SQL Server 2008 Date Teaser

It has been a while since my last teaser but here we go

What do you think the following returns?


SELECT CONVERT(datetime,'1/1/1') -CONVERT(datetime,1) + CONVERT(datetime,0)


How about this on SQL Server 2008


SELECT CONVERT(datetime2,'1/1/1'),CONVERT(datetime2,'01/01/01'),CONVERT(datetime2,'0001/01/01')


Now run this on SQL Server 2008

SELECT
ISDATE('1/1/1'),
ISDATE('01/01/01'),
ISDATE('001/01/01'),
ISDATE('0001/01/01')


Now just for fun run these 4 on SQL Server 2008, one of them will fail so run them one by one

SELECT CONVERT(datetime2,'1/1/1')
SELECT CONVERT(datetime2,'01/01/01')
SELECT CONVERT(datetime2,'001/01/01')
SELECT CONVERT(datetime2,'0001/01/01')

Compare the isdate output to the select statement, see the inconsistency?

SQL Teaser: Guess the output

What do you think will be the output?


DECLARE @d datetime
SET @d = '20071010'

SELECT DATEADD(yy, DATEDIFF(yy, 0, @d)+1, -1)

After that run this to see how to get first and last days for years, quarters, months and weeks(be careful of ISO weeks!)

DECLARE @d datetime
SET @d = '20071010'

SELECT DATEADD(yy, DATEDIFF(yy, 0, @d), 0) as FirstDayOfYear,
DATEADD(yy, DATEDIFF(yy, 0, @d)+1, -1) as LastDayOfYear,
DATEADD(qq, DATEDIFF(qq, 0, @d), 0) as FirstDayOfQuarter,
DATEADD(qq, DATEDIFF(qq, 0, @d)+1, -1) as LastDayOfQuarter,
DATEADD(mm, DATEDIFF(mm, 0, @d), 0) as FirstDayOfMonth,
DATEADD(mm, DATEDIFF(mm, 0, @d)+1, -1) as LastDayOfMonth,
DATEADD(wk, DATEDIFF(wk, 0, @d), 0) as FirstDayOfWeek,
DATEADD(wk, DATEDIFF(wk, 0, @d)+1, -1) as LastDayOfWeek

Do You Know How Between Works With Dates?

Do you use between to return data that has dates? Do you know that between will get everything since midnight from the first criteria and up to midnight exactly from the second criteria. If you do BETWEEN 2006-10-01 AND 2006-10-02 then all the values that are greater or equal than 2006-10-01 and less or equal to 2006-10-02 will be returned. So no values after 2006-10-02 midnight will be returned.

Let's test this out, first let's create this table

CREATE TABLE SomeDates (DateColumn DATETIME)

Insert 2 values
INSERT INTO SomeDates VALUES('2006-10-02 00:00:00.000')
INSERT INTO SomeDates VALUES('2006-10-01 00:00:00.000')

Return everything between '2006-10-01' and '2006-10-02'
SELECT *
FROM SomeDates
WHERE DateColumn BETWEEN '20061001' AND '20061002'
ORDER BY DateColumn
This works without a problem

Let's add some more dates including the time portion
INSERT INTO SomeDates VALUES('2006-10-02 00:01:00.000')
INSERT INTO SomeDates VALUES('2006-10-02 00:00:59.000')
INSERT INTO SomeDates VALUES('2006-10-02 00:00:01.000')
INSERT INTO SomeDates VALUES('2006-10-01 00:01:00.000')
INSERT INTO SomeDates VALUES('2006-10-01 00:12:00.000')
INSERT INTO SomeDates VALUES('2006-10-01 23:00:00.000')


Return everything between '2006-10-01' and '2006-10-02'
SELECT *
FROM SomeDates
WHERE DateColumn BETWEEN '20061001' AND '20061002'
ORDER BY DateColumn
Here is where it goes wrong; for 2006-10-02 only the midnight value is returned the other ones are ignored


Now if we change 2006-10-02 to 2006-10-03 we get what we want
SELECT *
FROM SomeDates
WHERE DateColumn BETWEEN '20061001' AND '20061003'
ORDER BY DateColumn

Now insert a value for 2006-10-03 (midnight)

INSERT INTO SomeDates VALUES('2006-10-03 00:00:00.000')

Run the query again
SELECT *
FROM SomeDates
WHERE DateColumn BETWEEN '20061001' AND '20061003'
ORDER BY DateColumn

We get back 2006-10-03 00:00:00.000; between will return the date if it is exactly midnight

If you use >= and < then you get exactly what you need
SELECT *
FROM SomeDates
WHERE DateColumn >= '20061001' AND DateColumn < '20061003'
ORDER BY DateColumn

--Clean up
DROP TABLE SomeDates


So be careful when using between because you might get back rows that you did not expect to get back and it might mess up your reporting if you do counts or sums

How Are Dates Stored Internally In SQL Server?

