Question

I am observing a strange behaviour. The following code that is related to conversion of a substring of nvarchar to tinyint works fine:

DECLARE @s nvarchar(50) = N'Line 1'
DECLARE @n tinyint = SUBSTRING(@s, 6, 10)

SELECT
        N'Line 1' AS explicit_nvarchar,
        @s AS nvarchar_variable,
        SUBSTRING(@s, 6, 10) AS nvarchar_substring
        , @n AS tinyint_var_converted_earlier
        , CAST(SUBSTRING(@s, 6, 10) AS tinyint) AS cast_sub_var
        , CAST(SUBSTRING(N'Line 1', 6, 10) AS tinyint) AS cast_nvarchar_substr

I can observe the following output in the Microsoft SQL Server Management Studio:

.

However, the code that tries to do with the SELECT from the tables fails if the commented line is uncommented:

Select 
    [order].[identifier] As [order_identifier],
    [plan_data].[starts_on] As [from],
    [plan_data].[ends_on] As [to],
    [workplace].[identifier] AS line_identifier,
    SUBSTRING([workplace].[identifier], 6, 10) AS line_no_str
    --, CAST(SUBSTRING([workplace].[identifier], 6, 10) AS int) AS line_no
From 
    ...

With commented line…

but with uncommented line (both for conversion to tinyint or int)…

Update: Strange. I have added the last line to the WHERE to check whether all lines
contain the 'Line...

Where
    [plan].[identifier] = @lPlanIdentifier And
    (
        [plan_data].[starts_on] Between @fromUTC And @toUTC Or
        [plan_data].[ends_on] Between @fromUTC And @toUTC)
    AND LEFT([workplace].[identifier], 4) = N'Line'

… and all of a sudden it works also with the uncommented line. How can I discover what causes the problem?

Update2: I should have followed the Hamlet Hakobyan’s answer exactly as it was published. When looking inside the table, I can see:

Answer 1

You have value in identifier column for which SUBSTRING([workplace].[identifier], 6, 10) returns ined.

Try this query:

SELECT * FROM [workplace]
WHERE SUBSTRING([identifier], 6, 10) = N'ined'