SQL multi-table query to eliminate duplicate content in a table

See a friend write another SQL statement: Two tables A1 table has SN, SN2, TN,B1 table has SM, SM2, TN2, if the data in the SN of A1 and B1 of the data in the SM is consistent, then the corresponding data in A1 is modified to the corresponding data in B1.

Update a1 Set SN2 = (select B1. SM2 from B1 where A1. SN = B1. SM), TN = (select B1. TN2 from B1 where A1. SN = B1. SM) where  A1. SN in (select Smfrom B1);

SQL multi-table query to eliminate duplicate content in a table