Add lookup function.

Author: pohuan

query-languages/dax/lookup-function-dax.md

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+---
+description: "Learn more about: LOOKUP"
+title: "LOOKUP function (DAX) | Microsoft Docs"
+---
+# LOOKUP
+
+Used in visual calculation only. The Lookup function allows user to retrieve a value in a cell in the visual matrix using absolute navigation. User can specify a value as filter for any axis on the visual matrix and anything not specified will be inferred from the context. If Lookup could not result in single value, it will return an error.  
+
+## Syntax
+
+```dax
+LOOKUP(<column>[, <filter1> [, <filter2> [, …]]])
+```
+
+### Parameters
+
+|Term|Definition|
+|--------|--------------|
+|column| Only column reference is allowed,  For example [Sales] is allowed, but [Sales] – [Cost] is not, nor is SUM([Sales]) |
+|filter1, filter2,…|(Optional) Filter has to be either: 1. An equality filter. For example [Year]=2019 or [Year]=MAX([Year]). 2.
+The Collapse function. For example, LOOKUP([Sales], Collapse([Country])) returns the subtotal value for Sales for all Countries|
+
+## Return value
+
+The value of **column** at the row after filters are applied.
+
+If there isn't a match that satisfies all the search values, an error is returned.
+
+If multiple rows match the filters, an error is returned.
+
+## Example 1
+
+In this example, LOOKUP retrieves the sum of sale easily when [Year] = 2006, [MonthNumberOfYear] = 1, [Month] = "January".
+
+```dax
+// GB column references
+Define
+var _Core = SUMMARIZECOLUMNS(
+        'DimDate'[Year],
+		'DimDate'[Month], 'DimDate'[MonthNumberOfYear],
+        'DimProduct'[ProductCategoryName],
+        'DimProduct'[ProductSubcategoryName],
+      "SumOfInternetSales", CALCULATE(SUM('FactInternetSales'[SalesAmount]))
+    )
+table t = _Core
+	with visual shape
+	axis rows 
+		group [Year]
+		group [Month], [MonthNumberOfYear]
+		order by [Year], [MonthNumberOfYear] 
+	axis columns 
+		group [ProductCategoryName]
+		group [ProductSubcategoryName]
+		order by [ProductCategoryName], [ProductSubcategoryName] 
+	densify "isDensified"
+
+column t[ccLookup] = 
+lookup([SumOfInternetSales], [Year] = 2006, [MonthNumberOfYear] = 1, [Month] = "January")
+
+evaluate selectcolumns(
+	filter(t, not isblank([SumOfInternetSales])),
+	[ProductCategoryName], [ProductSubcategoryName], [Year], [MonthNumberOfYear], [SumOfInternetSales], [ccLookup]
+)
+order by [ProductCategoryName], [ProductSubcategoryName], [Year], [MonthNumberOfYear]
+```