My server's default collation is Latin1_General_CI_AS, as determined by this query:
SELECT SERVERPROPERTY('Collation') AS Collation;
I was surprised to discover that with this collation I can match non-digit characters in strings using the predicate LIKE '[0-9]'.
Why in the default collation does this happen? I can't think of a case where this would be useful. I know I can work around the behavior using a binary collation, but it seems like a strange way to implement the default collation.
Filtering digits produces non-digit caracters
I can demonstrate the behavior by creating a column that contains all possible single-byte character values and filtering the values with the digit-matching predicate.
The following statement creates a temporary table with 256 rows, one for each code point in the current code page:
WITH P0(_) AS (SELECT 0 UNION ALL SELECT 0),
P1(_) AS (SELECT 0 FROM P0 AS L CROSS JOIN P0 AS R),
P2(_) AS (SELECT 0 FROM P1 AS L CROSS JOIN P1 AS R),
P3(_) AS (SELECT 0 FROM P2 AS L CROSS JOIN P2 AS R),
Tally(Number) AS (
SELECT -1 + ROW_NUMBER() OVER (ORDER BY (SELECT 0))
FROM P3
)
SELECT Number AS CodePoint, CHAR(Number) AS Symbol
INTO #CodePage
FROM Tally
WHERE Number >= 0 AND Number <= 255;
Each row contains the integer value of the code point, and the character value of the code point. Not all of the character values are displayable - some of the code points are strictly control characters. Here is a selective sample of the output of SELECT CodePoint, Symbol FROM #CodePage:
0
1
2
...
32
33 !
34 "
35 #
...
48 0
49 1
50 2
...
65 A
66 B
67 C
...
253 ý
254 þ
255 ÿ
I would expect to be able to filter on the Symbol column to find digit characters using a LIKE predicate and specifying the range of characters '0' thru '9':
SELECT CodePoint, Symbol
FROM #CodePage
WHERE Symbol LIKE '[0-9]';
It produces a surprising output:
CodePoint Symbol
48 0
49 1
50 2
51 3
52 4
53 5
54 6
55 7
56 8
57 9
178 ²
179 ³
185 ¹
188 ¼
189 ½
190 ¾
The set of code points 48 thru 57 are the ones I expect. What surprises me is that the symbols for superscripts and fractions are also included in the result set!
There might be a mathematical reason to think of exponents and fractions as numbers, but it seems wrong to call them digits.
Using binary collation as a workaround
I understand that to get the result I expect, I can force the corresponding binary collation Latin1_General_BIN:
SELECT CodePoint, Symbol
FROM #CodePage
WHERE Symbol LIKE '[0-9]' COLLATE Latin1_General_BIN;
The result set includes only the code points 48 thru 57:
CodePoint Symbol
48 0
49 1
50 2
51 3
52 4
53 5
54 6
55 7
56 8
57 9
