Character Processing Functions and Operators

String functions and operators provided by MogDB are for concatenating strings with each other, concatenating strings with non-strings, and matching the patterns of strings.

• bit_length(string)

  Description: Specifies the number of bits occupied by a string.

  Return type: int

  Example:

  mogdb=# SELECT bit_length('world');
   bit_length
  ------------
           40
  (1 row)

• btrim(string text [, characters text])

  Description: Removes the longest string consisting only of characters in characters (a space by default) from the start and end of string.

  Return type: text

  Example:

  mogdb=# SELECT btrim('sring' , 'ing');
   btrim
  -------
   sr
  (1 row)

• char_length(string) or character_length(string)

  Description: Specifies the number of characters in a string.

  Return type: int

  Example:

  mogdb=# SELECT char_length('hello');
   char_length
  -------------
             5
  (1 row)

• instr(text,text,int,int)

  Description: instr(string1,string2,int1,int2) returns the text from int1 to int2 in string1. The first int indicates the start position for matching, and the second int indicates the number of matching times.

  Return type: int

  Example:

  mogdb=# SELECT instr( 'abcdabcdabcd', 'bcd', 2, 2 );
   instr
  -------
       6
  (1 row)

• lengthb(text/bpchar)

  Description: Obtains the number of bytes of a specified string.

  Return type: int

  Example:

  mogdb=# SELECT lengthb('hello');
   lengthb
  ---------
         5
  (1 row)

• left(str text, n int)

  Description: Returns the first n characters in a string. When n is negative, all but the last |n| characters are returned.

  Return type: text

  Example:

  mogdb=# SELECT left('abcde', 2);
   left
  ------
   ab
  (1 row)

• length(string bytea, encoding name )

  Description: Specifies the number of characters in string in the given encoding. string must be valid in this encoding.

  Return type: int

  Example:

  mogdb=# SELECT length('jose', 'UTF8');
   length
  --------
        4
  (1 row)

  NOTE:

  If the length of the bytea type is queried and UTF8 encoding is specified, the maximum length can only be 536870888.

• lpad(string text, length int [, fill text])

  Description: Fills up string to length by appending the characters fill (a space by default). If string is already longer than length, then it is truncated.

  Return type: text

  Example:

  mogdb=# SELECT lpad('hi', 5, 'xyza');
   lpad
  -------
   xyzhi
  (1 row)

• notlike(x bytea name text, y bytea text)

  Description: Compares x and y to check whether they are inconsistent.

  Return type: Boolean

  Example:

  mogdb=# SELECT notlike(1,2);
      notlike
  --------------
              t
  (1 row)
  mogdb=# SELECT notlike(1,1);
      notlike
  --------------
              f
  (1 row)

• octet_length(string)

  Description: Specifies the number of bytes in a string.

  Return type: int

  Example:

  mogdb=# SELECT octet_length('jose');
   octet_length
  --------------
              4
  (1 row)

• overlay(string placing string FROM int [for int])

  Description: Replaces substrings. FROM int indicates the start position of the replacement in the first string. for int indicates the number of characters replaced in the first string.

  Return type: text

  Example:

  mogdb=# SELECT overlay('hello' placing 'world' from 2 for 3 );
   overlay
  ---------
   hworldo
  (1 row)

• position(substring in string)

  Description: Specifies the position of a substring. Parameters are case-sensitive.

  Return type: int. If the character string does not exist, 0 is returned.

  Example:

  mogdb=# SELECT position('ing' in 'string');
   position
  ----------
          4
  (1 row)

• pg_client_encoding()

  Description: Specifies the current client encoding name.

  Return type: name

  Example:

  mogdb=# SELECT pg_client_encoding();
   pg_client_encoding
  --------------------
   UTF8
  (1 row)

• quote_ident(string text)

  Description: Returns the given string suitably quoted to be used as an identifier in an SQL statement string (quotation marks are used as required). Quotation marks are added only if necessary (that is, if the string contains non-identifier characters or would be case-folded). Embedded quotation marks are properly doubled.

