How do I combine a substring and trim right in SQL - sql

Question

I am trying to extract the data between two underscore characters. In some situations, the 2nd underscore may not exist.
MyFld
P_36840
U_216137
C_203134_H
C_203134_W
I tried this:
substring(i.[MyFld],
CHARINDEX ('_',i.[MyFld])+1,len(i.[MyFld])
-CHARINDEX ('_',i.[MyFld])
) [DerivedPrimaryKey]
And I get this:
DerivedPrimaryKey
36840
216137
203134_H
203134_W
https://dbfiddle.uk/uPKC6oX4
I want to remove the second underscore and data that follows it. I'm trying to combine it with a trim right, but I'm unsure where to start.
How can I do this?

Answer 1

We can start by simplifying what you have so far. I will also add enough to make this a complete query, so we can see it in context for later steps:
SELECT
right(i.MyFld, len(i.MyFld) - charindex('_', i.MyFld)) [DerivedPrimaryKey]
FROM I
With this much done, we can now use it as the source for removing the trailing portion of the field:
SELECT
reverse(substring(reverse(step1)
, charindex('_', reverse(step1))+1
, len(step1)
)) [DerivedPrimaryKey]
FROM (
SELECT right(i.MyFld, len(i.MyFld) - charindex('_', i.MyFld)) [step1]
FROM I
) T
Notice the layer of nesting. You can, of course, remove the nesting, but it means replicating the entire inner expression every time you see step1 (good thing I took the time to simplify it):
SELECT
reverse(substring(reverse(right(i.MyFld, len(i.MyFld) - charindex('_', i.MyFld)))
, charindex('_', reverse(right(i.MyFld, len(i.MyFld) - charindex('_', i.MyFld))))+1
, len(right(i.MyFld, len(i.MyFld) - charindex('_', i.MyFld)))
))
FROM I
And now back to just the expression:
reverse(substring(reverse(right(i.MyFld, len(i.MyFld) - charindex('_', i.MyFld)))
, charindex('_', reverse(right(i.MyFld, len(i.MyFld) - charindex('_', i.MyFld))))+1
, len(right(i.MyFld, len(i.MyFld) - charindex('_', i.MyFld)))
))
See it work here:
https://dbfiddle.uk/nFO4Vwhm
There is also this alternate expression that saves one function call:
left( right(i.MyFld,len(i.MyFld)-charindex('_',i.MyFld)),
coalesce(
nullif(
charindex('_',
right(i.MyFld,len(i.MyFld)-charindex('_',i.MyFld))
) -1, -1,
),
len( right(i.MyFld,len(i.MyFld)-charindex('_',i.MyFld)) )
)
)

Answer 2

Just a two more options. One using parsename() provided your data does not have more than 4 segments. The second using a JSON array
Example
Declare #YourTable Table ([MyFld] varchar(50)) Insert Into #YourTable Values
('P_36840')
,('U_216137')
,('C_203134_H')
,('C_203134_W')
Select *
,UsingParseName = reverse(parsename(reverse(replace(MyFld,'_','.')),2))
,UsingJSONValue = json_value('["'+replace(MyFld,'_','","')+'"]','$[1]')
From #You
Results
MyFld UsingParseName UsingJSONValue
P_36840 36840 36840
U_216137 216137 216137
C_203134_H 203134 203134
C_203134_W 203134 203134

Answer 3

We can do this:
Declare #testData Table ([MyFld] varchar(50));
Insert Into #testData (MyFld)
Values ('P_36840')
, ('U_216137')
, ('C_203134_H')
, ('C_203134_W');
Select *
, second_element = substring(v.MyFld, p1.pos, p2.pos - p1.pos - 1)
From #testData As td
Cross Apply (Values (concat(td.MyFld, '__'))) As v(MyFld) -- Make sure we have at least 2 delimiters
Cross Apply (Values (charindex('_', v.MyFld, 1) + 1)) As p1(pos) -- First Position
Cross Apply (Values (charindex('_', v.MyFld, p1.pos) + 1)) As p2(pos) -- Second Position
If you actually have a fixed number of characters in the first element, then it could be simplified to:
Select *
, second_element = substring(v.MyFld, 3, charindex('_', v.MyFld, 4) - 3)
From #testData td
Cross Apply (Values (concat(td.MyFld, '_'))) As v(MyFld)

Answer 4

Often I try to fake out SQL if an expected character isn't always present and I don't need the resulting value:
SELECT SUBSTRING(field_Calculated, 1, CHARINDEX('_', field_Calculated) - 1)
FROM (SELECT SUBSTRING(MyFld, CHARINDEX('_', MyFld) + 1, LEN(MyFld)) + '_' As field_Calculated
FROM MyTable) T
I think this is clear, but I really like the ParseName solution #JohnCappalletti suggests.
If it's only ever one numeric value you can use string_split:
SELECT * FROM MyTable
CROSS APPLY string_split(MyFld, '_')
WHERE ISNUMERIC(value) = 1
Either way you have to be careful of the data before deciding the best approach.

Answer 5

your data
Declare #Table Table ([MyFld] varchar(100))
Insert Into #Table
([MyFld] ) Values
('P_36840')
,('U_216137')
,('C_203134_H')
,('C_203134_W')
use SubString,Left and PatIndex
select
Left(
SubString(
[MyFld],
PatIndex('%[0-9.-]%', [MyFld]),
8000
),
PatIndex(
'%[^0-9.-]%',
SubString(
[MyFld],
PatIndex('%[0-9.-]%', [MyFld]),
8000
) + 'X'
)-1
) as DerivedPrimaryKey
from
#Table