Converting a NVARCHAR into a UK date - sql-server

Question

I'm looking to convert a users input that will be a NVARCHAR to a UK date
The user's input will always be a maximum of 6 digits.
For example:
151119 (DDMMYY) --> 15NOV19
120119 (DDMMYY) --> 12JAN19

Answer 1

You need a valid DATE or DATETIME / DATETIME2 value first. You can get this value using this SELECT query with CONVERT:
SELECT CONVERT(DATE,
SUBSTRING('151119', 3, 2) + '-'
+ SUBSTRING('151119', 1, 2) + '-'
+ SUBSTRING('151119', 5, 2)
, 10);
Now you have two possibilities to get the date in the expected format (DDMMMYY).
1 - solution using DATENAME:
-- set the date value (from custom date value).
DECLARE #dDateValue AS DATE = CONVERT(DATE,
SUBSTRING('150419', 3, 2) + '-'
+ SUBSTRING('150419', 1, 2) + '-'
+ SUBSTRING('150419', 5, 2)
, 10);
-- format the date value to the expected format.
SELECT DATENAME(DD, #dDateValue) +
UPPER(LEFT(DATENAME(MM, #dDateValue), 3)) +
RIGHT(DATENAME(YY, #dDateValue), 2);
2 - solution using CONVERT:
-- set the date value (from custom date value).
DECLARE #dDateValue AS DATE = CONVERT(DATE,
SUBSTRING('150419', 3, 2) + '-'
+ SUBSTRING('150419', 1, 2) + '-'
+ SUBSTRING('150419', 5, 2)
, 10);
-- format the date value to the expected format.
SELECT UPPER(REPLACE(CONVERT(VARCHAR, #dDateValue, 6), ' ', ''));
demo on dbfiddle.uk
Note: I recommend to store the date values on columns with DATE or DATETIME data type. You can format the date value on your application or using format functions on SQL-Server directly.

Answer 2

You need to alter the string into something that looks even little like a normal date format, then you can use convert:
declare #textDate nvarchar(6) = '120119';
select #textDate
, stuff(stuff(#textDate, 3,0,'/'), 6,0,'/')
, convert(date, stuff(stuff(#textDate, 3,0,'/'), 6,0,'/'), 3);
Gives:120119,12/01/19,2019-01-12
As that last one is a valid date datatype, you can now use Format().
Though obviously 2 digit years are a bad idea in the first place.
Forgot to say, the 3 is for UK 2 digit format(dd/mm/yy), and you can see the other choices here.

Answer 3

I highly recommend to work with date values on columns AS DATE or DATETIME type. This is a simple implementation for your need :
SQL Fiddle
MS SQL Server 2017 Schema Setup:
create table test (mydate nvarchar(6))
insert into test (mydate)values('151119')
insert into test (mydate)values('120119')
Query 1:
select FORMAT(CONVERT(DATE, dates), 'dd/MM/yyyy ') from (select left(mydate,2)
+
CASE SUBSTRING(mydate, 3, 2)
WHEN '01' THEN 'JAN'
WHEN '02' THEN 'FEB'
WHEN '03' THEN 'MAR'
WHEN '04' THEN 'APR'
WHEN '05' THEN 'MAY'
WHEN '06' THEN 'JUN'
WHEN '07' THEN 'JUL'
WHEN '08' THEN 'AUG'
WHEN '09' THEN 'SEP'
WHEN '10' THEN 'OCT'
WHEN '11' THEN 'NOV'
WHEN '12' THEN 'DEC'
ELSE 'error'
END
+ right(mydate,2) AS 'dates'
from test ) AS T
Results:
| |
|-------------|
| 15/11/2019 |
| 12/01/2019 |