/*
Usage
select dbo.STR_TO_DATE('20121105','%Y%m%d')
select dbo.STR_TO_DATE('2012-11','%Y-%m')
select dbo.STR_TO_DATE('05:06:07','%H:%i:%s')
select dbo.STR_TO_DATE('05','%m')
select dbo.STR_TO_DATE('Jun','%m')
select dbo.STR_TO_DATE('June','%M')
select dbo.STR_TO_DATE('90','%y')
select dbo.STR_TO_DATE('1990','%Y')


*/

CREATE function STR_TO_DATE(@dateValue varchar(50), @MySQLStyleformat varchar(200))
returns datetime
as
begin
declare @dt datetime;

if @dateValue is not null
select @dt= 
case rtrim(ltrim(@MySQLStyleformat)) on collate Latin1_General_CS_AI
when '%Y%m%d' then convert(datetime,@dateValue,120)
when '%Y-%m-%d' then convert(datetime,@dateValue,120)
when '%m%d%Y' then convert(datetime,@dateValue,120)
when '%Y-%m' then convert(datetime,@dateValue + '-01',120)
when '%Y-%m-%d' then convert(datetime,@dateValue ,120)
when '%Y%m%d %H:%i:%s' /*yyyyMMdd H:mm:ss*/ then convert(datetime,@dateValue ,120)
when '%y%m%d %H:%i:%s' /*yyMMdd H:mm:ss*/ then convert(datetime,@dateValue ,120)
when '%H:%i:%s' /*H:mm:ss*/ then convert(datetime,@dateValue ,120)
when '%H:%i' /*H:mm*/ then convert(datetime,@dateValue ,120) 
when '%m' /*MM*/ then convert(datetime,@dateValue + '01 1900' ,120) 
when '%M' /*MMMM*/ then convert(datetime,@dateValue + '01 1900' ,120)  
when '%b' /*MMM*/ then convert(datetime,@dateValue + '01 1900',120)  
when '%y' /*yy*/ then convert(datetime,substring(cast(datepart(yy, getdate()) as varchar(4)),1,2) + @dateValue + '/1/1' ,120)  
when '%Y' /*yyyy*/ then convert(datetime,@dateValue + '1/1' ,120)  when '%m%d' /*MMDD*/ then convert(datetime,@dateValue + '1900' ,120)  
when '%H:%i ' /*h:mm tt*/ then convert(datetime,@dateValue ,120)  
end

The input for '%y' will be two digit and for '%Y' of four, if we can make case statement to differentiate between y and Y i.e. upper and lowercase then  our STR_TO_DATE will work as desired. 

Now this is throwing exception when we execute 

select dbo.STR_TO_DATE('1990','%Y')

as the statement when '%y' /*yy*/ then convert(datetime,substring(cast(datepart(yy, getdate()) as varchar(4)),1,2) + @dateValue + '/1/1' ,120)  executed first and date became '2019090/1/1' 

We do not need case sensitiveness for whole DB, therefor we can not switch to Latin1_general_CI_AS. 

All suggestions are welcome.