ms access - SQL Update query on query result -

i have 2 tables like:

table 1

sid     sdefinition   cvalue 4057    s1             32 4058    s2              4059    s3             6 4060    s4 

mapping_tbl

sid     sinid   ecfid   sid-sinid 4057    1099    4027e   1099_4057 4058    1099    4027e   1099_4058 4059    1121    4003e   1121_4059 4060    1121    4003e   1121_4060 

query1

  select mapping_tbl.sid, table1.sdefinition, table1.cvalue   table1 inner join mapping_tbl on table1.sid= mapping_tbl.sid; 

query1(result)

   sid  sdefinition cvalue    4057    s1      32    4058    s2          4059    s3      6    4060    s4         

i have situation wanted update query table (query1) i.e set field(cvalue) 0 if contains null. using update query like

   update query1 set cvalue = 0 cvalue null; 

the query table (query1) gets updated , sets cvalue 0 if contains nulls, , updates(set 0) table1 cvalues null.

how can avoid updating table1? suggestions.

it seems don't want change values stored in table, , want query display 0 when cvalue null. can have query substitute 0 null in result set (without changing values in table) ...

select m.sid, t.sdefinition, iif(t.cvalue null, 0, t.cvalue) [cvalue] table1 t inner join mapping_tbl m on t.sid= m.sid; 

for query in access session, use vba nz() function ...

select m.sid, t.sdefinition, nz(t.cvalue null, 0) [cvalue] table1 t inner join mapping_tbl m on t.sid= m.sid; 

i used aliases table names. don't need them null substitution work. prefer aliases.