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字符串函数

📄️ SUBSTRING

SUBSTRING function is used to extract a string containing a specific number of characters from a particular position of a given string. The forms without a len argument return a substring from string str starting at position pos. The forms with a len argument return a substring len characters long from string str, starting at position pos. It is also possible to use a negative value for pos. In this case, the beginning of the substring is pos characters from the end of the string, rather than the beginning. A negative value may be used for pos in any of the forms of this function. A value of 0 for pos returns an empty string. The position of the first character in the string from which the substring is to be extracted is reckoned as 1.