SQL: Select 2 tables without join - sql

Question

I wrote this SQL query:
SELECT *
FROM
dbo.RDB_LOG_ITEM,
(
SELECT '000' + CAST(operatore as varchar) + cast(scontrino as varchar) search
FROM
(
SELECT
N0_XACT_NO scontrino,
N0_OPERATOR_NO operatore
FROM
dbo.RDB_SCALE_ITEM
WHERE
BL_RECORD_EXPLODED = 0 AND
N0_COUNTER_NO = 1 AND
DT_TIME_STAMP LIKE '20160526%'
) db
) db2
WHERE
DT_TIME_STAMP > '2016-05-26T00:00:00.000' AND
SZ_SCALE_LABEL LIKE db2.search + '%'
But this query is executed in 3+ sec. The result of this query is a single row. The result of the select db2 are only 7 rows.
I think when I use from data1,db2 that SQL does a cross join (data1 is a big db with something like 300k+ rows) and slows the process.
If I try to write the select hard-coded with the result from the 2nd select I get the result in 0.01 sec like this: select * from data1 where DT_TIME_STAMP > '2016-05-26T00:00:00.000' and SZ_SCALE_LABEL like '0001013530%'
How can I use the db2 without joining it with the other db?
edit
the subquery:
(
SELECT '000' + CAST(operatore as varchar) + cast(scontrino as varchar) search
FROM
(
SELECT
N0_XACT_NO scontrino,
N0_OPERATOR_NO operatore
FROM
dbo.RDB_SCALE_ITEM
WHERE
BL_RECORD_EXPLODED = 0 AND
N0_COUNTER_NO = 1 AND
DT_TIME_STAMP LIKE '20160526%'
) db
) db2
give me X rows like
0001013530
0001013531
0001013532
0001013533
0001013534
what i need is a query like this select * from dbo.RDB_LOG_ITEM where DT_TIME_STAMP > '2016-05-26T00:00:00.000' and (SZ_SCALE_LABEL like '0001013530%' or SZ_SCALE_LABEL like '0001013531%' or SZ_SCALE_LABEL like '0001013532%' or SZ_SCALE_LABEL like '0001013533%' or SZ_SCALE_LABEL like '0001013534%')
i think is something near the subquery IN http://www.dofactory.com/sql/subquery but with the LIKE
PS sorry for the incomplete post but i was at work and they was kicking me for close :-)

Answer 1

First, you should be using explicit JOINs. Your WHERE clause should not have more than one table (in this case, considering the subquery as a "table").
Second, there is no need for a subquery here at all. You need to get out of the habit of leaning on subqueries and think in a set-based way when writing SQL.
Finally, use your aliases to prefix all of your columns in a query. It's impossible to tell where some of these columns are coming from without prefixing them.
I believe that this will get you the same results and will ideally be done in a performant way. If it's not, then you will need to post the full table structures (including indexes) as well as the query plans for anyone to be able to help you with performance issues.
SELECT
LI.dt_time_stamp,
LI.sz_scale_label,
<list other columns here, because we **never** use SELECT *>
FROM
dbo.RDB_LOG_ITEM LI
INNER JOIN dbo.RDB_SCALE_ITEM SI ON
SI.bl_record_exploded = 0 AND
SI.no_counter_no = 1 AND
SI.dt_time_stamp LIKE '20160526%' AND -- You should be using date datatypes for your date columns
LI.sz_scale_label LIKE '000' + CAST(operatore AS VARCHAR(20)) + cast(scontrino AS VARCHAR(20)) + '%' -- I guessed on appropriate VARCHAR sizes, which you should have
WHERE
LI.dt_time_stamp > '2016-05-26T00:00:00.000'

Answer 2

How much data does your tables have? If they have huge data, without join your execution time will be more. I had similar experience. Using join reduced my execution time significantly.