what is the result [duplicate] - sql

Question

This question already has answers here:
Why is casting from float to varchar being rounded in SQL Server?
(3 answers)
Closed 3 years ago.
The following query gives the output as 123.5
SELECT STR(123.45, 6, 1);
GO
But.. The following query gives the output as 123.3
SELECT STR(123.35, 6, 1);
GO
Why it is not giving the result as 123.4 ?
SELECT STR(123.45, 6, 1);
GO
SELECT STR(123.35, 6, 1);
GO
The following query gives the output as 123.3
SELECT STR(123.35, 6, 1);
GO
Why it is not giving the result as 123.4 ?

Answer 1

From the documentation for SQL Server's STR function:
[the first parameter] Is an expression of approximate numeric (float) data type with a decimal point.
From my local testing, even if I pass in a DECIMAL value, the imprecision you are seeing continues, and the input parameter still gets treated as a float.
That is:
SELECT STR(CAST(123.45 AS DECIMAL(10,2)), 6, 1)
still returns 123.5.
If you want to truncate a numerical value exactly in SQL Server, then just try casting to a DECIMAL type, e.g.
SELECT CAST(123.45 AS DECIMAL(10,1))
returns 123.4 as you would expect.

Answer 2

Long story short, use FORMAT instead of STR, only to format strings. There is an inconsistency here indeed.
STR is rounding half to odd which is an .... odd behavior. The more common approach is round half to even, also known as Banker's rounding. Both strategies minimize the aggregated rounding error.
That's why
SELECT STR(123.45, 6, 1),STR(123.35, 6, 1);
Returns
123.5 123.3
STR is not a rounding function though, it's a string formatting function. The strings it produces aren't meant to be added. What's more, T-SQL's ROUND function rounds away from zero and
SELECT round(123.45,1), round(123.35,1);
Produces
123.50 123.40
Oracle, PostgreSQL and MySQL's ROUND behave the same way, rounding away from zero. I suspect this is part of the standard but I haven't found a relevant link yet.
The FORMAT is far more powerful. It uses the same format strings as .NET and rounds away from zero.
SELECT FORMAT(123.45, 'n1'), FORMAT(123.35, 'n1');
Produces
123.5 123.4