Is SQL Server 'MONEY' data type a decimal floating point or binary floating point?

问题

I couldn't find anything that rejects or confirms whether SQL Server 'MONEY' data type is a decimal floating point or binary floating point.

In the description it says that MONEY type range is from -2^63 to 2^63 - 1 so this kind of implies that it should be a binary floating point.

But on this page it lists MONEY as "exact" numeric. Which kind of suggests that MONEY might be a decimal floating point (otherwise how is it exact? or what is the definition of exact?)

Then if MONEY is a decimal floating point, then what is the difference between MONEY and DECIMAL(19,4) ?

回答1:

Neither. If it were an implementation of floating point it would be subject to the same inaccuracies as FLOAT and REAL types. See Floating Point on wikipedia.

MONEY is a fixed point type.

It's one byte smaller than a DECIMAL(19,4), because it has a smaller range (922,337,203,685,477.5808 to 922,337,203,685,477.5807) as opposed to (-10^15+1 to 10^15-1).

回答2:

To see the differences we can look at the documentation:

Documentation for money:

Data type  Range                                                 Storage
money      -922,337,203,685,477.5808 to 922,337,203,685,477.5807 8 bytes
smallmoney -214,748.3648 to 214,748.3647                         4 bytes

The money and smallmoney data types are accurate to a ten-thousandth of the monetary units that they represent.

Compare to decimal:

When maximum precision is used, valid values are from -10^38 + 1 through 10^38 - 1.

Precision    Storage
1 - 9        5 bytes
10 - 19      9 bytes
20 - 28      13 bytes
29 - 38      17 bytes

So they're not exactly equivalent, just similar. A DECIMAL(19,4) has a slightly greater range than MONEY (it can store from -10^15 + 0.0001 to 10^15 - 0.0001), but also needs one more byte of storage.

In other words, this works:

CREATE TABLE Table1 (test DECIMAL(19,4) NOT NULL);
INSERT INTO Table1 (test) VALUES
(999999999999999.9999);
SELECT * FROM Table1 

999999999999999.9999

But this doesn't:

CREATE TABLE Table1 (test MONEY NOT NULL);
INSERT INTO Table1 (test) VALUES
(999999999999999.9999);
SELECT * FROM Table1 

Arithmetic overflow error converting numeric to data type money.

There's also a semantic difference. If you want to store monetary values, it makes sense to use the type money.