Internally dates are stored as 2 integers. The first integer is the number of dates before or after the base date (1900/01/01). The second integer stores the number of clock ticks after midnight, each tick is 1/300 of a second.


So if we run the following code for the base date (1900/01/01)

DECLARE @d DATETIME
SELECT @d = '1900-01-01 00:00:00.000'


SELECT CONVERT(INT,SUBSTRING(CONVERT(VARBINARY(8),@d),1,4)) AS DateInt,
SUBSTRING(CONVERT(VARBINARY(8),@d),1,4) AS DateBinary
SELECT CONVERT(INT,SUBSTRING(CONVERT(VARBINARY(8),@d),5,4)) AS TimeInt, SUBSTRING(CONVERT(VARBINARY(8),@d),5,4) AS TimeBinary
Go

The results are

DateInt DateBinary
----------- ----------
0 0x00000000


TimeInt TimeBinary
----------- ----------
0 0x00000000


If we use the max date 9999/12/31

DECLARE @d DATETIME
SELECT @d = '9999-12-31 23:59:59.997'


SELECT CONVERT(INT,SUBSTRING(CONVERT(VARBINARY(8),@d),1,4)) AS DateInt,
SUBSTRING(CONVERT(VARBINARY(8),@d),1,4) AS DateBinary
SELECT CONVERT(INT,SUBSTRING(CONVERT(VARBINARY(8),@d),5,4)) AS TimeInt, SUBSTRING(CONVERT(VARBINARY(8),@d),5,4) AS TimeBinary
Go


we get the following result

DateInt DateBinary
----------- ----------
2958463 0x002D247F

TimeInt TimeBinary
----------- ----------
25919999 0x018B81FF



If you take binary values and convert to datetime you get the following results

SELECT CONVERT(DATETIME,0x0000000000000001) --1 Tick 1/300 of a second
------------------------------------------------------
--1900-01-01 00:00:00.003

SELECT CONVERT(DATETIME,0x000000000000012C) -- 1 minute = 300 ticks
------------------------------------------------------
--1900-01-01 00:00:01.000

SELECT CONVERT(INT,0x12C) --= 300
SELECT CONVERT(VARBINARY(3),300) --= 0x00012C

SELECT CONVERT(DATETIME,0x0000000100000000) --add 1 day
------------------------------------------------------
--1900-01-02 00:00:00.000

For smalldatetime the time is stored as the number of minutes after midnight

Now here is some fun stuff

DECLARE @d DATETIME
SELECT @d = .0
SELECT @d
GO
------------------------------------------------------
--1900-01-01 00:00:00.000


DECLARE @d DATETIME
SELECT @d = .1
SELECT @d
GO
------------------------------------------------------
--1900-01-01 02:24:00.000

DECLARE @d DATETIME
SELECT @d = .12
SELECT @d
GO
------------------------------------------------------
--1900-01-01 02:52:48.000

DECLARE @d DATETIME
SELECT @d = '0'
SELECT @d
GO
Server: Msg 241, Level 16, State 1, Line 2
Syntax error converting datetime from character string.

DECLARE @d DATETIME
SELECT @d = 0
SELECT @d
GO
------------------------------------------------------
--1900-01-01 00:00:00.000

So there is no implicit conversion, o is fine 'o' is not


DECLARE @d DATETIME
SELECT @d = 20061030
SELECT @d
GO
Server: Msg 8115, Level 16, State 2, Line 2
Arithmetic overflow error converting expression to data type datetime.

DECLARE @d DATETIME
SELECT @d = '20061030'
SELECT @d
GO
------------------------------------------------------
--2006-10-30 00:00:00.000

Here we have the reverse, the varchar value is fine but the int is not.
This happens because the max integer value that a datetime can take is 36523
If we run the following we are okay

DECLARE @d DATETIME
SELECT @d = 2958463
SELECT @d
GO
------------------------------------------------------
--9999-12-31 00:00:00.000