[![uniq][1]][1]


  [1]: https://i.sstatic.net/icnml.png

i tried alot by field calculator but it gives me null as u show in table 


and another try it gives me value 1 in all rows .

most of examples,on internet they are exlaining old version and i have QGIS version 3  and iam not sure how can i add id for all rows .

if i want to generate random id ,how can i do it also 
Thanks in advance