Avoid use of the MONEY and SMALLMONEY datatypes
The MONEY
data type confuses the storage of data values with their display, though its name clearly suggests the sort of data it holds. It is proprietary to SQL Server and allows you to specify monetary values preceded by a currency symbol, but SQL Server doesn’t store any currency information at all with the actual numeric values, so the purpose of this is unclear.
It has limited precision; the underlying type is a BIGINT
or, in the case of SMALLMONEY
, an INT
, so you can unintentionally get a loss of precision due to rounding errors. While simple addition or subtraction is fine, more complicated calculations that can be done for financial reports can show errors. Although the MONEY
datatype generally takes less storage, and takes less bandwidth when sent over networks, via TDS, it is generally far better to use a data type such as the DECIMAL
or NUMERIC
type, which is less likely to suffer from rounding errors or scale overflow.
A recommendation to avoid use of the MONEY
or SMALLMONEY
datatypes is included as a “Best Practice” code analysis rule in SQL Prompt (BP022).
Rounding errors when using MONEY datatype
The MONEY
and SMALLMONEY
data types are accurate to roughly a tenthousandth of the monetary units that they represent. SMALLMONEY
is accurate between – 214,748.3648 and 214,748.3647 whereas MONEY
is accurate between 922,337,203,685,477.5808 (922,337 billion) and 922,337,203,685,477.5807 (922,337 billion).
Although MONEY
can be represented with a currency symbol, this information isn’t stored. Under the covers, MONEY
is stored as an integer data type. A decimal number, the more usual choice for storing a monetary value, can range accurately between 10^38 +1 through 10^38 – 1. Several sqillion!
The scientific world can tolerate tiny rounding errors and margins of error, but in finance a monetary calculation is either right or wrong. It is futile to argue that the odd cent or pence isn’t worth worrying about; I have, myself, been laughed at when I was a smidgen out from the right answer.
Take the calculation in Listing 1, which is the simplest I can think of that illustrates the problem.
1 2 3 4 5 6 7 8 
DECLARE @MoneyTable TABLE (Total MONEY, Portion MONEY); INSERT INTO @MoneyTable (Total, Portion) VALUES ($271.00, $199.50), ($4639.00, $4316.00), ($8031.00, $7862.00), ($7558.00, $7081.00), ($9912.00, $9547.00), ($389.00, $179.00), ($4495.00, $4214.00), ($2844.00, $2398.00), ($265.67, $124.33), ($4936.56, $967.54); SELECT Portion, Total, (Portion / total)*100 AS Percentage FROM @MoneyTable 
Listing 1
Here are the results:
Notice the lack of any currency symbols for the Portion
and Total
values. The currency isn’t stored. It was useful in the VALUES
clause because it indicated to SQL Server that it should parse the scalar literal values such as $124.33 into the MONEY
datatype. Aside from that, though, are the percentage values correct? Let’s check that in Excel:
Hmm: doesn’t look quite right. Let’s rerun the calculation with decimal arithmetic (you’ll need to round the total or cast to numeric with a scale of two).
1 2 3 4 5 6 7 8 9 10 
DECLARE @MoneyTable TABLE (Total DECIMAL(19, 4), Portion DECIMAL(19, 4)); INSERT INTO @MoneyTable (Total, Portion) VALUES (271.00, 199.50), (4639.00, 4316.00), (8031.00, 7862.00), (7558.00, 7081.00), (9912.00, 9547.00), (389.00, 179.00), (4495.00, 4214.00), (2844.00, 2398.00), (265.67, 124.33), (4936.56, 967.54); SELECT Portion,Total, cast((Portion / Total) * 100 as numeric(19,2)) AS percentage FROM @MoneyTable 
Listing 2
This time, the answers are the same as we saw in Excel:
Incidentally, if you rerun Listing 2 but with the currency symbol in front of each of the values we’re inserting for Total
and Portion
, then you’ll still get the correct percentage values (under the covers, SQL Server implicitly converts the monetary values to DECIMAL
(19,4)
before inserting them into the table variable).
The following figure shows the results of doing the reverse calculations i.e. calculating the portion
values, from the total
and percentage
. The fact that we cannot calculate the portion
(or total
) exactly, from the percentage
values produced using the MONEY
datatype (Listing 1), confirms that there are rounding errors in those percentage values.
Other errors when using MONEY
You can also get scale overflow errors if you try to calculate correlations the classical way, when using MONEY
values. The values stored in the intermediate sum of the squares calculations can get enormous, if you are trying to find relationships between monetary values and other variables over many rows. This will cause errors in the value of the correlation. If you cannot avoid the MONEY
datatype then it is far better to use the builtin StDevP()
aggregate function to get the correlation.
Summary
Basically, it pays to do calculations in DECIMAL
(a.k.a. NUMERIC
) with as many digits to the right of the decimal point as practical, and only using two or three decimal places to display the result. A scale of four digits to the right of the decimal point isn’t always sufficient for a datatype that is involved in any operations beyond addition or subtraction. Be aware also of ‘Bankers rounding’ in calculations.
MONEY
can be made to perform well and accurately if you know all the constraints and workarounds, such as using the NUMERIC
datatype within calculations using division or multiplication, or employing the builtin aggregate functions. MONEY
uses integers under the covers, so it is fast, and will generally use less storage, and is particularly suited to being transmitted across a network as TDS. However, it is for experts only.
Tools in this post
SQL Prompt
Write, format, and refactor SQL effortlessly in SQL Server Management Studio and Visual Studio.
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