Substring in Where clause

Question

Below is my query, I am looking to pull records where only the substring value (which will be YYYY) is less than current year - 25 years.... and I should mention that this field is varchar and probably needs to be converted which I haven't been able to do successfully either.

SELECT 
            AccountNumber,
            LoanPrimeLongName,
            convert (varchar,LoanOpenDate,103)LoanOpenDate,
            LoanOriginalBalance,
            LoanBalance,
            LoanInterestRate,
            LoanRemainingTermMonths,
            LoanDelqDays,
            LoanDescription
FROM 
            ARCU.ARCULoanDetailed 
WHERE 
            (((LOANTYPE = '15'          OR
            LOANTYPE = '16'         OR
            LoanType = '17')            AND
            LoanStatus = 'Open')        AND
            ProcessDate = (CONVERT(VARCHAR, GETDATE(), 112)-1)) AND
            (SUBSTRING (loandescription,1,4) not like '%[^0-9]%')

ORDER BY AccountNumber

Answer 1

Since we don't know which RDBMS you are using, I'm going to go with the simplest answer. This assumes you're using MSSQL.

Use the ISNUMERIC() function to determine if SUBSTRING(loandescription,1,4) is actually a number. If it is, then you can cast/convert it at that point and compare it to the "year" you're interested in. IE:

        ...
        ProcessDate = (CONVERT(VARCHAR, GETDATE(), 112)-1)) AND
        (ISNUMERIC(SUBSTRING(loandescription,1,4)) = 1 AND 
        CAST(SUBSTRING(loandescription,1,4) AS INT) = (YEAR(GETDATE()) - 25))

Due to boolean short circuiting, if the first 4 characters AREN'T numeric, then it won't bother casting to compare. Same rule applies with other RDBMS systems (such as MySQL, PostgreSQL, SQLite, etc), but the methods might be a bit different. In fact, I don't think MySQL or PostgreSQL even have the ISNUMERIC function, meaning you need to find other ways (Regex) to test.