MAQL syntax is used to define metrics in the Custom Metric Editor.
Every metric definition is preceded with the SELECT command. Think of this command in terms of making a request to the data warehouse, and selecting which information you'd like to be returned.
The following is the case when you are using MAQL syntax to aggregate a fact into a metric:
In this example, the MAQL statement defines a metric that computes the sum total of all Payment fact values across all available records in the current project.
You also use SELECT to introduce one or more predefined metrics within a metric definition: