compare two tables if tableb coloum is null repalce table a

Question

I HAVE TEO TABLES LIKE..

TABLE_A

SNO   NAME         MOBILE            EMPID
1       RAJU        9014494876       A12345
2       VAMSHI    9765432123       B12367
3       KARTHIK   986623456         C34567


TABLE_B
SNO     NAME           MOBILE           EMPID
1         SRINU        970123454         D123489
2         RAJENDER      'NULL'             B12367
3         KARTHIK     934642389         C34567     

COMPARE TWO TABLE A & B  EMPID AND  IF TABLE B MOBILE FIELD 'NULL' UPADATE  TABLE A MOBILE TO TABLE B

i want o/p like table_b
SNO     NAME           MOBILE           EMPID
1         SRINU        970123454         D123489
2         RAJENDER      9765432123     B12367
3         KARTHIK     934642389         C34567   

plz write quarie ...................