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Date, Math and Text Functions in SQL Server 2000

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In this third article in his series on functions, Baya Pavliashvili continues to explore various types of built-in functions, including the most frequently used math, text, date, and time functions.
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In the previous articles in this series, I introduced you to aggregate and system-related functions. This article continues to explore various types of built-in functions. I will walk you through the most useful functions that fall into date and time, math, and text function categories.

Date and Time Functions

Date and time functions enable a programmer to get the system date, as well as to manipulate the date and time values stored in the database. Because date and time functions are useful in many different circumstances, it's difficult to emphasize one particular usage as being more essential than the others.

One complaint that you might hear is that SQL Server does not allow storing only the date or only the time—you must store both date and time in the same column if you use the DATETIME or SMALLDATETIME data type. Of course, you have the alternative of storing date values as strings, as in '1/1/2003'. Another alternative is to use the DATETIME data type and then use one of the date and time functions to retrieve only the needed portion (date or time) from the table.

One of my other InformIT.com articles covered the functions DATEPART and DATENAME in detail. DATEPART retrieves a portion of the date and time value (month, day, weekday, year, hour, minute, and so on) as an integer; DATENAME returns the character (for weekdays) or integer (for everything but weekdays) representation. DATEPART is deterministic in all cases except when it refers to weekdays. DATENAME, on the other hand is non-deterministic. (Please refer to my article "SQL Server: Determining Whether a Date is a Business Day" for an extensive coverage of these functions.)

The DAY(), MONTH(), and YEAR() functions are deterministic. Each of these functions accepts a single date value as a parameter and returns respective portions of the date as an integer. All three of these functions can be duplicated by retrieving the same portions of the date using the DATEPART function, as shown in the following example:

SELECT 
    DAY('1/1/2003'), 
    MONTH('1/1/2003'), 
    YEAR('1/1/2003'), 
    DATEPART(DAY, '1/1/2003'), 
    DATEPART(MONTH, '1/1/2003'), 	
    DATEPART(YEAR, '1/1/2003')

Results:

----------- -----------  -----------  -----------  -----------  ----------- 
1           1            2003         1            1            2003

GETDATE() and GETUTCDATE() both return the current date and time. However, GETUTCDATE() returns the current Universal Time Coordinate (UTC) time, whereas GETDATE() returns the date and time on the computer where SQL Server is running. By the way, GETUTCDATE() does not have any magic power for determining the appropriate UTC time—it simply compares the time zone of SQL Server computer with the UTC time zone. Note that neither of these functions accepts parameters, and they are both non-deterministic. Here is an example:

SELECT 
    GETDATE() AS local_date, 
    GETUTCDATE() AS UTC_date

Results:

local_date                      UTC_date                       
-------------------------       ---------------------------- 
2003-01-26 14:32:35.713         2003-01-26 20:32:35.713

The DATEADD() and DATEDIFF() functions are both deterministic and can be very helpful in reporting applications (among other uses). The DATEADD() function adds a certain period of time to the existing date and time value. For instance, you can use the following query to determine the date six months from today:

SELECT DATEADD(MONTH, 6, GETDATE())AS '6_months_from_now'

Results:

6_months_from_now    
------------------------
2003-07-26 14:38:55.960

DATEADD() is also often used to determine which rows qualify for a particular report. Suppose that you want to see the report of all titles that have sold in the past nine years. you can effectively use DATEADD() with -9 as a parameter, as follows:

SELECT 
    DISTINCT 
    a.title_id, 
    title, 
    YEAR(ord_date) AS year_sold 
FROM sales a INNER JOIN titles b
ON a.title_id = b.title_id
WHERE 
ord_date > = DATEADD(YEAR, -9, GETDATE())

Results:

title_id   title                                          year_sold  
--------   ----------------------------------------       ----------- 
BU1032     The Busy Executive's Database Guide            1994
BU2075     You Can Combat Computer Stress!                2002
MC3021     The Gourmet Microwave                          1994
PS2091     Is Anger the Enemy?                            1994

The DATEDIFF() function accepts two DATETIME values and a date portion (minute, hour, day, month, and so on) as parameters. DATEDIFF() determines the difference between the two date values passed, expressed in the date portion specified. Notice that the start date should come before the end date if you want to see positive numbers in the result set. For instance, the following query determines the time difference between today and when each sale occurred in terms of months (the output is limited to the first five rows to save some room):

SELECT TOP 5
    ord_date,
    DATEDIFF(MONTH, ord_date, GETDATE()) AS no_of_months_since
FROM sales

Results:

ord_date                         no_of_months_since 
----------------------------     ----------------- 
1994-09-14 00:00:00.000          100
1994-09-13 00:00:00.000          100
1993-05-24 00:00:00.000          116
1994-09-13 00:00:00.000          100
1994-09-14 00:00:00.000          100

DATEDIFF() will work even if the end date is earlier than the start date—you will simply get negative values in the output. Keep in mind that DATEDIFF() returns an INTEGER; it does not calculate fractions for you. This might not seem relevant at first glance, but check out what happens when you compare the beginning and end of 2002 with the first day of 2003:

SELECT 
    DATEDIFF (YEAR, '1/1/2002', '1/1/2003'), 
    DATEDIFF (YEAR, '12/31/2002', '1/1/2003')

Results:

----------- ----------- 
1           1

So whether it is the first or last day of 2002, the difference between years is still 1, regardless of what you might expect.

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