MySQL常见的字符串函数

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整理自官档。

1.1     SUBSTR or SUBSTRING

SUBSTR(str,pos), SUBSTR(str FROM pos),SUBSTR(str,pos,len), SUBSTR(str FROM

pos FOR len)

SUBSTR() is a synonym for SUBSTRING().

SUBSTRING(str,pos), SUBSTRING(str FROM pos),SUBSTRING(str,pos,len),

SUBSTRING(str FROM pos FOR len)

The forms without a len argument return asubstring from string str starting at position pos.

The forms with a len argument return asubstring len characters long from string str, starting at

position pos. The forms that use FROM arestandard SQL syntax. It is also possible to use a negative

value for pos. In this case, the beginningof the substring is pos characters from the end of the

string, rather than the beginning. Anegative value may be used for pos in any of the forms of this

function.

For all forms of SUBSTRING(), the positionof the first character in the string from which the

substring is to be extracted is reckoned as1.

mysql> SELECTSUBSTRING('Quadratically',5);

-> 'ratically'

mysql> SELECT SUBSTRING('foobarbar' FROM4);

-> 'barbar'

mysql> SELECTSUBSTRING('Quadratically',5,6);

-> 'ratica'

mysql> SELECT SUBSTRING('Sakila', -3);

-> 'ila'

mysql> SELECT SUBSTRING('Sakila', -5,3);

-> 'aki'

mysql> SELECT SUBSTRING('Sakila' FROM -4FOR 2);

-> 'ki'

This function is multibyte safe.

If len is less than 1,the result is the empty string.

1.2     SUBSTRING_INDEX

SUBSTRING_INDEX(str,delim,count)

Returns the substring from string strbefore count occurrences of the delimiter delim. If count

is positive, everything to the left of thefinal delimiter (counting from the left) is returned. If count

is negative, everything to the right of thefinal delimiter (counting from the right) is returned.

SUBSTRING_INDEX() performs a case-sensitivematch when searching for delim.

mysql> SELECTSUBSTRING_INDEX('www.mysql.com', '.', 2);

-> 'www.mysql'

mysql> SELECTSUBSTRING_INDEX('www.mysql.com', '.', -2);

-> 'mysql.com'

This function is multibyte safe.

1.3     CHAR_LENGTH

CHAR_LENGTH(str) 返回字符数

Returns the length of the string str,measured in characters. A multibyte character counts as a

single character. This means that for astring containing five 2-byte characters, LENGTH() returns

10, whereas CHAR_LENGTH() returns 5.

1.4     LENGTH

LENGTH(str) 返回字节数

Returns the length of the string str,measured in bytes. A multibyte character counts as multiple

bytes. This means that for a stringcontaining five 2-byte characters, LENGTH() returns 10, whereas

CHAR_LENGTH() returns 5.

mysql> SELECT LENGTH('text');

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