It's much more useful, though, for aggregation functions to ignore null values and only combine those that are non-null. By extension, if we use sum to add all the values in a set, and any of those values are null, the result should also be null. If we add two values, and one of them is null, the result is null. One final important feature of aggregation functions is that they are inconsistent with the rest of SQL in a very useful way. It might use the first one processed, the last one, or something else entirely.Īnother important fact is that when there are no values to aggregate, aggregation's result is "don't know" rather than zero or some other arbitrary value: select person, max(reading), sum(reading) from Survey where quant= "missing" person Why does Lake's name appear rather than Roerich's or Dyer's? The answer is that when it has to aggregate a field, but isn't told how to, the database manager chooses an actual value from the input set. We can also combine aggregated results with raw results, although the output might surprise you: select person, count(*) from Survey where quant= "sal" and reading<= 1.0 We can, for example, find the range of sensible salinity measurements: select min(reading), max(reading) from Survey where quant= "sal" and reading<= 1.0 min(reading) SQL lets us do several aggregations at once. We used count(reading) here, but we could just as easily have counted quant or any other field in the table, or even used count(*), since the function doesn't care about the values themselves, just how many values there are. Select sum(reading) from Survey where quant= "sal" sum(reading) Select count(reading) from Survey where quant= "sal" count(reading) Three others are avg, count, and sum: select avg(reading) from Survey where quant= "sal" avg(reading) Min and max are just two of the aggregation functions built into SQL. Select max(dated) from Visited max(dated) Each of these functions takes a set of records as input, and produces a single record as output: select min(dated) from Visited min(dated)
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We know how to select all of the dates from the Visited table: select dated from Visited datedīut to combine them, we must use an aggregation function such as min or max. We now want to calculate ranges and averages for our data. Explain how missing data is handled during aggregation.Trace the execution of a query that performs aggregation.Write queries that compute aggregated values.Define "aggregation" and give examples of its use.
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