  Return type: text

  Example:

  mogdb=# SELECT quote_ident('hello world');
   quote_ident
  --------------
   "hello world"
  (1 row)

• quote_literal(string text)

  Description: Returns the given string suitably quoted to be used as a string literal in an SQL statement string (quotation marks are used as required).

  Return type: text

  Example:

  mogdb=# SELECT quote_literal('hello');
   quote_literal
  ---------------
   'hello'
  (1 row)

  If a command similar to the following exists, the text will be escaped.

  mogdb=# SELECT quote_literal(E'O\'hello');
   quote_literal
  ---------------
   'O''hello'
  (1 row)

  If a command similar to the following exists, the backslash will be properly doubled.

  mogdb=# SELECT quote_literal('O\hello');
   quote_literal
  ---------------
   E'O\\hello'
  (1 row)

  If the parameter is null, NULL is returned. If the parameter may be null, you are advised to use quote_nullable.

  mogdb=# SELECT quote_literal(NULL);
   quote_literal
  ---------------
  
  (1 row)

• quote_literal(value anyelement)

  Description: Converts the given value to text and then quotes it as a literal.

  Return type: text

  Example:

  mogdb=# SELECT quote_literal(42.5);
   quote_literal
  ---------------
   '42.5'
  (1 row)

  If a command similar to the following exists, the given value will be escaped.

  mogdb=# SELECT quote_literal(E'O\'42.5');
   quote_literal
  ---------------
   '0''42.5'
  (1 row)

  If a command similar to the following exists, the backslash will be properly doubled.

  mogdb=# SELECT quote_literal('O\42.5');
   quote_literal
  ---------------
   E'O\\42.5'
  (1 row)

• quote_nullable(string text)

  Description: Returns the given string suitably quoted to be used as a string literal in an SQL statement string (quotation marks are used as required).

  Return type: text

  Example:

  mogdb=# SELECT quote_nullable('hello');
   quote_nullable
  ----------------
   'hello'
  (1 row)

  If a command similar to the following exists, the text will be escaped.

  mogdb=# SELECT quote_nullable(E'O\'hello');
   quote_nullable
  ----------------
   'O''hello'
  (1 row)

  If a command similar to the following exists, the backslash will be properly doubled.

  mogdb=# SELECT quote_nullable('O\hello');
   quote_nullable
  ----------------
   E'O\\hello'
  (1 row)

  If the parameter is null, NULL is returned.

  mogdb=# SELECT quote_nullable(NULL);
   quote_nullable
  ----------------
   NULL
  (1 row)

• quote_nullable(value anyelement)

  Description: Converts the given value to text and then quotes it as a literal.

  Return type: text

  Example:

  mogdb=# SELECT quote_nullable(42.5);
   quote_nullable
  ----------------
   '42.5'
  (1 row)

  If a command similar to the following exists, the given value will be escaped.

  mogdb=# SELECT quote_nullable(E'O\'42.5');
   quote_nullable
  ----------------
   'O''42.5'
  (1 row)

  If a command similar to the following exists, the backslash will be properly doubled.

  mogdb=# SELECT quote_nullable('O\42.5');
   quote_nullable
  ----------------
   E'O\\42.5'
  (1 row)

  If the parameter is null, NULL is returned.

  mogdb=# SELECT quote_nullable(NULL);
   quote_nullable
  ----------------
   NULL
  (1 row)

• substring_inner(string [from int] [for int])

  Description: Extracts a substring. from int indicates the start position of the truncation. for int indicates the number of characters truncated.

  Return type: text

  Example:

  mogdb=# select substring_inner('adcde', 2,3);
   substring_inner
  -----------------
   dcd
  (1 row)

• substring(string [from int] [for int])

  Description: Extracts a substring. from int indicates the start position of the truncation. for int indicates the number of characters truncated.

  Return type: text

  Example:

  mogdb=# SELECT substring('Thomas' from 2 for 3);
   substring
  -----------
   hom
  (1 row)

• substring(string from pattern)

  Description: Extracts substrings matching the POSIX regular expression. It returns the text that matches the pattern. If no match record is found, a null value is returned.

