Assuming you are using Mule JDBC transport, you do not need to quote the column value, so the correct version is:
select * from emp where emp_no = #[flowVars['emp-no']]
If your flowVars['emp-no'] is numerical and you really need the quotes, set the flowVars['emp-no'] to type String instead.
If you are not using Mule JDBC, and just want to set the example payload, use:
<set-payload value="select * from emp where emp_no = '#[flowVars['emp-no']]'" />