SQL statements will be missed some parts and causes the query failed

There is a simple schema 'test', a table 'test1', a column 'test1c' for char(10).

There is one record '123' in 'test'.'test1'

use ADODB RecordSet to retrieve data.

set a = "123"
sql = "select test1c from test.test1 where test1c = '" & a & "';"
rs = conn.execute(sql)

it will produce the error 'near testc = '123 error'

and if I debug for this problem by use response.write, the strange output occurs.

set a = "123"
sql = "select test1c from test.test1 where test1c = '" & a & "';"
response.write sql
rs = conn.execute(sql)

the result is

select test1c from test.tesmt1 where test1c = '123
(the following is error message for lacking the last part of sql statements)

once I add response.end after response.write sql, the result is the same.

set a = "123"
sql = "select test1c from test.test1 where test1c = '" & a & "';"
response.write sql
response.end
rs = conn.execute(sql)

and the last time I comment out the rs, and sql statements will be fine.

set a ="'123"
sql = "select test1c from test.test1 where test1c = '" & a & "';"
response.write sql
response.end
//rs = conn.execute(sql)

select test1c from test.test1 where tect1c = '123';

why is the rs affect the sql statements?