声明标量和表格变量，在较低的查询中只能识别1

Question

有人能向我解释为什么联合查询可以访问orderRef变量，而orderId不能访问吗？运行整个查询会显示一条错误消息"'其中"operator:未能解析名为"orderId"的标量表达式">

**我知道第二个let分配的工作方式就像我在前两个let分配之后立即用orderId语句执行orderId一样。我将获得我希望在最终联合查询中使用的orderId。

// <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;

Answer 1

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);