declare @表tbl table (name varchar(10),cn varchar(10))
insert into @表tbl
select 'fda,o',null union all
select 'aa,o=',null union all
select 'bb,3,o',null

update @表tbl set cn=left(name,charindex(',o',name)-1)
select * from @表tbl
/*
name       cn
---------- ----------
fda,o      fda
aa,o=      aa
bb,3,o     bb,3
*/

------解决方案--------------------
SQL code

update t set cn=substring(name,1,charindex(',o',name)-1)

------解决方案--------------------
SQL code

update tb 
set cn=substring(name,1,charindex(',o',name)-1) 
where charindex(',o',name) > 0