Does SQL BETWEEN work with string dates?

问题

I have a large SQL which has to generate some dates to use and compare with a DATETIME column that I also convert to a string for the purposes of this to compare strings with strings. But does BETWEEN work with strings?

...
    AND ((pur.StudyYearID <= @StudyYear
        AND CONVERT(varchar, pur.StartDate, 112) BETWEEN CONVERT(varchar, GETDATE(), 112)
                                                     AND CONVERT(varchar, CAST(CAST(YEAR(DATEADD(YEAR, 1, GETDATE())) AS varchar) + '1231' AS DATETIME), 112))
         OR (pur.StudyYearID > @StudyYear
        AND CONVERT(varchar, pur.StartDate, 112) BETWEEN STR(YEAR(GETDATE()) + SUBSTRING(pur.StudyYearID, 2, 1) - SUBSTRING(@Workgroup, 1, 1)) + '0101' 
                                                     AND STR(YEAR(GETDATE()) + SUBSTRING(pur.StudyYearID, 2, 1) - SUBSTRING(@Workgroup, 1, 1)) + '1231'))
    ...

I cannot pin point it, but I "think" this is working. However, I am dubious about the BETWEEN. Does BETWEEN only work with real dates such as DATETIME data types, or can it work like I did above with Strings? And if not, I assume I would have to remove the BETWEEN and replace it with dreaded >= and <=, right?

By the way each date string calculation above does work, as I've tested them individually and I do get dates in the YYYYMMDD format.

Thanks

UPDATE The point of the above was to avoid using >= and <= when comparing strings. I wanted to use BETWEEN, but with DATEs as strings as I wanted to make sure I am comparing "apples with apples". However, it was "poor programming" as I could have simply CAST()ed to strip off the time and used BETWEEN, as mentioned below.

@StudyYearID can be anything from [S1,S2,S3 or S4] @WorkGroup can be anything from '1A,1B,1C,1D,1E up to 4E'

Sorry for the confusion

回答1:

Oh my gosh, so many things going wrong here.

Converting all of these date/datetime values to strings. Don't do it. Date and datetime values are dates and datetime values - converting them to strings causes all kinds of bad things like lack of validation, inability to use indexes for seeks or range scans, dropping of all kinds of in-built date function support, etc.
Converting to varchar without length. This is a bad habit and should be avoided so you aren't a victim to silent truncation.
Using BETWEEN for date range queries. You should only do this if the underlying column is DATE, and even then I'd be careful. In order to properly use underlying indexes (that exist today, or that may exist tomorrow), you should use an open-ended date range.

I think this query is much tidier by avoiding all of the conversions to strings, and also gives you a much better shot at index usage, should an index ever exist on StartDate (today or in the future).

  AND 
  (
    (
      pur.StudyYearID <= @StudyYear
      AND pur.StartDate >= @d AND 
      AND pur.StartDate < DATEADD(YEAR, 1, @next_year)
    )
    OR 
    (
      pur.StudyYearID > @StudyYear
      AND pur.StartDate >= DATEADD(YEAR, YEAR(GETDATE()) 
          + SUBSTRING(pur.StudyYearID, 2, 1) - LEFT(@WorkGroup, 1) - 1900, 0)
      AND pur.StartDate < DATEADD(YEAR, 1 + YEAR(GETDATE()) 
          + SUBSTRING(pur.StudyYearID, 2, 1) - LEFT(@WorkGroup, 1) - 1900, 0)
    )
  )

回答2:

Why are you trying to compare dates as strings? Dates should be DATETIME objects. It's not clear what you're doing with StudyYearID or @Workgroup, but if a part of the date needs to be built up some other way, then convert it to a DATETIME object before doing your BETWEEN comparison.

If there is no other way to come up with your "between" values other than building them up as strings first, then at least cast before comparing, like this:

    AND (pur.StartDate BETWEEN GETDATE()
                AND CAST(CAST(YEAR(DATEADD(YEAR, 1, GETDATE())) AS varchar) + '-12-31' AS DATETIME))

... same with your other between values.

回答3:

Yes, dates converted to a strings in ISO 8601 format will sort in chronological order using any of the collation settings that ship with SQL Server. The question everyone is going to ask you is, "Why in the world are you converting dates to strings before comparing them with each other?"

来源：https://stackoverflow.com/questions/20412334/does-sql-between-work-with-string-dates

标签

sql-server

string

datetime

between