T-sql - determine if value is integer

With sqlserver 2005 and later you can use regex-like character classes with LIKE operator. See here.

To check if a string is a non-negative integer (it is a sequence of decimal digits) you can test that it doesn't contain other characters.

SELECT numstr
  FROM table
 WHERE numstr NOT LIKE '%[^0-9]%'

Note1: This will return empty strings too.

Note2: Using LIKE '%[0-9]%' will return any string that contains at least a digit.

See fiddle

Here's a blog post describing the creation of an IsInteger UDF.

Basically, it recommends adding '.e0' to the value and using IsNumeric. In this way, anything that already had a decimal point now has two decimal points, causing IsNumeric to be false, and anything already expressed in scientific notation is invalidated by the e0.

In his article Can I convert this string to an integer?, Itzik Ben-Gan provides a solution in pure T-SQL and another that uses the CLR.

Which solution should you choose?

Is the T-SQL or CLR Solution Better? The advantage of using the T-SQL solution is that you don’t need to go outside the domain of T-SQL programming. However, the CLR solution has two important advantages: It's simpler and faster. When I tested both solutions against a table that had 1,000,000 rows, the CLR solution took two seconds, rather than seven seconds (for the T-SQL solution), to run on my laptop. So the next time you need to check whether a given string can be converted to an integer, you can include the T-SQL or CLR solution that I provided in this article.

If you only want to maintain T-SQL, then use the pure T-SQL solution. If performance is more important than convenience, then use the CLR solution.

The pure T-SQL Solution is tricky. It combines the built-in ISNUMERIC function with pattern-matching and casting to check if the string represents an int.

SELECT keycol, string, ISNUMERIC(string) AS is_numeric,
  CASE
    WHEN ISNUMERIC(string) = 0     THEN 0
    WHEN string LIKE '%[^-+ 0-9]%' THEN 0
    WHEN CAST(string AS NUMERIC(38, 0))
      NOT BETWEEN -2147483648. AND 2147483647. THEN 0
    ELSE 1
  END AS is_int
FROM dbo.T1;

The T-SQL part of the CLR solution is simpler. You call the fn_IsInt function just like you would call ISNUMERIC.

SELECT keycol, string, ISNUMERIC(string) AS is_numeric,
  dbo.fn_IsInt(string) AS is_int
FROM dbo.T1;

The C# part is simply a wrapper for the .NET's parsing function Int32.TryParse. This works because the SQL Server int and the .NET Int32 are both 32-bit signed integers.

using System;
using System.Data.SqlTypes;

public partial class UserDefinedFunctions
{
    [Microsoft.SqlServer.Server.SqlFunction]
    public static SqlBoolean fn_IsInt(SqlString s)
    {
        if (s.IsNull)
            return SqlBoolean.False;
        else
        {
            Int32 i = 0;
            return Int32.TryParse(s.Value, out i);
        }
    }
};

Please read Itzik's article for a full explanation of these code samples.