In the second example you have to set the order to inv

 use sales in 0 alias sales order inv

>select sales
>mkey='003333'

 seek mkey

>scan while inv=mkey
>    disp PK
>endscan
>use in sales

>I wonder why these two codes show difference results:
>
>

>use sales in 0 alias sales
>select sales
>mkey='003333'
>disp PK for inv=mkey
>use in sales
>

>
>result:
>--------
>0001
>0002
>0045
>0046
>
>

>use sales in 0 alias sales
>select sales
>mkey='003333'
>seek mkey order inv
>scan while inv=mkey
>    disp PK
>endscan
>use in sales
>

>
>result:
>-------
>0001
>0002
>
>
>thanks for the help