expression

This field can contain a metric math expression to be performed on the other metrics that you are retrieving within this MetricDataQueries structure.

A math expression can use the Id of the other metrics or queries to refer to those metrics, and can also use the Id of other expressions to use the result of those expressions. For more information about metric math expressions, see Metric Math Syntax and Functions in the Amazon CloudWatch User Guide.

Within each MetricDataQuery object, you must specify either Expression or MetricStat but not both.