Functions let you apply arithmetic, string, or other computations and transformations to Impala data. You typically use them in SELECT lists and WHERE clauses to filter and format query results so that the result set is exactly what you want, with no further processing needed on the application side.
Scalar functions return a single result for each input row. See Functions.
Aggregate functions combine the results from multiple rows. See Aggregate Functions.
User-defined functions let you code your own logic. They can be either scalar or aggregate functions. See User-Defined Functions (UDFs).