Populate random data from another table

Question

update dataset1.test
   set column4 = (select column1 
                 from dataset2
                 order by random()
                 limit 1
                 )          


        

Answer 1

Better is to reference the outer table from the subquery. Then the subquery has to be evalued for every row:

update dataset1.test
   set column4 = (select
        case when dataset1.test.column4 = dataset1.test.column4
             then column1 end
        from dataset2
        order by random()
        limit 1
   )