
Explanation/Reference:
Note: The behavior of SUMMARIZE is similar to the GROUP BY syntax of a SELECT statement in SQL.
For example, consider the following query.
EVALUATE
SUMMARIZE(
'Internet Sales',
'Internet Sales'[Order Date],
"Sales Amount", SUM( 'Internet Sales'[Sales Amount] )
)
This query calculates the total of Sales Amount for each date in which there is at least one order, producing this result.
Reference: https://msdn.microsoft.com/en-us/library/gg492171.aspx