from withing Excel you can use this...
=SQL.REQUEST("dsn=yourdatabasename;uid=yourUserId;pwd=yourPassword",,2,"select field1,field3 from yourtable1 where table1key=100 and field2='"&$A1&"'")
(You'll need to have Excel ODBC addin installed)
with regard to knowledge of SQL, you'll need some basics to manipulate the data if the information is stored in multiple tables.
You can find out a little more on the sql.Request in Microsoft office forum.
Basically, the above command uses a value in cell A1 to match with column field2 in the table to return 2 fields from the table
As Claire mentioned prior, you can also use DTS. This is great if you want need to schedule the file on a regular basis.
Fred