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1. ASCII
   Returns the ASCII code value of the leftmost character of a character expression.
   返回字符串最左侧字符的ASCII值
   ASCII ( character_expression )

2. CHAR
   Returns the single-byte character with the specified integer code, as defined by the character set and encoding of the default collation of the current database.
   返回具有ASCII码对应的单字节字符，该代码由当前数据库的默认排序规则的字符集和编码定义。
   CHAR ( integer_expression )

3. CHARINDEX
   This function searches for one character expression inside a second character expression, returning the starting position of the first expression if found.
   返回字符串中第一个符合目标字符串的index
   CHARINDEX ( expressionToFind , expressionToSearch [ , start_location ] )

4. CONCAT
   This function returns a string resulting from the concatenation, or joining, of two or more string values in an end-to-end manner. (To add a separating value during concatenation, see CONCAT_WS.)
   返回字符串组连接
   CONCAT ( string_value1, string_value2 [, string_valueN ] )

5. CONCAT_WS
   This function returns a string resulting from the concatenation, or joining, of two or more string values in an end-to-end manner. It separates those concatenated string values with the delimiter specified in the first function argument. (CONCAT_WS indicates concatenate with separator.)
   拼接字符串，但是添加了分隔符
   CONCAT_WS ( separator, argument1, argument2 [, argumentN]... )

6. DIFFERENCE
   This function returns an integer value measuring the difference between the SOUNDEX() values of two different character expressions.
   根据soundex的值判断两个字符串的相似度，4最大，0最小
   DIFFERENCE ( character_expression , character_expression )

7. FORMAT
   Returns a value formatted with the specified format and optional culture. Use the FORMAT function for locale-aware formatting of date/time and number values as strings. For general data type conversions, use CAST or CONVERT.
   根据格式类型返回日期和时间
   FORMAT( value, format [, culture ] )

8. LEFT
   Returns the left part of a character string with the specified number of characters.
   保留左侧开始的N的字符
   LEFT ( character_expression , integer_expression )

9. LEN
   返回字符串长度
   Returns the number of characters of the specified string expression, excluding trailing spaces.
   LEN ( string_expression )

10. LOWER
    Returns a character expression after converting uppercase character data to lowercase.
    返回字符串中的英文转为小写
    LOWER ( character_expression )

11. LTRIM
    Removes space character char(32) or other specified characters from the start of a string.
    移除字符串开头的空格或者其他指定符号。
    LTRIM ( character_expression , [ characters ] )

12. NCHAR
    Returns the Unicode character with the specified integer code, as defined by the Unicode standard.
    根据ASCII码返回Unicode指定的符号
    NCHAR ( integer_expression )

13. PATINDEX
    Returns the starting position of the first occurrence of a pattern in a specified expression, or zero if the pattern is not found, on all valid text and character data types.
    返回第一个符合pattern表达式的字符串的位置，如果没有返回0
    PATINDEX ( '%pattern%' , expression )

14. QUOTENAME
    Returns a Unicode string with the delimiters added to make the input string a valid SQL Server delimited identifier.
    默认分隔符为[],可以自定义，将字符串外加一个分隔符，用于将普通字符串变为标识符
    QUOTENAME ( 'character_string' [ , 'quote_character' ] )

15. REPLACE
    Replaces all occurrences of a specified string value with another string value.
    替换所有符合pattern的字符。
    REPLACE ( string_expression , string_pattern , string_replacement )

16. REPLICATE
    Repeats a string value a specified number of times.
    重复字符串指定次数。
    REPLICATE ( string_expression , integer_expression )

17. REVERSE
    Returns the reverse order of a string value.
    反转字符串
    REVERSE ( string_expression )

18. RIGHT
    Returns the right part of a character string with the specified number of characters.
    返回从右起数的指定个数的字符
    RIGHT ( character_expression , integer_expression )

19. RTRIM
    Removes space character char(32) or other specified characters from the end of a string.
    trim字符串右部分
    RTRIM ( character_expression , [ characters ] )

20. SOUNDEX
    Returns a four-character (SOUNDEX) code to evaluate the similarity of two strings.
    返回表示两个字符串相似度的一个四字符的值
    SOUNDEX ( character_expression )

21. SPACE
    Returns a string of repeated spaces.
    返回指定个数的空格
    SPACE ( integer_expression )

22. STR
    Returns character data converted from numeric data. The character data is right-justified, with a specified length and decimal precision.
    返回从数字转来的字符
    STR ( float_expression [ , length [ , decimal ] ] )

23. STRING_AGG
    Concatenates the values of string expressions and places separator values between them. The separator isn't added at the end of string.
    不懂，应该与存储过程有关
    STRING_AGG ( expression, separator ) [ <order_clause> ]
    <order_clause> ::= WITHIN GROUP ( ORDER BY <order_by_expression_list> [ ASC | DESC ] )

24. STRING_ESCAPE
    Escapes special characters in texts and returns text with escaped characters. STRING_ESCAPE is a deterministic function, introduced in SQL Server 2016.
    转义文本中的指定字符后返回字符串
    STRING_ESCAPE( text , type )

25. STRING_SPLIT
    A table-valued function that splits a string into rows of substrings, based on a specified separator character.
    STRING_SPLIT ( string , separator [ , enable_ordinal ] )

26. STUFF
    The STUFF function inserts a string into another string. It deletes a specified length of characters in the first string at the start position and then inserts the second string into the first string at the start position.
    删除指定长度的字符后，插入新的字符串到被删除的部分
    STUFF ( character_expression , start , length , replaceWith_expression )

27. SUBSTRING
    Returns part of a character, binary, text, or image expression in SQL Server.
    返回字符/二进制/文本或者图片类型的表达式的一部分
    SUBSTRING ( expression ,start , length )

28. TRANSLATE
    Returns the string provided as a first argument, after some characters specified in the second argument are translated into a destination set of characters, specified in the third argument.
    将输入的变量的字符串到第一个变量，第二个变量为指定被替换的字符，第三个是即将新的字符，返回变化后的字符串
    TRANSLATE ( inputString, characters, translations )

29. TRIM
    Removes the space character char(32) or other specified characters from the start and end of a string.
    删除左右两侧空格
    Starting with SQL Server 2022 (16.x) Preview, optionally remove the space character char(32) or other specified characters from the start, end, or both sides of a string.
    TRIM ( [ characters FROM ] string )
    2022 preview：TRIM ( [ LEADING | TRAILING | BOTH ] [characters FROM ] string )

30. UNICODE
    Returns the integer value, as defined by the Unicode standard, for the first character of the input expression.
    根据Unicode标准为输入表达式的第一个字符返回其int值
    UNICODE ( 'ncharacter_expression' )

31. UPPER
    Returns a character expression with lowercase character data converted to uppercase.
    将字符串中的小写字符转为大写。
    UPPER ( character_expression )