Getting Started – the Select Statement
If you understand the SELECT statement thoroughly, you have understood 80% of SQL, because this is the most commonly used statement, as well as the most complex (when you understand its ramifications in terms of all the things it lets you do). The original definition of SQL included only the SELECT statement.
Select * from tblClientData
means all rows and all columns.
this will be alot of data. in fact, this query would be almost useless, because it would return too much data, as well as kill the server. Generally this kind of a query is a bad idea.