  Return type: text

  Example:

  mogdb=# SELECT substring('Thomas' from '...$');
   substring
  -----------
   mas
  (1 row)
  mogdb=# SELECT substring('foobar' from 'o(.)b');
   result
  --------
   o
  (1 row)
  mogdb=# SELECT substring('foobar' from '(o(.)b)');
   result
  --------
   oob
  (1 row)

  NOTE: If the POSIX pattern contains any parentheses, the portion of the text that matched the first parenthesized sub-expression (the one whose left parenthesis comes first) is returned. You can put parentheses around the whole expression if you want to use parentheses within it without triggering this exception.

• substring(string from pattern for escape)

  Description: Extracts substrings matching the SQL regular expression. The declared schema must match the entire data string; otherwise, the function fails and returns a null value. To indicate the part of the pattern that should be returned on success, the pattern must contain two occurrences of the escape character followed by a double quotation mark ("). The text matching the portion of the pattern between these marks is returned.

  Return type: text

  Example:

  mogdb=# SELECT substring('Thomas' from '%#"o_a#"_' for '#');
   substring
  -----------
   oma
  (1 row)

• rawcat(raw,raw)

  Description: Indicates the string concatenation function.

  Return type: raw

  Example:

  mogdb=# SELECT rawcat('ab','cd');
   rawcat
  --------
   ABCD
  (1 row)

• regexp_like(text,text,text)

  Description: Indicates the mode matching function of a regular expression.

  Return type: Boolean

  Example:

  mogdb=# SELECT regexp_like('str','[ac]');
   regexp_like
  -------------
   f
  (1 row)

• regexp_substr(text,text)

  Description: Extracts substrings from a regular expression. Its function is similar to substr. When a regular expression contains multiple parallel brackets, it also needs to be processed.

  Return type: text

  Example:

  mogdb=# SELECT regexp_substr('str','[ac]');
   regexp_substr
  ---------------
  
  (1 row)

• regexp_matches(string text, pattern text [, flags text])

  Description: Returns all captured substrings resulting from matching a POSIX regular expression against string. If the pattern does not match, the function returns no rows. If the pattern contains no parenthesized sub-expressions, then each row returned is a single-element text array containing the substring matching the whole pattern. If the pattern contains parenthesized sub-expressions, the function returns a text array whose _n_th element is the substring matching the _n_th parenthesized sub-expression of the pattern.

  The optional flags argument contains zero or multiple single-letter flags that change function behavior. i indicates that the matching is not related to uppercase and lowercase. g indicates that each matching substring is replaced, instead of replacing only the first one.

  NOTICE: If the last parameter is provided but the parameter value is an empty string ('') and the SQL compatibility mode of the database is set to A, the returned result is an empty set. This is because the A compatibility mode treats the empty string ('') as NULL. To resolve this problem, you can:

  • Change the database SQL compatibility mode to C.
  • Do not provide the last parameter or do not set the last parameter to an empty string.

  Return type: SETOF text[]

  Example:

  mogdb=# SELECT regexp_matches('foobarbequebaz', '(bar)(beque)');
   regexp_matches
  ----------------
   {bar,beque}
  (1 row)
  mogdb=# SELECT regexp_matches('foobarbequebaz', 'barbeque');
   regexp_matches
  ----------------
   {barbeque}
  (1 row)
  mogdb=#  SELECT regexp_matches('foobarbequebazilbarfbonk', '(b[^b]+)(b[^b]+)', 'g');
      result
  --------------
   {bar,beque}
   {bazil,barf}
  (2 rows)

• regexp_split_to_array(string text, pattern text [, flags text ])

  Description: Splits string using a POSIX regular expression as the delimiter. The regexp_split_to_array function behaves the same as regexp_split_to_table, except that regexp_split_to_array returns its result as an array of text.

  Return type: text[]

  Example:

  mogdb=# SELECT regexp_split_to_array('hello world', E'\\s+');
   regexp_split_to_array
  -----------------------
   {hello,world}
  (1 row)

• regexp_split_to_table(string text, pattern text [, flags text])

  Description: Splits string using a POSIX regular expression as the delimiter. If there is no match to the pattern, the function returns the string. If there is at least one match, for each match it returns the text from the end of the last match (or the beginning of the string) to the beginning of the match. When there are no more matches, it returns the text from the end of the last match to the end of the string.

