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Some questions can be answered from this information using a single table. To determine the number of companies you have contracts with, you need count only the rows in the company table. Similarly, to determine the number of hits during a given time period, only the hit table need be examined. Other questions are more complex and it's necessary to consult multiple tables to determine the answers. For example, to determine how many times each of the ads for Pickles, Inc. was served on July 14, you'd use all three tables as follows:
Look up the company name (Pickles, Inc.) in the company table to find the company number (14).
Use the company number to find matching records in the ad table so you can determine the associated ad numbers. There are two such ads, 48 and 101.
For each of the matched records in the ad table, use the ad number in the record to find matching records in the hit table that fall within the desired date range, and then count the number of matches. There are three matches for ad 48 and two matches for ad 101.
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