All applications need to retrieve data in SQL Server tables based on DATETIME and/or SMALLDATETIME columns. In your particular application, you may need to select records that were entered on a particular date. On the other hand, you might need to select a set of records that have a DATETIME column value for a particular month, or year. In other case, you might want to find all the records between two different dates. Possibly, you might need to find the first, or last record entered in a given month, day, or year. This article will discuss selecting records from a database table based on values in a DATETIME, or SMALLDATETIME column.
Prior to discussing selecting records for a particular DATETIME value, let's review what specific values are stored in a given DATETIME and SMALLDATETIME column. From my first article in this series you should recall that a DATETIME column contains a date and time value, where time is accurate to milliseconds and SMALLDATETIME columns hold a date and time value, but the time portion is only accurate to one minute. Since these date/time columns store the time portion you will need to consider this when searching for records where the column holds a specific date. You will need to provide the date and time portion in the search criteria or you may not return any records or the records you wish to return. If you are not sure of the exact time associated with the records you want to retrieve you should search based on a date and/or time range. Let's go through a couple of examples to show you what I am talking about.
In order to show you different methods of searching SQL Server tables, I will need a sample table. The table I will be using is a very simple table called DATE_SAMPLE and here is a list of records in that table.
Common Mistakes When Searching for Dates:
When searching for dates there are a number of common mistakes that new SQL Server programmers sometimes make. In this section, I will show you two common date/time pitfalls.
The intent of this first example is to select all the records in the DATE_SAMPLE table that have a SAMPLE_DATE equal to '2003-04-09'. Here is the code:
SELECT * FROM DATE_SAMPLE WHERE SAMPLE_DATE = '2003-04-09'
When this code is run only record 4 is returned. Why are records 5, 6 & 7 not returned? Can you tell why? Remember DATETIME, or SMALLDATE columns contain not only the date but also the time. In this particular example SAMPLE_DATE is a DATETIME column, so all the dates store contain a time, down to the milliseconds. When you specify a search criteria that only contains a date, like the above example, SQL Server needs to first convert the string expression '2003-04-09' to a date and time value, prior to matching the string with the values in the SAMPLE_DATE column. This conversion creates a value of '2003-04-09 00:00:00.000', which matches with only record 4.
Another common mistake is to use the BETWEEN verb like so:
SELECT * FROM DATE_SAMPLE WHERE SAMPLE_DATE between '2003-04-09'
When using the BETWEEN verb all records that are between or equal to the dates specified are returned. Now if in my example above I only wanted to return records that have a SAMPLE_DATE in '2003-04-09'. This example returns all the records that have a SAMPLE_DATE in '2003-04-09' (records 4 - 7), but also returns record 8 that has a SAMPLE_DATE of '2003-04-10'. Since the BETWEEN clause is inclusive of the two dates specified, record 8 is also returned.
Now if you really desire to select all the records in the DATE_SAMPLE table that have a SAMPLE_DATE sometime in '2003-04-09' you have a couple of options. Let me go through each option and then explain why one might be better than another might.
Using the Convert Function:
This first example selects all records from the DATE_SAMPLE where the date portion of the SAMPLE_DATE is equal to '2003-04-09'.
SELECT * FROM DATE_SAMPLE
WHERE CONVERT(CHAR(10),SAMPLE_DATE,120) = '2003-04-09'
The reason this example works, and the first example above does not, is because this example removes the time portion of the SAMPLE_DATE column prior to the comparison with string '2003-04-09' being performed. The CONVERT function removes the time portion by truncating the value of the SAMPLE_DATE field to only the first 10 characters.