ทั้งนี้คำสั่ง LENGTH() จะนับจำนวนสตริงในคอลัมน์ที่ระบุ และจะมีผลลัพธ์ผิดพลาดทันทีถ้าหากคอลัมน์นั้นมีข้อมูลลักษณะนี้ เช่น Final Fantasy, Goofy. ตัวข้อมูล Goofy จะถูกนำขึ้นมาก่อน Final Fantasy เพราะจำนวนสตริงน้อยกว่า คำสั่งนี้จึงไม่เหมาะกับสถานการณ์ดังที่กล่าว.
วิธีที่ 2
สร้าง function, routine ขึ้นมาช่วย
Code (SQL)
/**
* Returns a string formatted for natural sorting. This function is very useful when having to sort alpha-numeric strings.
*
* @author Alexandre Potvin Latreille (plalx)
* @param {nvarchar(4000)} instring The formatted string.
* @param {int} numberLength The length each number should have (including padding). This should be the length of the longest number. Defaults to 10.
* @param {char(50)} sameOrderChars A list of characters that should have the same order. Ex: '.-/'. Defaults to empty string.
*
* @return {nvarchar(4000)} A string for natural sorting.
* Example of use:
*
* SELECT Name FROM TableA ORDER BY Name
* TableA (unordered) TableA (ordered)
* ------------ ------------
* ID Name ID Name
* 1. A1. 1. A1-1.
* 2. A1-1. 2. A1.
* 3. R1 --> 3. R1
* 4. R11 4. R11
* 5. R2 5. R2
*
*
* As we can see, humans would expect A1., A1-1., R1, R2, R11 but that's not how SQL is sorting it.
* We can use this function to fix this.
*
* SELECT Name FROM TableA ORDER BY dbo.udf_NaturalSortFormat(Name, default, '.-')
* TableA (unordered) TableA (ordered)
* ------------ ------------
* ID Name ID Name
* 1. A1. 1. A1.
* 2. A1-1. 2. A1-1.
* 3. R1 --> 3. R1
* 4. R11 4. R2
* 5. R2 5. R11
*/
DROP FUNCTION IF EXISTS `udf_FirstNumberPos`;
DELIMITER ;;
CREATE FUNCTION `udf_FirstNumberPos` (`instring` varchar(4000))
RETURNS int
LANGUAGE SQL
DETERMINISTIC
NO SQL
SQL SECURITY INVOKER
BEGIN
DECLARE position int;
DECLARE tmp_position int;
SET position = 5000;
SET tmp_position = LOCATE('0', instring); IF (tmp_position > 0 AND tmp_position < position) THEN SET position = tmp_position; END IF;
SET tmp_position = LOCATE('1', instring); IF (tmp_position > 0 AND tmp_position < position) THEN SET position = tmp_position; END IF;
SET tmp_position = LOCATE('2', instring); IF (tmp_position > 0 AND tmp_position < position) THEN SET position = tmp_position; END IF;
SET tmp_position = LOCATE('3', instring); IF (tmp_position > 0 AND tmp_position < position) THEN SET position = tmp_position; END IF;
SET tmp_position = LOCATE('4', instring); IF (tmp_position > 0 AND tmp_position < position) THEN SET position = tmp_position; END IF;
SET tmp_position = LOCATE('5', instring); IF (tmp_position > 0 AND tmp_position < position) THEN SET position = tmp_position; END IF;
SET tmp_position = LOCATE('6', instring); IF (tmp_position > 0 AND tmp_position < position) THEN SET position = tmp_position; END IF;
SET tmp_position = LOCATE('7', instring); IF (tmp_position > 0 AND tmp_position < position) THEN SET position = tmp_position; END IF;
SET tmp_position = LOCATE('8', instring); IF (tmp_position > 0 AND tmp_position < position) THEN SET position = tmp_position; END IF;
SET tmp_position = LOCATE('9', instring); IF (tmp_position > 0 AND tmp_position < position) THEN SET position = tmp_position; END IF;
IF (position = 5000) THEN RETURN 0; END IF;
RETURN position;
END
;;
DROP FUNCTION IF EXISTS `udf_NaturalSortFormat`;
DELIMITER ;;
CREATE FUNCTION `udf_NaturalSortFormat` (`instring` varchar(4000), `numberLength` int, `sameOrderChars` char(50))
RETURNS varchar(4000)
LANGUAGE SQL
DETERMINISTIC
NO SQL
SQL SECURITY INVOKER
BEGIN
DECLARE sortString varchar(4000);
DECLARE numStartIndex int;
DECLARE numEndIndex int;
DECLARE padLength int;
DECLARE totalPadLength int;
DECLARE i int;
DECLARE sameOrderCharsLen int;
SET totalPadLength = 0;
SET instring = TRIM(instring);
SET sortString = instring;
SET numStartIndex = udf_FirstNumberPos(instring);
SET numEndIndex = 0;
SET i = 1;
SET sameOrderCharsLen = CHAR_LENGTH(sameOrderChars);
WHILE (i <= sameOrderCharsLen) DO
SET sortString = REPLACE(sortString, SUBSTRING(sameOrderChars, i, 1), ' ');
SET i = i + 1;
END WHILE;
WHILE (numStartIndex <> 0) DO
SET numStartIndex = numStartIndex + numEndIndex;
SET numEndIndex = numStartIndex;
WHILE (udf_FirstNumberPos(SUBSTRING(instring, numEndIndex, 1)) = 1) DO
SET numEndIndex = numEndIndex + 1;
END WHILE;
SET numEndIndex = numEndIndex - 1;
SET padLength = numberLength - (numEndIndex + 1 - numStartIndex);
IF padLength < 0 THEN
SET padLength = 0;
END IF;
SET sortString = INSERT(sortString, numStartIndex + totalPadLength, 0, REPEAT('0', padLength));
SET totalPadLength = totalPadLength + padLength;
SET numStartIndex = udf_FirstNumberPos(RIGHT(instring, CHAR_LENGTH(instring) - numEndIndex));
END WHILE;
RETURN sortString;
END
;;
โดยคำสั่งนี้จะเป็นการสร้าง function ขึ้นมา ให้เราสั่งเพียงครั้งเดียวแล้วฐานข้อมูลจะสร้าง function ไว้ใน routines ซึ่งการเรียกใช้ครั้งต่อไปก็เรียกใช้ได้เลย.
ตัวอย่างการเรียกใช้.
Code (SQL)
SELECT * FROM `products` ORDER BY udf_NaturalSortFormat(`product_name`, 10, ".") ASC
You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'RETURNING id,t' at line 1