What's the meaning of the percent characters in this line:
Console.WriteLine(50% + 50%)
Outputs:
100
The link in Hans Passant comment mentions it:
Literals can also use the identifier type characters (%, &, #, !, #,
$), as can variables, constants, and expressions.
Related
I have the following value in a db column:
some%thing
I'm using h2 and postgres, but can't figure out why this query, where I have escaped the % to perform a literal search, is not matching the row highlighted?:
LIKE 'some\%thing'
Because 'value' is a character string literal and not the name of your column. You need to use double quotes instead.
"value" LIKE 'some\%hing'
One option is to use regular expressions:
where value ~ '%'
LIKE also supports an explicit escape character:
where value like '%$%%' escape '$'
Or the default one, \.
I am trying to replace all special symbols with a space (' ').
What I have thus far is:
select regexp_replace(column_name, '-|"|~|!|(|)|-', ' ') from tablename;
Starting COLUMN_NAME : He!lo
What I end up with : H e l o
What am I doing wrong? Thanks in advance!
Your issue is the |(|)| parentheses test. Parentheses are a special character in regex so you are actually testing (|) which is just capturing every character (the pipe does nothing here).
For example, you are capturing the character 'H' and then replacing that capture with the capture H and a space ' '. It also means that any character appearing after this test (your hyphen) isn't going to get replaced since it was already caught in the capture.
That being said a better way to do this would be:
REGEXP_REPLACE(COLUMN_NAME, '[^0-9a-zA-Z]', ' ')
The caret in the bracket says "If a character is found that isn't in these ranges"
Or, of your list of characters to test for is limited, then:
REGEXP_REPLACE(COLUMN_NAME, '["~!\(\)]', ' ')
Taking special care to escape regex special characters in the list with a backslash. Here's a good resource for identifying special characters
there are 12 characters with special meanings: the backslash \, the
caret ^, the dollar sign $, the period or dot ., the vertical bar or
pipe symbol |, the question mark ?, the asterisk or star *, the plus
sign +, the opening parenthesis (, the closing parenthesis ), the
opening square bracket [, and the opening curly brace {,
alternatively you may use this :
select regexp_replace(column_name, '[^[:alpha:][:digit:]]+', '') from tablename;
or
more abbreviately
select regexp_replace(column_name, '[^[:alnum:]]', '') from tablename;
as #ctwheels suggested.
D e m o
Escape metacharacters ( and ).
regexp_replace(column_name, '-|"|~|!|\(|\)|-', ' ')
I can guesss it may be easy to answer type question but I am facing it first time, so any help will be more appreciated.
My Query:
SELECT remarks FROM enroll WHERE remarks LIKE 'REC_%'
OUTPUT:
remarks
REC_59161
Reclassify Hedge
Expected Output is only REC_59161. Yes _ is used for matching any single character but I am just looking for achieving my expected output.
_ is a wildcard Character. So that you have to escape it using [].
Query
select remarks
from enroll
where remarks like 'REC[_]%';
The underscore _ character is actually a special character with the LIKE operator, as are %, [] and ^:
https://learn.microsoft.com/en-us/sql/t-sql/language-elements/like-transact-sql
In that article, you'll see that an underscore _ matches any single character.
Use the ESCAPE keyword to define an escape character to allow you to escape the pattern-matching characters. We can use ! as the escape character, so:
WHERE remarks LIKE 'REC!_%' ESCAPE '!'
_ is a wildcard character, Try this :
declare #enroll table (remarks varchar(50));
insert into #enroll values ('REC_59161') , ('Reclassify Hedge');
SELECT remarks FROM #enroll WHERE remarks LIKE 'REC[_]%';
Demo
There are two issues:
It seems that the column is case insensitive, see Is the LIKE operator case-sensitive with MS SQL server? for details. That's why Rec in Reclassify Hedge fits REC in the Like
_ (as well as %) is a wild card, you should escape _ in the pattern
Query:
SELECT remarks
FROM enroll
WHERE remarks LIKE 'REC*_%' ESCAPE '*' /* SQL 92 standard */
http://www.contrib.andrew.cmu.edu/~shadow/sql/sql1992.txt
uses the triadic operator LIKE (or the inverse, NOT
LIKE), operating on three character strings and returning a Boolean.
LIKE determines whether or not a character string "matches" a given
"pattern" (also a character string). The characters '%' (percent)
and '_' (underscore) have special meaning when they occur in the
pattern. The optional third argument is a character string
containing exactly one character, known as the "escape character", for
use when a percent or underscore is required in the pattern without
its special meaning.
I try to extract the '930' from 'EM 930' with following Regexp
REGEXP_REPLACE(info,'^[:space:]*[ABCDEFGHIJKLMNOPQRSTUVWXYZ]*[:space:]*([0-9]+)[:space:]*$','\1')
But it returns me the original string.
An idea why ?
Subsidiary question:
Why does the "\1" returned the original string when the pattern is not matched ? I expected it to return NULL, as in my other regexp experiences (eg Perl).
Who I can re-write this in a performant way so that I get of wel the matched string of well NULL ?
Your space character class was not exactly correct. If we change [:space:] to [[:space:]], your regexp_replace works as you expect:
REGEXP_REPLACE(info, '^[[:space:]]*[ABCDEFGHIJKLMNOPQRSTUVWXYZ]*[[:space:]]*([0-9]+)[[:space:]]*$','\1')
For the sake of succinctness, we could use the upper character class, [[:upper:]], for [ABCDEFGHIJKLMNOPQRSTUVWXYZ]. This changes the function invocation to:
regexp_replace(info, '^[[:space:]]*[[:upper:]]*[[:space:]]*([0-9]+)[[:space:]]*$','\1')
Or escape characters could be used in lieu of character classes:
\s space
\w word character
\d digit character
regexp_replace(info, '^\s*\w*\s*(\d+)\s*$','\1')
Explanation:
Since your malformed character class, [:space:], does not match the space that exists between 'EM' and '930', your search by parameter does not match any characters in the source parameter.