  The flags parameter is a text string containing zero or more single-letter flags that change the function's behavior. i indicates case-insensitive matching.

  Return type: SETOF text

  Example:

  mogdb=# SELECT regexp_split_to_table('hello world', E'\\s+');
   regexp_split_to_table
  -----------------------
   hello
   world
  (2 rows)

• repeat(string text, number int )

  Description: Repeats string the specified number of times.

  Return type: text

  Example:

  mogdb=# SELECT repeat('Pg', 4);
    repeat
  ----------
   PgPgPgPg
  (1 row)

  NOTE: The maximum size of memory allocated at a time cannot exceed 1 GB due to the memory allocation mechanism of the database. Therefore, the maximum value of number cannot exceed (1 GB - x)/lengthb (string) - 1. x indicates the length of the header information, which is usually greater than 4 bytes. The value varies among different scenarios.

• replace(string text, from text, to text)

  Description: Replaces all occurrences in string of substring from with substring to.

  Return type: text

  Example:

  mogdb=# SELECT replace('abcdefabcdef', 'cd', 'XXX');
      replace
  ----------------
   abXXXefabXXXef
  (1 row)

• reverse(str)

  Description: Returns the reversed string.

  Return type: text

  Example:

  mogdb=# SELECT reverse('abcde');
   reverse
  ---------
   edcba
  (1 row)

• right(str text, n int)

  Description: Returns the last n characters in a string. When n is negative, all but the first |n| characters are returned.

  Return type: text

  Example:

  mogdb=# SELECT right('abcde', 2);
   right
  -------
   de
  (1 row)
  
  mogdb=# SELECT right('abcde', -2);
   right
  -------
   cde
  (1 row)

• rpad(string text, length int [, fill text])

  Description: Fills up string to length by appending the characters fill (a space by default). If string is already longer than length, then it is truncated.

  Return type: text

  Example:

  mogdb=# SELECT rpad('hi', 5, 'xy');
   rpad
  -------
   hixyx
  (1 row)

• rtrim(string text [, characters text])

  Description: Removes the longest string containing only characters from characters (a space by default) from the end of string.

  Return type: text

  Example:

  mogdb=# SELECT rtrim('trimxxxx', 'x');
   rtrim
  -------
   trim
  (1 row)

• substrb(text,int,int)

  Description: Extracts a substring. The first intindicates the start position of the subtraction. The second int indicates the number of characters extracted.

  Return type: text

  Example:

  mogdb=# SELECT substrb('string',2,3);
   substrb
  ---------
   tri
  (1 row)

• substrb(text,int)

  Description: Extracts a substring. int indicates the start position of the extraction.

  Return type: text

  Example:

  mogdb=# SELECT substrb('string',2);
   substrb
  ---------
   tring
  (1 row)

• substr(bytea,from,count)

  Description: Extracts a substring from bytea. from specifies the position where the extraction starts. count specifies the length of the extracted substring.

  Return type: text

  Example:

  mogdb=# SELECT substr('string',2,3);
   substr
  --------
   tri
  (1 row)

• string || string

  Description: Concatenates strings.

  Return type: text

  Example:

  mogdb=# SELECT 'MPP'||'DB' AS RESULT;
   result
  --------
   MPPDB
  (1 row)

• string || non-string or non-string || string

  Description: Concatenates strings and non-strings.

  Return type: text

  Example:

  mogdb=# SELECT 'Value: '||42 AS RESULT;
    result
  -----------
   Value: 42
  (1 row)

• split_part(string text, delimiter text, field int)

  Description: Splits string on delimiter and returns the fieldth column (counting from text of the first appeared delimiter).

  Return type: text

  Example:

  mogdb=# SELECT split_part('abc~@~def~@~ghi', '~@~', 2);
   split_part
  ------------
   def
  (1 row)

• strpos(string, substring)

  Description: Specifies the position of a substring. It is the same as position(substring in string). However, the parameter sequences of them are reversed.

