问题
If the database row is like this: country = 'usa' and i query "select * from data where country = 'usa '" it also returns this row. So its not an exact match.
Why MySQL does this? And in what other cases it will also return TRUE when its not really true?
回答1:
The trailing spaces are omitted if the column is of type char or varchar; using like 'usa ' resolves the issue
回答2:
As mentioned in the manual:
All MySQL collations are of type
PADSPACE. This means that allCHARandVARCHARvalues in MySQL are compared without regard to any trailing spaces.
In the definition of the LIKE operator, it states:
In particular, trailing spaces are significant, which is not true for CHAR or VARCHAR comparisons performed with the = operator:
As mentioned in this answer:
This behavior is specified in SQL-92 and SQL:2008. For the purposes of comparison, the shorter string is padded to the length of the longer string.
From the draft (8.2 <comparison predicate>):
If the length in characters of X is not equal to the length in characters of Y, then the shorter string is effectively replaced, for the purposes of comparison, with a copy of itself that has been extended to the length of the longer string by concatenation on the right of one or more pad characters, where the pad character is chosen based on CS. If CS has the NO PAD characteristic, then the pad character is an implementation-dependent character different from any character in the character set of X and Y that collates less than any string under CS. Otherwise, the pad character is a <space>.
In addition to the other excellent solutions:
select binary 'a' = 'a '
回答3:
try it with like or with this
country = 'usa ' AND LENGTH(country) = LENGTH('usa ')
回答4:
The trailing spaces are omitted if the column is of type char or varchar,
Can read more on it on following link:
http://blogs.agilefaqs.com/2011/06/22/trailing-white-spaces-are-ignored-by-mysql-for-comparison-in-a-select-statement/
回答5:
The manual says this:
All MySQL collations are of type PADSPACE. This means that all CHAR and VARCHAR values in MySQL are compared without regard to any trailing spaces [...] This is true for all MySQL versions, and is not affected by the server SQL mode.
http://dev.mysql.com/doc/refman/5.5/en/char.html
If you need to consider blank space, you basically have to get rid of language-aware sorting. A simple way is to force a binary collation. Please run and compare:
SELECT 'ab'='ab ', BINARY 'ab'='ab '
来源：https://stackoverflow.com/questions/10495692/mysql-comparison-operator-spaces