Your search by parameter, '^[[:space:]]*[ABCDEFGHIJKLMNOPQRSTUVWXYZ]*[[:space:]]*([0-9]+)[[:space:]]*$', is anchored to the beginning and end of the column, info, thus it can only match the column, info, one time at most.
In your case, there is no match and the character group, '\1' which is associated with '([0-9]*)', has no value.
Consequently, no characters are replaced and you are left with original value of the column, info, 'EM 930'.
interesting variations to better understand this function:
-If your corrected function invocation had no pattern_to_replace_by parameter, '\1', then a NULL would be returned:
regexp_replace(info, '^\s*\w*\s*(\d+)\s*$' ) FROM dual;
-Since you have a pattern_to_replace_by parameter, '\1', and now it has the matching character group, the repeating digit group is returned:
930
What is the complete list of all special characters for a SQL (I'm interested in SQL Server but other's would be good too) LIKE clause?
E.g.
SELECT Name FROM Person WHERE Name LIKE '%Jon%'
SQL Server:
%
_
[specifier] E.g. [a-z]
[^specifier]
ESCAPE clause E.g. %30!%%' ESCAPE '!' will evaluate 30% as true
' characters need to be escaped with ' E.g. they're becomes they''re
MySQL:
% - Any string of zero or more characters.
_ - Any single character
ESCAPE clause E.g. %30!%%' ESCAPE '!' will evaluate 30% as true
Oracle:
% - Any string of zero or more characters.
_ - Any single character
ESCAPE clause E.g. %30!%%' ESCAPE '!' will evaluate 30% as true
Sybase
%
_
[specifier] E.g. [a-z]
[^specifier]
Progress:
% - Any string of zero or more characters.
_ - Any single character
Reference Guide here [PDF]
PostgreSQL:
% - Any string of zero or more characters.
_ - Any single character
ESCAPE clause E.g. %30!%%' ESCAPE '!' will evaluate 30% as true
ANSI SQL92:
%
_
An ESCAPE character only if specified.
PostgreSQL also has the SIMILAR TO operator which adds the following:
[specifier]
[^specifier]
| - either of two alternatives
* - repetition of the previous item zero or more times.
+ - repetition of the previous item one or more times.
() - group items together
The idea is to make this a community Wiki that can become a "One stop shop" for this.
For SQL Server, from http://msdn.microsoft.com/en-us/library/ms179859.aspx :
% Any string of zero or more characters.
WHERE title LIKE '%computer%' finds all book titles with the word 'computer' anywhere in the book title.
_ Any single character.
WHERE au_fname LIKE '_ean' finds all four-letter first names that end with ean (Dean, Sean, and so on).
[ ] Any single character within the specified range ([a-f]) or set ([abcdef]).
WHERE au_lname LIKE '[C-P]arsen' finds author last names ending with arsen and starting with any single character between C and P, for example Carsen, Larsen, Karsen, and so on. In range searches, the characters included in the range may vary depending on the sorting rules of the collation.
[^] Any single character not within the specified range ([^a-f]) or set ([^abcdef]).
WHERE au_lname LIKE 'de[^l]%' all author last names starting with de and where the following letter is not l.
ANSI SQL92:
%
_
an ESCAPE character only if specified.
It is disappointing that many databases do not stick to the standard rules and add extra characters, or incorrectly enable ESCAPE with a default value of ‘\’ when it is missing. Like we don't already have enough trouble with ‘\’!
It's impossible to write DBMS-independent code here, because you don't know what characters you're going to have to escape, and the standard says you can't escape things that don't need to be escaped. (See section 8.5/General Rules/3.a.ii.)
Thank you SQL! gnnn
You should add that you have to add an extra ' to escape an exising ' in SQL Server:
smith's -> smith''s
Sybase :
% : Matches any string of zero or more characters.
_ : Matches a single character.
[specifier] : Brackets enclose ranges or sets, such as [a-f]
or [abcdef].Specifier can take two forms:
rangespec1-rangespec2:
rangespec1 indicates the start of a range of characters.
- is a special character, indicating a range.
rangespec2 indicates the end of a range of characters.
set:
can be composed of any discrete set of values, in any
order, such as [a2bR].The range [a-f], and the
sets [abcdef] and [fcbdae] return the same
set of values.
Specifiers are case-sensitive.
[^specifier] : A caret (^) preceding a specifier indicates
non-inclusion. [^a-f] means "not in the range
a-f"; [^a2bR] means "not a, 2, b, or R."
Potential answer for SQL Server
Interesting I just ran a test using LinqPad with SQL Server which should be just running Linq to SQL underneath and it generates the following SQL statement.
Records
.Where(r => r.Name.Contains("lkjwer--_~[]"))
-- Region Parameters
DECLARE #p0 VarChar(1000) = '%lkjwer--~_~~~[]%'
-- EndRegion
SELECT [t0].[ID], [t0].[Name]
FROM [RECORDS] AS [t0]
WHERE [t0].[Name] LIKE #p0 ESCAPE '~'
So I haven't tested it yet but it looks like potentially the ESCAPE '~' keyword may allow for automatic escaping of a string for use within a like expression.