Description
In database management, an aggregate function or aggregation function is a function where the values of multiple rows in a single column are processed together to form a single summary value.
Common aggregate functions include:
- Average (i.e., arithmetic mean)
- Count
- Maximum
- Median
- Minimum
- Mode
- Range
- Sum
Others include:
- Nanmean (mean ignoring NaN values, also known as "nil" or "null")
- Stddev
Formally, an aggregate function takes as input a set, a multiset (bag), or a
list from some input domain I and outputs an element of an output domain
O. The input and output domains may be the same, such as for SUM, or may
be different, such as for COUNT.
Aggregate functions occur commonly in numerous programming languages, in spreadsheets, and in relational algebra.
The listagg function, as defined in the SQL:2016 standard aggregates data
from multiple rows into a single concatenated string.
In the entity relationship diagram, aggregation is represented as seen in Figure 1 with a rectangle around the relationship and its entities to indicate that it is being treated as an aggregate entity.
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