count_distinct() (aggregation function) - (preview)
Counts unique values specified by the scalar expression per summary group, or the total number of unique values if the summary group is omitted.
Null values are ignored and don't factor into the calculation.
Note
This function is used in conjunction with the summarize operator.
If you only need an estimation of unique values count, we recommend using the less resource-consuming dcount
aggregation function.
To count only records for which a predicate returns true
, use the count_distinctif aggregation function.
Note
- This function is limited to 100M unique values. An attempt to apply the function on an expression returning too many values will produce a runtime error (HRESULT: 0x80DA0012).
- Function performance can be degraded when operating on multiple data sources from different clusters.
Syntax
count_distinct
(
expr)
Learn more about syntax conventions.
Parameters
Name | Type | Required | Description |
---|---|---|---|
expr | scalar | ✔️ | The expression whose unique values are to be counted. |
Returns
Long integer value indicating the number of unique values of expr per summary group.
Example
This example shows how many types of storm events happened in each state.
StormEvents
| summarize UniqueEvents=count_distinct(EventType) by State
| top 5 by UniqueEvents
Output
State | UniqueEvents |
---|---|
TEXAS | 27 |
CALIFORNIA | 26 |
PENNSYLVANIA | 25 |
GEORGIA | 24 |
NORTH CAROLINA | 23 |
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