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How to Search for Date and Time Values Using SQL Server 2000


SQL ServerBefore you can effectively query date/time (or temporal) data, you have to know something about how date/time values are stored.  In this article, Bryan demonstrates the various ways in which to query an SQL Server to successfully retrieve the records you may be seeking.

Author Info:
By: Bryan Syverson
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December 04, 2003
TABLE OF CONTENTS:
  1. · How to Search for Date and Time Values Using SQL Server 2000
  2. · How Dates and Times are Stored in SQL Server
  3. · Date/Time Values are Approximate Numerics
  4. · Dates Without Times and Times Without Dates
  5. · The Effect of Database Design on Querying
  6. · Performance Considerations in Querying
  7. · How to Search by Date
  8. · How to Search by Time

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How to Search for Date and Time Values Using SQL Server 2000 - Date/Time Values are Approximate Numerics
(Page 3 of 8 )

Datetime and smalldatetime are like the floating-point data types, float and real, in that they’re approximate numerics. That means the value retrieved from SQL Server may be different from the value that was originally stored. For example, if you store the expression 10/3.0 in a column of data type float, you’ll retrieve a value 3.3333330000000001. Although this is a reasonable representation of ten thirds, it’s not exact since it’s rounded past the 6th digit. In fact, if you add three such values together, you get 9.9999990000000007, not 10. Of course, most programmers understand this as a rounding error. And it’s a persistent problem for all digital computers, not just those running SQL Server. Still, you need to be aware of it as you code search conditions.

In contrast, when working with exact numeric data, the value retrieved from SQL Server is exactly the value that was originally stored. For example, if you store 10/3.0 in a column of data type int, it’s stored as 3 and retrieved as 3. In this case, SQL Server implicitly casts the result of the expression as a real value, 3.333333. Then, SQL Server implicitly casts 3.333333 as an integer because it’s being stored in a column of type int. Although this is still a rounding error, it occurs before the value is stored, not as a result of the physical limitations of computer storage. In other words, the error was introduced by using the wrong data type, not by the inherent limitation of the data type itself. Since the system always returns the same value as was stored, the data type is exact.

Now, to see how this affects date/time values, consider the date and time value for 8:00AM on January 4, 1900. As you saw above, noon on this day is stored as 3.5, or halfway through the fourth day. In contrast, 8:00AM is one third of the way through the day, so its representation will be approximate. To see this for yourself, submit the following query:

SELECT CAST(CAST('1900-01-04 08:00' AS datetime) AS float)

You’ll get the following result:

3.3333333333333335

But if you submit this query:

SELECT CAST(3.3333333 AS datetime), CAST(3.3333334 AS datetime)

you’ll get the following results:

1900-01-04 07:59:59.997             1900-01-04 08:00:00.003

As you can see, these three values are all quite close. In fact, they’re close enough to be considered 8:00AM for most applications. However, in a search condition based on a single value, such as:

WHERE (DTValue = '1900-01-04 08:00')

you’d only match those rows where the stored value exactly matches 3.3333333333333335. You’ll see how to get around this later in this article.


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