  Return type: int

  Example:

  mogdb=# SELECT strpos('source', 'rc');
   strpos
  --------
        4
  (1 row)

• to_hex(number int or bigint)

  Description: Converts a number to a hexadecimal expression.

  Return type: text

  Example:

  mogdb=# SELECT to_hex(2147483647);
    to_hex
  ----------
   7fffffff
  (1 row)

• translate(string text, from text, to text)

  Description: Any character in string that matches a character in the from set is replaced by the corresponding character in the to set. If from is longer than to, extra characters occurred in from are removed.

  Return type: text

  Example:

  mogdb=# SELECT translate('12345', '143', 'ax');
   translate
  -----------
   a2x5
  (1 row)

• length(string)

  Description: Obtains the number of characters in a string.

  Return type: integer

  Example:

  mogdb=# SELECT length('abcd');
   length
  --------
        4
  (1 row)

• lengthb(string)

  Description: Obtains the number of characters in a string. The value depends on character sets (GBK and UTF8).

  Return type: integer

  Example:

  mogdb=# SELECT lengthb('Chinese');
   lengthb
  ---------
         7
  (1 row)

• substr(string,from)

  Description:

  Extracts substrings from a string.

  from indicates the start position of the extraction.

  • If from starts at 0, the value 1 is used.
  • If the value of from is positive, all characters from from to the end are extracted.
  • If the value of from is negative, the last n characters in the string are extracted, in which n indicates the absolute value of from.

  Return type: varchar

  Example:

  If the value of from is positive:

  mogdb=# SELECT substr('ABCDEF',2);
   substr
  --------
   BCDEF
  (1 row)

  If the value of from is negative:

  mogdb=# SELECT substr('ABCDEF',-2);
   substr
  --------
   EF
  (1 row)

• substr(string,from,count)

  Description:

  Extracts substrings from a string.

  from indicates the start position of the extraction.

  count indicates the length of the extracted substring.

  • If from starts at 0, the value 1 is used.
  • If the value of from is positive, extract count characters starting from from.
  • If the value of from is negative, extract the last n count characters in the string, in which n indicates the absolute value of from.
  • If the value of count is smaller than 1, null is returned.

  Return type: varchar

  Example:

  If the value of from is positive:

  mogdb=# SELECT substr('ABCDEF',2,2);
   substr
  --------
   BC
  (1 row)

  If the value of from is negative:

  mogdb=# SELECT substr('ABCDEF',-3,2);
   substr
  --------
   DE
  (1 row)

• substrb(string,from)

  Description: The functionality of this function is the same as that of SUBSTR(string,from). However, the calculation unit is byte.

  Return type: bytea

  Example:

  mogdb=# SELECT substrb('ABCDEF',-2);
   substrb
  ---------
   EF
  (1 row)

• substrb(string,from,count)

  Description: The functionality of this function is the same as that of SUBSTR(string,from,count). However, the calculation unit is byte.

  Return type: bytea

  Example:

  mogdb=# SELECT substrb('ABCDEF',2,2);
   substrb
  ---------
   BC
  (1 row)

• trim([leading |trailing |both] [characters] from string)

  Description: Removes the longest string containing only the characters (a space by default) from the start/end/both ends of the string.

  Return type: varchar

  Example:

  mogdb=# SELECT trim(BOTH 'x' FROM 'xTomxx');
   btrim
  -------
   Tom
  (1 row)

  mogdb=# SELECT trim(LEADING 'x' FROM 'xTomxx');
   ltrim
  -------
   Tomxx
  (1 row)

  mogdb=# SELECT trim(TRAILING 'x' FROM 'xTomxx');
   rtrim
  -------
   xTom
  (1 row)

• rtrim(string [, characters])

  Description: Removes the longest string containing only characters from characters (a space by default) from the end of string.

  Return type: varchar

  Example:

  mogdb=# SELECT rtrim('TRIMxxxx','x');
   rtrim
  -------
   TRIM
  (1 row)

• ltrim(string [, characters])

  Description: Removes the longest string containing only characters from characters (a space by default) from the start of string.

  Return type: varchar

  Example:

  mogdb=# SELECT ltrim('xxxxTRIM','x');
   ltrim
  -------
   TRIM
  (1 row)

• upper(string)

  Description: Converts the string into the uppercase.

