The LIKE predicate expects a pattern to be applied to a varchar or nvarchar column to qualify the results to be returned from a query.

If the pattern does not begin with an at-sign (@) or with two asterisks (**), then we test the equality of the string and pattern with ordinary wildcard matching, which behaves approximately like the filename pattern matching in the Unix shell. (But not like the regular expression matching in utilities like grep and sed).

The following characters have special significance in the pattern:

? Matches any single character.
* Matches zero or more of any characters.

[ ] (Called a group-expression here) Matches any one of the enclosed characters, unless the first character following the opening [ is ^, then matches only if the character (in the datum string) is not any one of those specified after the ^. (i.e. the ^ negates the meaning of this expression.)

You can use character ranges like 0-9 (shorthand for 0123456789) inside the brackets, in which case the character in the datum string must be lexically within the inclusive range of that pair (of course the character at the left side of hyphen must be lexically (that is, its ASCII value) less than the character at the right side).

The hyphen can be included in the character set by putting it as the first or last character. The right bracket (]) can be included by putting it as the first character in the expression, i.e. immediately after the opening bracket ([) or the caret (^) following it.

Example 9.32. Examples:

[abc]          Matches any of the letters a, b and c.
[^0123456789]  Matches anything, except digits. (same as [^0-9])
[[]            Matches [
[]]            Matches ]
[][]           Matches ] and [
[^]]           Matches anything except ]
[A-Za-z0-9]    Matches all the alphanumeric characters.
[-*+/]         Matches the four basic arithmetic operators.
[-]            Matches to single hyphen.
[]-]           Matches to ] or -
[-[] or [[-]   Matches to - or [

That is, the hyphen indicates a range between characters, unless it is the first or the last character in the group expression, in which case it matches just to itself.

@ Matches the character last matched to ? or group-expression. For example ?*@ matches to all strings which begin with the same character they end. However, if there is neither ? nor [] expression at the left side of @ in the pattern, then @ matches just to itself. (e.g. *@* should match to all e-mail addresses).

Any other characters match ONLY to themselves, that is, not even to the upper- or lowercase variants of the same letter. Use expression like [Wo][Oo][Rr][Dd] if you want to find any mixed-case variant of the word "word", or use the substring search explained below.

However, if the pattern begins with an at-sign (@) then we compare the rest of pattern to string with the fuzzy matching, allowing differences of few characters in quality and quantity (length). If there is more than one @ in the beginning of pattern they are all skipped, and so many additional liberties are given for the match function. The more @-signs there are in the beginning, the more fuzzy (liberal) is the search. For example: pattern "@Johnson" will match to string "Jonsson" and pattern "@@Johnson" will match also to "Jansson".

If the pattern begins with two asterisks, then we do diacritic- and case insensitive substring search, trying to find the string given in the rest of pattern from the datum string.

Example 9.33. Example:

"**escort" will match to "Ford Escort vm. 1975".

If there are any ISO8859.1 diacritic letters (e.g. vowels with accents or umlaut-signs, or letters like the Spanish n with ~ (tilde)) present in the datum string, then the plain unaccented (7-bit ASCII) variant of the same letter in the pattern string will match to it. But if there are any diacritic letter specified in the pattern string, then it will match only to the upper- or lowercase variant of exactly the same diacritic letter.

The rationale behind this is that the people entering the information to database can use the exact spelling for the word, for example writing the word "Citroen" with the umlaut-e (e with two dots above it), as it is actually written in French, and the people who search for the Citroens can still find it without need to remember the exact orthography of the French, by just giving a word "citroen". And this allows also the people who have just plain 7-bit ASCII keyboards to search for the words like Ra"a"kkyla" (place in Finland, a" means umlaut-a, i.e. a with two dots above it), just by entering the word raakkyla.

So the following holds with the substring searches:

1) Any non-alphabetic character in the pattern matches just to itself in the datum string (e.g. ? to ? and 3 to 3).
2) Any 7-bit ASCII letter (A-Z and a-z without any diacritic signs) in the pattern matches to any diacritic variant of the same letter (as well as to same 7-bit ASCII letter) in the datum string, either in the upper- or lowercase.
3) Any diacritic letter (8-bit ISO8859.1 letter) in the pattern matches only to the same letter (in the upper- or lowercase) in the datum string.
[Note] Note:

because the internal matching functions use macros which consider also the characters like: @, [, \, ], and ^ to be letters, they will match against characters `, {, |, }, and ~ respectively, which is just all right, because in some older implementations of European character sets those characters mark the uppercase and lowercase variants of certain diacritic letters.

It is generally better to match too liberally and so maybe sometimes give something entirely off the wall to the user, than to miss something important because of too strict criteria.

Of course, when searching from the data which contains text in some wide-character format (like certain coding systems for Japanese and Chinese where one character is coded with two bytes) neither fuzzy matching function nor nc_strstr function presented here should be used, as they would often match on entirely spurious cases.