Declare scalar and tabular variables, only 1 being recognised in lower query-CodePudding

Can someone explain to me why the orderRef variable is accessible to the union query but orderId is not? Running the entire query gives an error message "'where' operator: Failed to resolve scalar expression named 'orderId'"

** I know the second let assignment works as if I execute orderId with an orderId statement immediately following the first 2 let assignments I will get the orderId that I am looking to use in the final union query.

// <config>
let orderRef = "ABCDEF";
// </config>

// get orderId
let orderId = app('my-function').traces
| where message has orderRef
| order by timestamp asc
| extend msg=parse_json(substring(message, 6, strlen(message))) 
| project msg.OrderId;

union *,
    app('my-function').traces,
    app('my-function2').traces,
    app('my-function3').traces,
| where message has orderRef or message has orderId 
| project timestamp, cloud_RoleName, operation_Name, message;

CodePudding user response：

toscalar()

let orderId = toscalar(app('my-function').traces
| where message has orderRef
| order by timestamp asc
| extend msg=parse_json(substring(message, 6, strlen(message))) 
| project msg.OrderId);