ISDATE() revisited

ISDATE is a wonderful function used to test if the underlying value is a valid date. I have often found developers wink-their-eyes in shock over ISDATE’s non-deterministic behaviour. I hope this small presentation will help developers use ISDATE with care.

What this function does?

ISDATE function returns 1 if the value/expression under test is a valid datetime and returns 0 for all other values.

Syntax

ISDATE (value/expression under test)

Let us know take a deep-dive into application of this function. Though it would be very easy for me to list down what is valid (when function returns 1) and what is invalid (when function returns 0) date; it would be better to apply and check how this function behaves in different scenarios (build over different application sets, given below, with varied complexity). These will build-up our learning base very well.

Application Set – 1

Below is the list of queries where ISDATE is applied to different expressions. For all the these cases, consider that dateformat is set to DMY and language is set to English (US).

SELECT ISDATE('15/01/2012') -- Result returned = 1
SELECT ISDATE('15-01-2012') -- Result returned = 1
SELECT ISDATE('15-2012-01') -- Result returned = 1
SELECT ISDATE('01-15-2012') -- Result returned = 0
SELECT ISDATE('01-2012-15') -- Result returned = 0
SELECT ISDATE('2012-01-15') -- Result returned = 0
SELECT ISDATE('2012-15-01') -- Result returned = 1

As per ISDATE logic it is clear that for #1 and #2 the return value should be 1 and for all other it should be 0. But this is not the case!

#3 and #7 also have return value of 1. Why??

Because, SQL Server can implicitly convert expression (character string specifically) in #3 and #7 into valid datetime and for others (#4, #5 and #6) implicit conversion will fail with out-of-range conversion error. To demonstrate this, just try to execute the following code snippet in SSMS.

PRINT '@DT1'
DECLARE @DT1 DATETIME = '15/01/2012';

PRINT '@DT2'
DECLARE @DT2 DATETIME = '15-01-2012';

PRINT '@DT3'
DECLARE @DT3 DATETIME = '15-2012-01';

PRINT '@DT4'
DECLARE @DT4 DATETIME = '01-15-2012';

PRINT '@DT5'
DECLARE @DT5 DATETIME = '01-2012-15';

PRINT '@DT6'
DECLARE @DT6 DATETIME = '2012-01-15';

PRINT '@DT7'
DECLARE @DT7 DATETIME = '2012-15-01';

You will find the following error message for @DT4, @DT5 and @DT6
The conversion of a varchar data type to a datetime data type resulted in an out-of-range value.

Application Set – 2

Let us try to re-execute the same set of statements executed in Application Set – 1, but this time dateformat is set to MDY (SET DATEFORMAT MDY)

SELECT ISDATE('15/01/2012') -- Result returned = 0
SELECT ISDATE('15-01-2012') -- Result returned = 0
SELECT ISDATE('15-2012-01') -- Result returned = 0
SELECT ISDATE('01-15-2012') -- Result returned = 1
SELECT ISDATE('01-2012-15') -- Result returned = 1
SELECT ISDATE('2012-01-15') -- Result returned = 1
SELECT ISDATE('2012-15-01') -- Result returned = 0

Return value of the function is just the opposite. This does not need any explanation. But it is worth mentioning here that

response of ISDATE function depends upon the current dateformat

Application Set – 3

Dateformat being MDY let us change the language setting to something other than English (US) and check the result. Say for example, we are changing the language to Italian (SET LANGUAGE Italian)

SELECT ISDATE('15/01/2012') -- Result returned = 1
SELECT ISDATE('15-01-2012') -- Result returned = 1
SELECT ISDATE('15-2012-01') -- Result returned = 1
SELECT ISDATE('01-15-2012') -- Result returned = 0
SELECT ISDATE('01-2012-15') -- Result returned = 0
SELECT ISDATE('2012-01-15') -- Result returned = 0
SELECT ISDATE('2012-15-01') -- Result returned = 1

Again we noticed that this has an impact on ISDATE response. Hence it is worth mentioning that

response of ISDATE function depends upon current language settings

Application Set – 4

Let us try to check the behaviour of ISDATE when expression is of the following types (dateformat being DMY and language being English (US)):

  • Varchar
  • N Varchar
  • Datetime
  • Datetime2
  • Date
  • Null expression

Here is the code snippet used to check this:

SET DATEFORMAT DMY;
SET LANGUAGE English;
-- When expression => VARCHAR
DECLARE @dt_varchar VARCHAR(10) = '15-01-2012'
SELECT ISDATE(@dt_varchar)
GO
-- When expression => N'VARCHAR
DECLARE @dt_NVarchar NVARCHAR(10) = N'15-01-2012'
SELECT ISDATE(@dt_NVarchar)
GO
-- When expression => NULL
DECLARE @dt_Null DATETIME = NULL
SELECT ISDATE(@dt_Null)
GO
-- When expression => DATETIME
DECLARE @dt_DateTime DATETIME = '15-01-2012 00:00:00.000'
SELECT ISDATE(@dt_DateTime)
GO
-- When expression => DATETIME2
DECLARE @dt_DateTime2 DATETIME2 = '15-01-2012 00:00:00.0000000'
SELECT ISDATE(@dt_DateTime2)
GO
-- When expression => DATE
DECLARE @dt_Date DATE = '15-01-2012'
SELECT ISDATE(@dt_Date)
GO

Here is the resultant

#
Expression
Return Value
1
VARCHAR
1
2
N'VARCHAR
1
3
Is Null
0
4
DATETIME
1
5
DATETIME2
ERROR: Invalid argument
6
DATE
ERROR: Invalid argument

From above, picture about the behaviour of ISDATE is very clear now. It is also crystal clear that Datatime2 and Date data type are not valid arguments for ISDATE function. But care should be taken when datetime2 value is directly passed as character string in ISDATE argument. Here return value will be zero rather than an error message.

SELECT ISDATE('15-01-2012 00:00:00.0000000')
-- Returns 0

In nutshell, we can summarize our discussion with following points:

  • Behaviour of ISDATE function is dependent on default dateformat and language
  • It returns 1 when the string argument can implicitly be converted into valid datetime value. Increasing the seconds precision of the same string value to represent datetime2 format (or even if the seconds precision exceeds 3 significant digits) will return a 0 value.
  • Date and Datetime2 arguments are not allowed, they result in invalid argument error
  • Null values inside expressions also results in 0 as return value
  • Values such as text, ntext and image data types also return 0 value

I hope you enjoyed reading this post and must have admitted few facts to your memory.

The motivation to write this post came from SQLServerCentral Question of the Day. Here is the link to the question posted by me –> link

Happy Learning
Lokesh Vij

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