toscalar()

Applies to: ✅ Microsoft Fabric ✅ Azure Data Explorer ✅ Azure Monitor ✅ Microsoft Sentinel

Returns a scalar constant value of the evaluated expression.

This function is useful for queries that require staged calculations. For example, calculate a total count of events, and then use the result to filter groups that exceed a certain percent of all events.

Any two statements must be separated by a semicolon.

Syntax

toscalar(expression)

Learn more about syntax conventions.

Parameters

Name	Type	Required	Description
expression	string	✔️	The value to convert to a scalar value.

Returns

A scalar constant value of the evaluated expression. If the result is a tabular, then the first column and first row will be taken for conversion.

Tip

You can use a let statement for readability of the query when using toscalar().

Limitations

toscalar() can't be applied on a scenario that applies the function on each row. This is because the function can only be calculated a constant number of times during the query execution. Usually, when this limitation is hit, the following error will be returned: can't use '<column name>' as it is defined outside its row-context scope.

In the following example, the query fails with the error:

'toscalar': can't use 'x' as it is defined outside its row-context scope.

let _dataset1 = datatable(x:long)[1,2,3,4,5];
let _dataset2 = datatable(x:long, y:long) [ 1, 2, 3, 4, 5, 6];
let tg = (x_: long)
{
    toscalar(_dataset2| where x == x_ | project y);
};
_dataset1
| extend y = tg(x)

This failure can be mitigated by using the join operator, as in the following example:

Run the query

let _dataset1 = datatable(x: long)[1, 2, 3, 4, 5];
let _dataset2 = datatable(x: long, y: long) [1, 2, 3, 4, 5, 6];
_dataset1
| join (_dataset2) on x 
| project x, y

Output

x	y
1	2
3	4
5	6

Examples

Evaluate Start, End, and Step as scalar constants, and use the result for range evaluation.

Run the query

let Start = toscalar(print x=1);
let End = toscalar(range x from 1 to 9 step 1 | count);
let Step = toscalar(2);
range z from Start to End step Step | extend start=Start, end=End, step=Step

Output

z	start	end	step
1	1	9	2
3	1	9	2
5	1	9	2
7	1	9	2
9	1	9	2

The following example shows how toscalar can be used to "fix" an expression so that it will be calculated precisely once. In this case, the expression being calculated returns a different value per evaluation.

Run the query

let g1 = toscalar(new_guid());
let g2 = new_guid();
range x from 1 to 2 step 1
| extend x=g1, y=g2

Output

x	y
e6a15e72-756d-4c93-93d3-fe85c18d19a3	c2937642-0d30-4b98-a157-a6706e217620
e6a15e72-756d-4c93-93d3-fe85c18d19a3	c6a48cb3-9f98-4670-bf5b-589d0e0dcaf5