Project allows you to select a subset of columns
//adds a calculated column to the result set
Perf
| where CounterName == "Free Megabytes"
|extend FreeGB = CounterValue / 1000
Perf
| project ObjectName
, CounterName
, InstanceName
, CounterValue
, TimeGenerated
//Combine project with extend
Perf
| where CounterName == "Free Megabytes"
| project ObjectName
, CounterName
, InstanceName
, CounterValue
, TimeGenerated
| extend FreeGB = CounterValue / 1000
, FreeMB = CounterValue
,FreeKB = CounterValue * 1000
//Combine project with extend
Perf
| where CounterName == "Free Megabytes"
| project ObjectName
, CounterName
, InstanceName
, CounterValue
, TimeGenerated
| extend FreeGB = CounterValue / 1000
, FreeMB = CounterValue
,FreeKB = CounterValue * 1000
//Hide the CounterValue column in the results
Perf
| where CounterName == "Free Megabytes"
| extend FreeGB = CounterValue / 1000
, FreeMB = CounterValue
,FreeKB = CounterValue * 1000
| project ObjectName
, CounterName
, InstanceName
, TimeGenerated
//Use project to build extend columns directly
Perf
| where CounterName == "Free Megabytes"
| project ObjectName
, CounterName
, InstanceName
, CounterValue
, TimeGenerated
, FreeGB = CounterValue / 1000
, FreeMB = CounterValue
, FreeKB = CounterValue * 1000
//remove columns from result set
Perf
| where TimeGenerated > ago(1h)
| project-away CounterValue
, SourceSystem
, CounterPath
, MG
//Renaming a column
Perf
| where TimeGenerated > ago(1h)
| project-rename myRenamedComputer = Computer
RootSecurity 