The following SQL Server string functions process on an input string and return a string or numeric value:
Function Description
ASCII Return the ASCII code value of a character
CHAR Convert an ASCII value to a character
CHARINDEX Search for a substring inside a string starting from a specified location and
return the position of the substring.
CONCAT Join two or more strings into one string
CONCAT_WS Concatenate multiple strings with a separator into a single string
DIFFERENCE Compare the SOUNDEX() values of two strings
FORMAT Return a value formatted with the specified format and optional culture
LEFT Extract a given a number of characters from a character string starting
from the left
LEN Return a number of characters of a character string
LOWER Convert a string to lowercase
LTRIM Return a new string from a specified string after removing all leading blanks
NCHAR Return the Unicode character with the specified integer code, as defined
by the Unicode standard
PATINDEX Returns the starting position of the first occurrence of a pattern in a string.
QUOTENAME Returns a Unicode string with the delimiters added to make the input
string a valid delimited identifier
REPLACE Replace all occurrences of a substring, within a string, with another
substring
REPLICATE Return a string repeated a specified number of times
REVERSE Return the reverse order of a character string
RIGHT Extract a given a number of characters from a character string starting
from the right
RTRIM Return a new string from a specified string after removing all trailing blanks
SOUNDEX Return a four-character (SOUNDEX) code of a string based on how it is
spoken
SPACE Returns a string of repeated spaces.
STR Returns character data converted from numeric data.
STRING_AGG Concatenate rows of strings with a specified separator into a new string
STRING_ESCAPE Escapes special characters in a string and returns a new string with escaped
characters
STRING_SPLIT A table-valued function that splits a string into rows of substrings based on
a specified separator.
STUFF Delete a part of a string and then insert another substring into the string
starting at a specified position.
SUBSTRING Extract a substring within a string starting from a specified location with a
specified length
TRANSLATE Replace several single-characters, one-to-one translation in one
operation.
TRIM Return a new string from a specified string after removing all leading and
trailing blanks
UNICODE Returns the integer value, as defined by the Unicode standard, of a
character.
UPPER Convert a string to uppercase
Source: guru99