两个比较有用的字符串函数

DELIMITER //
   CREATE
 FUNCTION substr_count(in_haystack mediumtext, in_needle varchar(255), in_offset int unsigned, in_length int unsigned) RETURNS int unsigned
 LANGUAGE SQL DETERMINISTIC NO SQL SQL SECURITY INVOKER
   BEGIN
      DECLARE v_count, v_haystack_len, v_needle_len, v_offset, v_endpos int unsigned DEFAULT 0;

      SET v_haystack_len = CHAR_LENGTH(in_haystack),
          v_needle_len   = CHAR_LENGTH(in_needle),
          v_offset       = IF(in_offset IS NOT NULL AND in_offset > 0, in_offset, 1),
          v_endpos       = IF(in_length IS NOT NULL AND in_length > 0, v_offset + in_length, v_haystack_len);

      -- The last offset to use with LOCATE is at v_endpos - v_needle_len.
      -- That also means that if v_needlen > v_endpos, the count is trivially 0
      IF (v_endpos > v_needle_len) THEN
         SET v_endpos = v_endpos - v_needle_len;
         WHILE (v_offset < v_endpos) DO
            SET v_offset = LOCATE(in_needle, in_haystack, v_offset);
            IF (v_offset > 0) THEN
               -- v_offset is now the position of the first letter in the needle.
               -- Skip the length of the needle to avoid double counting.
               SET v_count  = v_count  + 1,
                  v_offset = v_offset + v_needle_len;
            ELSE
               -- The needle was not found. Set v_offset = v_endpos to exit the loop.
               SET v_offset = v_endpos;
            END IF;
         END WHILE;
      END IF;

      RETURN v_count;
   END//

DELIMITER ;

例子：

mysql> SELECT substr_count('a/b/c/d/e', '/', 3, 5);
+--------------------------------------+
| substr_count('a/b/c/d/e', '/', 3, 5) |
+--------------------------------------+
|                                    2 |
+--------------------------------------+
1 row in set (0.00 sec)

2：

DELIMITER //
   CREATE
 FUNCTION substr_by_delim(in_set mediumtext, in_delimiter varchar(255), in_pos int) RETURNS mediumtext
  COMMENT 'Returns the Nth element from a delimited list.'
 LANGUAGE SQL DETERMINISTIC NO SQL SQL SECURITY INVOKER
    BEGIN
      DECLARE v_num_parts int unsigned DEFAULT 0;

      IF (in_pos < 0) THEN
         -- substr_count returns the number of delimiters, add 1 to get the number of parts
         SET v_num_parts = substr_count(in_set, in_delimiter, NULL, NULL) + 1;
         IF (v_num_parts >= ABS(in_pos)) THEN
            -- Add the requested position (which is negative, so is actually a subtraction)
            -- Add 1 as the position is 1 based.
            SET in_pos = v_num_parts + in_pos + 1;
         ELSE
            -- The requested position is out of range, so set in_pos to 0.
            SET in_pos = 0;
         END IF;
      END IF;
      IF (in_pos <= 0 OR in_pos IS NULL OR in_pos > substr_count(in_set, in_delimiter, 0, NULL)+1) THEN
         -- in_pos is not BETWEEN 1 AND #of elements.
         RETURN NULL;
      ELSE
         RETURN SUBSTRING_INDEX(SUBSTRING_INDEX(in_set, in_delimiter, in_pos), in_delimiter, -1);
      END IF;
   END//
DELIMITER ;

例子：

mysql> SELECT substr_by_delim('a,b,c,d,e', ',', 2);
+--------------------------------------+
| substr_by_delim('a,b,c,d,e', ',', 2) |
+--------------------------------------+
| b                                    |
+--------------------------------------+
1 row in set (0.00 sec)
 
mysql> SELECT substr_by_delim('a,b,c,d,e', ',', -2);
+---------------------------------------+
| substr_by_delim('a,b,c,d,e', ',', -2) |
+---------------------------------------+
| d                                     |
+---------------------------------------+
1 row in set (0.00 sec)
 
mysql> SELECT substr_by_delim('a||b||c||d||e', '||', 2);
+-------------------------------------------+
| substr_by_delim('a||b||c||d||e', '||', 2) |
+-------------------------------------------+
| b                                         |
+-------------------------------------------+
1 row in set (0.00 sec)