Convert string to date in MS SQL Server

I have table in which there is a column named period_start which is type of nvarchar. And it has different types of date formats.

Ex.

 1. 01122018 --ddmmyyyy

 2. 01132018 --mmddyyyy

 3. 20181802 --yyyyddmm

 4. 20180214 --yyyymmdd

 5. 2018-01-02--yyyy-mm-dd

 6. 01/02/2018 --dd-mm-yyyy

 7. 010218 --ddmmyy

These are the dates which i want to convert in database date format.

I tried writing this code but wasn't successful.

Create Function dbo.[ConvStrToDate]
(@str nvarchar(50))
Returns Date
As
Begin
Declare @ConvertedDate Date;
Set @str = convert(date,@str,104)
Return @ConvertedDate
End

Real Data Example..

This works for all the examples you've given us, and I've had to utterly guess your design, as you haven't posted that DDL and DML I requested. The CONVERT (Transact-SQL) documentation is your friend here, so if you have more styles you haven't supplied, I suggest using that page as a guide to get the results you need.

CREATE TABLE dbo.StringDates (NotDate varchar(12),
                              FormatString varchar(12));
GO

INSERT INTO dbo.StringDates (NotDate,
                             FormatString)
VALUES('01122018','ddmmyyyy'),
      ('01132018','mmddyyyy'),
      ('20181802','yyyyddmm'),
      ('20180214','yyyymmdd'),
      ('2018-01-02','yyyy-mm-dd'),
      ('01/02/2018','dd-mm-yyyy'),
      ('010218','ddmmyy');
GO

SELECT CASE FormatString WHEN 'ddmmyyyy' THEN TRY_CONVERT(date, STUFF(STUFF(NotDate,5,0,'/'),3,0,'/'),103)
                         WHEN 'mmddyyyy' THEN TRY_CONVERT(date, STUFF(STUFF(NotDate,5,0,'/'),3,0,'/'),101)
                         WHEN 'yyyyddmm' THEN TRY_CONVERT(date,CONCAT(LEFT(NotDate,4), RIGHT(NotDate,2),SUBSTRING(NotDate, 5,2)),112)
                         WHEN 'yyyymmdd' THEN TRY_CONVERT(date,NotDate,120)
                         WHEN 'yyyy-mm-dd' THEN TRY_CONVERT(date,NotDate,121)
                         WHEN 'dd-mm-yyyy' THEN TRY_CONVERT(date,REPLACE(NotDate,'-','/'),103)
                         WHEN 'ddmmyy' THEN TRY_CONVERT(date, STUFF(STUFF(NotDate,5,0,'/'),3,0,'/'),3)
       END,
       FormatString
FROM dbo.StringDates

There is, however, only one real solution here. Fix that data type:

UPDATE StringDates
SET NotDate = CONVERT(varchar(8),CASE FormatString WHEN 'ddmmyyyy' THEN TRY_CONVERT(date, STUFF(STUFF(NotDate,5,0,'/'),3,0,'/'),103)
                                                   WHEN 'mmddyyyy' THEN TRY_CONVERT(date, STUFF(STUFF(NotDate,5,0,'/'),3,0,'/'),101)
                                                   WHEN 'yyyyddmm' THEN TRY_CONVERT(date,CONCAT(LEFT(NotDate,4), RIGHT(NotDate,2),SUBSTRING(NotDate, 5,2)),112)
                                                   WHEN 'yyyymmdd' THEN TRY_CONVERT(date,NotDate,120)
                                                   WHEN 'yyyy-mm-dd' THEN TRY_CONVERT(date,NotDate,121)
                                                   WHEN 'dd-mm-yyyy' THEN TRY_CONVERT(date,REPLACE(NotDate,'-','/'),103)
                                                   WHEN 'ddmmyy' THEN TRY_CONVERT(date, STUFF(STUFF(NotDate,5,0,'/'),3,0,'/'),3)
                                        END,112);

ALTER TABLE dbo.StringDates ALTER COLUMN NotDate date;

SELECT *
FROM dbo.StringDates;

GO
--clean up
DROP TABLE dbo.StringDates;