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:

The group expression [ ] 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. In other words, the lexical (ASCII) value of the character at the left side of hyphen must be less than the lexical (ASCII) value of 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 (^) immediately following the opening bracket.

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.

[abc]          Matches any of the letters a, b, or 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 [

@ - Matches the character last matched to ? or group-expression. For example, ?*@ matches to all strings which begin with the same character as they end with. 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 lower-case variants of the same letter. Use expressions 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 @ at the beginning of pattern, they are all skipped, and many additional liberties are taken for the match function. The more @ signs there are at the beginning, the more fuzzy (liberal) is the search. For example, pattern "@Johnson" will match string "Jonsson", and pattern "@@Johnson" will also match "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.

If there are any ISO8859.1 diacritic letters (e.g., vowels with accents or umlauts, 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. However, if there is any diacritic letter specified in the pattern string, then it will match only to the upper- or lower-case variant of exactly the same diacritic letter.

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

So the following hold for substring searches:

	Note:
	Because the internal matching functions use macros which also consider characters like @, [, \, ], and ^ to be letters, they will match against characters `, {, |, }, and ~, respectively, 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.