Databases store data. That’s obvious, but the data itself is pretty inert—you save it, you retrieve it, and you sometimes change it. That’s OK for some things, but sometimes you want the data to work a little harder.
You can put the data to work when you start to summarize it. You can then see trends, see where it’s going, or just get an overview of the data.
Aggregate functions are used to calculate summaries of data. They have three contexts:
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Summarize the whole table.
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Summarize in groups, using GROUP BY.
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Include summaries ...