  Return type: varchar

  Example:

  mogdb=# SELECT upper('tom');
   upper
  -------
   TOM
  (1 row)

• lower(string)

  Description: Converts the string into the lowercase.

  Return type: varchar

  Example:

  mogdb=# SELECT lower('TOM');
   lower
  -------
   tom
  (1 row)

• rpad(string varchar, length int [, fill varchar])

  Description: Fills up string to length by appending the characters fill (a space by default). If string is already longer than length, then it is truncated.

  length in MogDB indicates the character length. One Chinese character is counted as one character.

  Return type: varchar

  Example:

  mogdb=# SELECT rpad('hi',5,'xyza');
   rpad
  -------
   hixyz
  (1 row)

  mogdb=# SELECT rpad('hi',5,'abcdefg');
   rpad
  -------
   hiabc
  (1 row)

• instr(string,substring[,position,occurrence])

  Description: Queries and returns the value of the substring position that occurs the occurrence (1 by default) times from the position (1 by default) in the string.

  • If the value of position is 0, 0 is returned.
  • If the value of position is negative, the search is performed backwards from the last _n_th character in the string, in which n indicates the absolute value of position.

  In this function, the calculation unit is character. One Chinese character is one character.

  Return type: integer

  Example:

  mogdb=# SELECT instr('corporate floor','or', 3);
   instr
  -------
       5
  (1 row)

  mogdb=# SELECT instr('corporate floor','or',-3,2);
   instr
  -------
       2
  (1 row)

• initcap(string)

  Description: Converts the first letter of each word in the string into the uppercase and the other letters into the lowercase.

  Return type: text

  Example:

  mogdb=# SELECT initcap('hi THOMAS');
    initcap
  -----------
   Hi Thomas
  (1 row)

• ascii(string)

  Description: Indicates the ASCII code of the first character in the string.

  Return type: integer

  Example:

  mogdb=# SELECT ascii('xyz');
   ascii
  -------
     120
  (1 row)

• replace(string varchar, search_string varchar, replacement_string varchar)

  Description: Replaces all search_string in the string with replacement_string.

  Return type: varchar

  Example:

  mogdb=# SELECT replace('jack and jue','j','bl');
      replace
  ----------------
   black and blue
  (1 row)

• lpad(string varchar, length int[, repeat_string varchar])

  Description: Adds a series of repeat_string (a space by default) on the left of the string to generate a new string with the total length of n.

  If the length of the string is longer than the specified length, the function truncates the string and returns the substrings with the specified length.

  Return type: varchar

  Example:

  mogdb=# SELECT lpad('PAGE 1',15,'*.');
        lpad
  -----------------
   *.*.*.*.*PAGE 1
  (1 row)

  mogdb=# SELECT lpad('hello world',5,'abcd');
   lpad
  -------
   hello
  (1 row)

• concat(str1,str2)

  Description: Connects str1 and str2 and returns the string.

  NOTICE:

  If the SQL compatibility mode is set to MY and str1 or str2 is set to NULL, NULL will be returned.

  Return type: varchar

  Example:

  mogdb=# SELECT concat('Hello', ' World!');
      concat
  --------------
   Hello World!
  (1 row)
  mogdb=# SELECT concat('Hello', NULL);
   concat
  --------
   Hello
  (1 row)

• chr(integer)

  Description: Specifies the character of the ASCII code.

  Return type: varchar

  Example:

  mogdb=# SELECT chr(65);
   chr
  -----
   A
  (1 row)

• regexp_substr(source_char, pattern)

  Description: Extracts substrings from a regular expression.

  Return type: varchar

  Example:

  mogdb=# SELECT regexp_substr('500 Hello World, Redwood Shores, CA', ',[^,]+,') "REGEXPR_SUBSTR";
    REGEXPR_SUBSTR
  -------------------
   , Redwood Shores,
  (1 row)

• regexp_replace(string, pattern, replacement [,flags ])

  Description: Replaces substrings matching the POSIX regular expression. The source string is returned unchanged if there is no match to the pattern. If there is a match, the source string is returned with the replacement string substituted for the matching substring.

