each perl cgi program acts like a client to the database-SQL-server program and gets monitored to avoid integrity problems. The same protection could have been completely coded in each perl program but that would generate a lot of extra coding and testing. This way one general purpose SQL-database-server program protects the database for all of the perl-cgi-client programs which in turn acts as a server to the HTML-web-client-program.
Back to the question, for a HTML-web-program to use a cgi-perl server program, the perls request will require that the SQL-database program have been started. A connect has to have a SQL-server program already running.
When testing MySQL on my PC I have to start a database-server, once each initial system load, to have it running when I try to test my perl programs.
Each program in a system may function as a client, server or both depending from what perspective you view it from. Even COBOL calls a SQL server program to get it'd data requests serviced. All systems are designed to run in client-server relationship environment. Hope this helps.