Export (0) Print
Expand All
ABS
ALL
AND
ANY
AVG
bit
COS
COT
DAY
EXP
GO
IN
LEN
LOG
MAX
MIN
NOT
OR
PI
SET
SIN
STR
SUM
TAN
USE
VAR
Expand Minimize

CHECKSUM

SQL Server 2000

Returns the checksum value computed over a row of a table, or over a list of expressions. CHECKSUM is intended for use in building hash indices.

Syntax

CHECKSUM ( * | expression [ ,...n ] )

Arguments

*

Specifies that computation is over all the columns of the table. CHECKSUM returns an error if any column is of noncomparable data type. Noncomparable data types are text, ntext, image, and cursor, as well as sql_variant with any of the above types as its base type.

expression

Is an expression of any type except a noncomparable data type.

Return Types

int

Remarks

CHECKSUM computes a hash value, called the checksum, over its list of arguments. The hash value is intended for use in building hash indices. If the arguments to CHECKSUM are columns, and an index is built over the computed CHECKSUM value, the result is a hash index, which can be used for equality searches over the columns.

CHECKSUM satisfies the properties of a hash function: CHECKSUM applied over any two lists of expressions returns the same value if the corresponding elements of the two lists have the same type and are equal when compared using the equals (=) operator. For the purpose of this definition, NULL values of a given type are considered to compare as equal. If one of the values in the expression list changes, the checksum of the list also usually changes. However, there is a small chance that the checksum will not change.

BINARY_CHECKSUM and CHECKSUM are similar functions: they can be used to compute a checksum value on a list of expressions, and the order of expressions affects the resultant value. The order of columns used in the case of CHECKSUM(*) is the order of columns specified in the table or view definition, including computed columns.

CHECKSUM and BINARY_CHECKSUM return different values for the string data types, where locale can cause strings with different representation to compare equal. The string data types are char, varchar, nchar, nvarchar, or sql_variant (if its base type is a string data type). For example, the BINARY_CHECKSUM values for the strings "McCavity" and "Mccavity" are different. In contrast, in a case-insensitive server, CHECKSUM returns the same checksum values for those strings. CHECKSUM values should not be compared against BINARY_CHECKSUM values.

Examples
Using CHECKSUM to build hash indices

The CHECKSUM function may be used to build hash indices. The hash index is built by adding a computed checksum column to the table being indexed, then building an index on the checksum column.

-- Create a checksum index.
SET ARITHABORT ON
USE Northwind
GO
ALTER TABLE Products 
ADD cs_Pname AS checksum(ProductName)
CREATE INDEX Pname_index ON Products (cs_Pname)

The checksum index can be used as a hash index, particularly to improve indexing speed when the column to be indexed is a long character column. The checksum index can be used for equality searches.

/*Use the index in a SELECT query. Add a second search 
condition to catch stray cases where checksums match, 
but the values are not identical.*/
SELECT * 
FROM Products
WHERE checksum(N'Vegie-spread') = cs_Pname
AND ProductName = N'Vegie-spread'

Creating the index on the computed column materializes the checksum column, and any changes to the ProductName value will be propagated to the checksum column. Alternatively, an index could be built directly on the column indexed. However, if the key values are long, a regular index is not likely to perform as well as a checksum index.

See Also

BINARY_CHECKSUM

CHECKSUM_AGG

Was this page helpful?
(1500 characters remaining)
Thank you for your feedback
Show:
© 2014 Microsoft