  The replacement string can contain \n, where n is 1 through 9, to indicate that the source substring matching the _n_th parenthesized sub-expression of the pattern should be inserted, and it can contain & to indicate that the substring matching the entire pattern should be inserted.

  The optional flags argument contains zero or multiple single-letter flags that change function behavior. i indicates that the matching is not related to uppercase and lowercase. g indicates that each matching substring is replaced, instead of replacing only the first one.

  Return type: varchar

  Example:

  mogdb=# SELECT regexp_replace('Thomas', '.[mN]a.', 'M');
   regexp_replace
  ----------------
   ThM
  (1 row)
  mogdb=# SELECT regexp_replace('foobarbaz','b(..)', E'X\\1Y', 'g') AS RESULT;
     result
  -------------
   fooXarYXazY
  (1 row)

• concat_ws(sep text, str"any" [, str"any" [, …] ])

  Description: Uses the first parameter as the separator, which is associated with all following parameters. The NULL parameter is ignored.

  NOTICE:

  • If the first parameter value is NULL, the returned result is NULL.
  • If the first parameter is provided but the parameter value is an empty string ('') and the SQL compatibility mode of the database is set to A, the returned result is NULL. This is because the A compatibility mode treats the empty string ('') as NULL. To resolve this problem, you can change the SQL compatibility mode of the database to B, C, or PG.

  Return type: text

  Example:

  mogdb=# SELECT concat_ws(',', 'ABCDE', 2, NULL, 22);
   concat_ws
  ------------
   ABCDE,2,22
  (1 row)

• convert(string bytea, src_encoding name, dest_encoding name)

  Description: Converts the bytea string to dest_encoding. src_encoding specifies the source code encoding. The string must be valid in this encoding.

  Return type: bytea

  Example:

  mogdb=# SELECT convert('text_in_utf8', 'UTF8', 'GBK');
            convert
  ----------------------------
   \x746578745f696e5f75746638
  (1 row)

  NOTE: If the rule for converting between source to target encoding (for example, GBK and LATIN1) does not exist, the string is returned without conversion. See the pg_conversion system catalog for details. Example:

  mogdb=# show server_encoding;
  server_encoding
  -----------------
  LATIN1
  (1 row)
  mogdb=# SELECT convert_from('some text', 'GBK');
  convert_from
  --------------
  some text
  (1 row)
  db_latin1=# SELECT convert_to('some text', 'GBK');
    convert_to
  ----------------------
  \x736f6d652074657874
  (1 row)
  db_latin1=# SELECT convert('some text', 'GBK', 'LATIN1');
     convert
  ----------------------
  \x736f6d652074657874
  (1 row)

• convert_from(string bytea, src_encoding name)

  Description: Converts the long bytea using the coding mode of the database.

  src_encoding specifies the source code encoding. The string must be valid in this encoding.

  Return type: text

  Example:

  mogdb=# SELECT convert_from('text_in_utf8', 'UTF8');
   convert_from
  --------------
   text_in_utf8
  (1 row)

• convert_to(string text, dest_encoding name)

  Description: Converts a string to dest_encoding.

  Return type: bytea

  Example:

  mogdb=# SELECT convert_to('some text', 'UTF8');
        convert_to
  ----------------------
   \x736f6d652074657874
  (1 row)

• string [NOT] LIKE pattern [ESCAPE escape-character]

  Description: Specifies the pattern matching function.

  If the pattern does not include a percentage sign (%) or an underscore (), this mode represents itself only. In this case, the behavior of LIKE is the same as the equal operator. The underscore () in the pattern matches any single character while one percentage sign (%) matches no or multiple characters.

  To match with underscores (_) or percent signs (%), corresponding characters in pattern must lead escape characters. The default escape character is a backward slash () and can be specified using the ESCAPE clause. To match with escape characters, enter two escape characters.

