Substring instead of Left - sql

Question

All,
I have a query like this:
SELECT
[RecName],
CHARINDEX('_',[Rec Name]),
CASE
WHEN CHARINDEX('_',[Rec Name]) >=1 THEN
LEFT([Rec Name],CHARINDEX('_',[Rec Name])-1)
ELSE
'NA'
END RecTrimmed
FROM calculated_data
WHERE [Rec Name] = 'T101184_7AB_C1_ABCDE'
GROUP BY [Rec Name]
When I execute the above I get the following result - T101184
How can I achieve the same result using Substring?
Any lead would be appreciated.
Thanks,
John

Answer 1

One way is
with cte (col) as
(select 'T101184_7AB_C1_ABCDE' union all
select 'T1011847ABC1ABCDE')
select coalesce(nullif(substring(col,0,charindex('_',col)),''),'NA')
from cte;
Having said that, there is no reason you shouldn't use left for the problem you have at hand

Answer 2

One simple and quick solution could be to use an ordinal splitter to divide the string at '_' into rows. Then there's no need to fiddle around with LEFT and SUBSTRING and CHARINDEX.
The ordinal splitter can be found here
CREATE FUNCTION dbo.DelimitedSplit8K
--===== Define I/O parameters
(#pString VARCHAR(8000), #pDelimiter CHAR(1))
--WARNING!!! DO NOT USE MAX DATA-TYPES HERE! IT WILL KILL PERFORMANCE!
RETURNS TABLE WITH SCHEMABINDING AS
RETURN
--===== "Inline" CTE Driven "Tally Table" produces values from 1 up to 10,000...
-- enough to cover VARCHAR(8000)
WITH E1(N) AS (
SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL
SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL
SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1
), --10E+1 or 10 rows
E2(N) AS (SELECT 1 FROM E1 a, E1 b), --10E+2 or 100 rows
E4(N) AS (SELECT 1 FROM E2 a, E2 b), --10E+4 or 10,000 rows max
cteTally(N) AS (--==== This provides the "base" CTE and limits the number of rows right up front
-- for both a performance gain and prevention of accidental "overruns"
SELECT TOP (ISNULL(DATALENGTH(#pString),0)) ROW_NUMBER() OVER (ORDER BY (SELECT NULL)) FROM E4
),
cteStart(N1) AS (--==== This returns N+1 (starting position of each "element" just once for each delimiter)
SELECT 1 UNION ALL
SELECT t.N+1 FROM cteTally t WHERE SUBSTRING(#pString,t.N,1) = #pDelimiter
),
cteLen(N1,L1) AS(--==== Return start and length (for use in substring)
SELECT s.N1,
ISNULL(NULLIF(CHARINDEX(#pDelimiter,#pString,s.N1),0)-s.N1,8000)
FROM cteStart s
)
--===== Do the actual split. The ISNULL/NULLIF combo handles the length for the final element when no delimiter is found.
SELECT ItemNumber = ROW_NUMBER() OVER(ORDER BY l.N1),
Item = SUBSTRING(#pString, l.N1, l.L1)
FROM cteLen l
;
Then the query is very simple and executes efficiently
with cte (col) as (
select 'T101184_7AB_C1_ABCDE' union all
select 'T1011847ABC1ABCDE')
select Item
from cte c
cross apply
(select case when charindex('_',c.col)=0 then 'NA' else c.col end chr) ndx
cross apply
dbo.DelimitedSplit8K(ndx.chr, '_')
where ItemNumber=1;
Output
Item
T101184
NA