関数を使用しない場合:
declare @t table(C varchar(18))
insert @t values('abc'), ('1234'), (' 1234a')
;with CTE as
(
select C, '[' + substring(c, a.n, 1) + ']' v, rn from
(select 1 n union all
select 2 union all
select 3 union all
select 4 union all
select 5 union all
select 6) a
cross apply
(select c, row_number() over (order by C) rn from @t group by c) b
where a.n <= len(C)
)
update t3
set C = t4.[value]
FROM @t t3
JOIN
(
select C,
(
select v
from CTE t1
where t1.rn = t2.rn
for xml path(''), type
).value('.', 'varchar(18)') [value]
from CTE t2
group by t2.rn, C
) t4
ON t3.C = t4.C
SELECT * FROM @t