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An aggregate function in SQL is a function that takes a group of rows and produces a single value. This value can be the sum, the average, the minimum, or the maximum of the values in the group.

1. SUM(): calculates the total sum of a numeric column.
2. AVG(): calculates the average of a numeric column.
3. MIN(): returns the minimum value in a column.
4. MAX(): returns the maximum value in a column.
5. COUNT(): returns the number of rows in a table, or the number of rows that match a specific condition.

Aggregate functions can be used with the GROUP BY clause in SQL to group the results by a specific column or set of columns, and apply the aggregate function to each group separately. This allows you to perform calculations and summarize data based on different criteria.

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