SQL - Group By Clause
The more data you start interrogating the more records you will return. Its always the same, you start off with basic criteria and before you know it you are referencing several tables to obtain the information your after. The questions being asked are no longer "what was the price of that product?" the questions are more generic, like "How many orders did we do this month?". The more tables you add to a query the more likley you are going to ask summary style questions of the data as you are no longer interested in specific records your more interested in trends.
To get the ball rolling on this aspect of SQL you need to start using the group by clause. This clause allows the user to summarise information in the form of aggregated values which you assign to specific fields (i.e. group to them).
This video will cover the basics of how you start grouping your data into summary information. The video will start by explaining on the small scale but then show you on multiple tables how it is achieved. This is not the end of the story with group by but this should give you the knowledge to plow into some rather meaty queries.
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