Do Not Concatenate VARCHAR and VARCHAR(MAX) Variables

Do Not Concatenate VARCHAR and VARCHAR(MAX) Variables, what happens is that the whole string will be implicitly converted to varchar(8000)

Run these examples to see what I mean

declare @v varchar(max)
select @v = (cast('a' as varchar)) + replicate('a', 9000)

select len(@v)
--8000
GO

declare @v varchar(max)
select @v = (cast('a' as varchar(1))) + replicate('a', 9000)

select len(@v)
--8000
GO

declare @v varchar(max)
select @v = (cast('a' as varchar)) +replicate (cast('a' as varchar(max)), 9000)

select len(@v)
--9001
GO

declare @v varchar(max)
select @v = (cast('a' as varchar(1))) + replicate(cast('a' as varchar(max)), 9000)

select len(@v)
--9001
GO


Or how about this? If you don't convert to varchar(max) while doing the LEN function it returns 8000

declare @v varchar(max)
select @v = replicate('a', 9000)
select len(@v)


declare @v varchar(max)
select @v = replicate(cast('a' as varchar(max)), 9000)
select len(@v)