Could not able to figure out the error near convert sql - sql

Question

It is giving me an error saying 'Incorrect syntax near the keyword 'convert'.' Could not able to figure out what could be done to remove it.
ALTER PROCEDURE [dbo].[sp_convert_unix_timestamp]
#table_name nvarchar(250), ---Staging Table
#DateTimeString nvarchar(max)
AS
BEGIN
if isnull(#DateTimeString,'') <> '' begin
declare #SQLString Nvarchar(max)
declare #UpdateString Nvarchar(max)
declare #initialepoch Nvarchar(10)
set #initialepoch = '19700101'
SELECT #SQLString = coalesce(#SQLString + ',', '') +
'convert(varchar, dateadd(ss, convert(Int, ' + value + '),' + #initialepoch + '), 120)'
FROM STRING_SPLIT(#DateTimeString, ',')
WHERE RTRIM(value) <> ''
select #UpdateString = 'Update [' + #table_name + '] set ' + #SQLString
exec (#SQLString)
end
END

Answer 1

Firstly, let's fix the SP so it's no open to Injection, and the datatype choice for #table_name:
ALTER PROCEDURE [dbo].[sp_convert_unix_timestamp]
#table_name sysname, ---Staging Table
#DateTimeString nvarchar(max)
AS
IF ISNULL(#DateTimeString, '') <> ''
BEGIN
DECLARE #SQLString nvarchar(MAX);
DECLARE #UpdateString nvarchar(MAX);
DECLARE #initialepoch nvarchar(10);
SET #initialepoch = N'19700101';
SELECT #SQLString = COALESCE(#SQLString + ',', '') + N'convert(varchar(10), dateadd(ss, convert(Int, ' + QUOTENAME(value,N'''') + N'), + #dinitialepoch), 120)'
FROM STRING_SPLIT(#DateTimeString, ',')
WHERE RTRIM(value) <> '';
SELECT #UpdateString = N'Update ' + QUOTENAME(#table_name) + N' set ' + #SQLString;
PRINT #UpdateString;
EXEC sp_executesql #SQLString, N'#dinitialepoch nvarchar(10)', #dinitialepoch = #initialepoch;
END;
If we were to run that SP, EXEC [dbo].[sp_convert_unix_timestamp] N'MyTable', N'20170720,20170721';, you'd get this output (and magcially, you know the reason why it's not working):
Update [MyTable] set convert(varchar, dateadd(ss, convert(Int, '20170720'), + #dinitialepoch), 120),convert(varchar, dateadd(ss, convert(Int, '20170721'), + #dinitialepoch), 120)
There is no column in your SET clause. You simply have UPDATE [Table] SET [Value]; The format of an UPDATE statement is UPDATE [Table] SET [Column] = [Value];. What column(s) that needs to be updated, however, I have no idea...