  Return type: Boolean

  Example:

  mogdb=# SELECT 'AA_BBCC' LIKE '%A@_B%' ESCAPE '@' AS RESULT;
   result
  --------
   t
  (1 row)

  mogdb=# SELECT 'AA_BBCC' LIKE '%A@_B%' AS RESULT;
   result
  --------
   f
  (1 row)

  mogdb=# SELECT 'AA@_BBCC' LIKE '%A@_B%' AS RESULT;
   result
  --------
   t
  (1 row)

• REGEXP_LIKE(source_string, pattern [, match_parameter])

  Description: Indicates the mode matching function of a regular expression.

  source_string indicates the source string and pattern indicates the matching pattern of the regular expression. match_parameter indicates the matching items and the values are as follows:

  • 'i': case-insensitive
  • 'c': case-sensitive
  • 'n': allowing the metacharacter "." in a regular expression to be matched with a linefeed.
  • 'm': allows source_string to be regarded as multiple rows.

  If match_parameter is ignored, case-sensitive is enabled by default, "." is not matched with a linefeed, and source_string is regarded as a single row.

  Return type: Boolean

  Example:

  mogdb=# SELECT regexp_like('ABC', '[A-Z]');
   regexp_like
  -------------
   t
  (1 row)

  mogdb=# SELECT regexp_like('ABC', '[D-Z]');
   regexp_like
  -------------
   f
  (1 row)

  mogdb=# SELECT regexp_like('ABC', '[a-z]','i');
   regexp_like
  -------------
   t
  (1 row)

• format(formatstr text [, str"any" [, …] ])

  Description: Formats a string.

  Return type: text

  Example:

  mogdb=# SELECT format('Hello %s, %1$s', 'World');
         format
  --------------------
   Hello World, World
  (1 row)

• md5(string)

  Description: Encrypts a string in MD5 mode and returns a value in hexadecimal form.

  NOTE: The MD5 encryption algorithm is not recommended because it has lower security and poses security risks.

  Return type: text

  Example:

  mogdb=# SELECT md5('ABC');
                 md5
  ----------------------------------
   902fbdd2b1df0c4f70b4a5d23525e932
  (1 row)

• decode(string text, format text)

  Description: Decodes binary data from textual representation.

  Return type: bytea

  Example:

  mogdb=# SELECT decode('MTIzAAE=', 'base64');
      decode
  --------------
   \x3132330001
  (1 row)

• similar_escape(pat text, esc text)

  Description: Converts a regular expression of the SQL:2008 style to the POSIX style.

  Return type: text

  Example:

  mogdb=# select similar_escape('\s+ab','2');
   similar_escape
  ----------------
   ^(?:\\s+ab)$
  (1 row)

• svals(hstore)

  Description: Obtains the value of the hstore type.

  Return type: SETOF text

  Example:

  mogdb=# select svals('"aa"=>"bb"');
   svals
  -------
   bb
  (1 row)

• tconvert(key text, value text)

  Description: Converts character strings to the hstore format.

  Return type: hstore

  Example:

  mogdb=# select tconvert('aa', 'bb');
    tconvert
  ------------
   "aa"=>"bb"
  (1 row)

• encode(data bytea, format text)

  Description: Encodes binary data into a textual representation.

  Return type: text

  Example:

  mogdb=# SELECT encode(E'123\\000\\001', 'base64');
    encode
  ----------
   MTIzAAE=
  (1 row)

NOTE:

• For a string containing newline characters, for example, a string consisting of a newline character and a space, the value of length and lengthb in MogDB is 2.
• In MogDB, n in the CHAR(n) type indicates the number of characters. Therefore, for multiple-octet coded character sets, the length returned by the LENGTHB function may be longer than n.
• MogDB supports multiple types of databases, including A, B, C, and PG. If the database type is not specified, A is used by default. The lexical analyzer of A database is different from that of the other three databases. In A database, an empty character string is considered as NULL. Therefore, when a type A database is used, if a NULL character string is used as a parameter in the preceding character operation function, no output is displayed. For example:

  mogdb=# SELECT translate('12345','123','');
    translate
   -----------

   (1 row)

This is because the kernel checks whether the input parameter contains NULL before calling the corresponding function. If yes, the kernel does not call the corresponding function. As a result, no output is displayed. In PG mode, the processing of character strings is the same as that of PostgreSQL. Therefore, the preceding problem does not occur.