UPDATE SET FORMAT not working in SQL Server 2016? - sql

Question

FORMAT instruction works in a SELECT but has no effect in an UPDATE:
SELECT ##VERSION
DROP TABLE IF EXISTS #t;
CREATE TABLE #t(DateMin datetime);
INSERT INTO #t VALUES ('2019-13-01 00:00:00')
SELECT * FROM #t
UPDATE #t SET DateMin = FORMAT(DateMin, 'dd/MM/yyyy');
SELECT * FROM #t;
SELECT #DateMin AS a, FORMAT(#DateMin, 'dd/MM/yyyy') AS b

Answer 1

A type like DATETIME isn't stored with a format.
So if one updates a DATETIME with a string in a certain format, it doesn't matter for the stored value in the DATETIME field.
The formatted string is implicitly converted to a datetime. At least if it's in a format that's valid.
The function FORMAT, which returns a NVARCHAR is rather used for representation of the datetime field in a query.
Or if one wants to INSERT/UPDATE a string field with a datetime in a certain format. But that should be avoided, because it's much easier to work with a datetime than a string.

Answer 2

If you want to change that format for the user use this:
set dateformat dmy;
By running this statement:
DBCC USEROPTIONS;
you will see your dateformat is ydm so you can alway back it up to that if this is not what you wanted :)

Answer 3

You cannot set the output format of a datetime in the datetime itselfs.
If you need to output the datetime as formatted char/varchar, you need to use the convert-function when you select the data:
SELECT CONVERT(char(10), CURRENT_TIMESTAMP, 101) -- format: MM/dd/yyyy
SELECT CONVERT(char(10), CURRENT_TIMESTAMP, 103) -- format: dd/MM/yyyy
In your case:
SELECT #DateMin AS a, CONVERT(char(10), #DateMin, 103) AS b
That works as expected.
If you want to have a mutable data-type, you need to declare it as sql_variant:
DROP TABLE IF EXISTS #t;
CREATE TABLE #t(DateMin sql_variant);
INSERT INTO #t VALUES ('2019-01-13T00:00:00')
UPDATE #t SET DateMin = FORMAT(CAST(DateMin AS datetime), 'dd''/''MM''/''yyyy');
SELECT * FROM #t;
Also, your format-expression needs to explicitly put the / into quotation marks, aka 'dd''/''MM''/''yyyy', otherwise sql-server replaces it with the date-separator specific to the current culture, which would be . in my case.
Just use convert with option 103 instead, it works on all versions of sql-server and it's probably faster.
Also, your insert-statement fails on some versions of sql-server, because iso-date-format is 2019-01-13T00:00:00 and not 2019-13-01 00:00:00
Correct is:
INSERT INTO #t VALUES ('2019-01-13T00:00:00')
Also
DROP TABLE IF EXISTS #t;
is sql-server 2016+ only, otherwise you need
IF OBJECT_ID('tempdb..#t') IS NOT NULL DROP TABLE #t
And post sql-server 2005, you should use datetime2 instead of datetime.
You shouldn't use datetime anymore, because datetime uses float, and as such is imprecise - if you insert an iso datetime value, it can do funny things because of the float-point-machine-epsilon, e.g. set it to the next day if you have 23:59:59.999, just as a scary example.
I advise you to never use the sql_variant type. If you have a temp-table with defined columns, just create another column where you will write the char/varchar value to.