MySQL uses user variables to update group sorting

The first requirement is sorted according to the A field, and the sort results are updated to the B field

After a simple search, you get answers quickly.

Http://dev.mysql.com/doc/refman/5.7/en/update.html

SET @a = 0 ; UPDATE SET = SELECT @a :=@a+1 )  ORDERbyASC 

The second requirement is to sort the B field according to the A field, sort the results to the C field

SET @c = 0;SET @A = 0;UPDATETableNameSETC= (SELECT  Case  when @A =A Then @c:= @c + 1ELSE     (@c:= @c + 1  and @A:=A)END)ORDER  byA, b

The result is correct, but I don't quite understand why ...

To be continue

MySQL uses user variables to update group sorting