Mysql | [ | sense of life

PHP-MySql: Select previous row and next row from current ID

Tuesday, July 15th, 2008

During the dark age, the simplest way to get previous row from current ID was using this query:

SELECT field1
FROM tablename
WHERE id = ($currentId - 1)

and to get next row:

SELECT field1
FROM tablename
WHERE id = ($currentId + 1)

But there’s a problem. Like common data, we need to delete some rows. For example, the current ID is 14 ($currentid = 14) , and i deleted row id number 13, when i run the query to get the previous row, the result will empty. The sql cannot find ID number 13 ($currentid - 1).

How to fix it?
Again, the first simplest way i got is to loop the ID. When it cannot find the first query, it calculate again by looping it with PHP.

//the bad way..

<?
$ok = false;
$prev = $currentid - 1;
while($ok == false){
$sql = "select * from tablename where id = $prev ";
$data = $db->query($sql);
if(empty($data)){
$prev = $prev-1;
}else{
$ok = true;
}
}
?>

This is really bad idea. It will consume more resources by continually query the database. What happen when you deleted 10 rows from current ID?
There will be 10 looping with 10 query to database. How about deleted 100 rows?? You must be insane doing this.

(more…)

Posted in coded | 4 Comments »

Mysql String Functions

Sunday, May 25th, 2008

Name	Description
`ASCII()`	Return numeric value of left-most character
`BIN()`	Return a string representation of the argument
`BIT_LENGTH()`	Return length of argument in bits
`CHAR_LENGTH()`	Return number of characters in argument
`CHAR()`	Return the character for each integer passed
`CHARACTER_LENGTH()`	A synonym for CHAR_LENGTH()
`CONCAT_WS()`	Return concatenate with separator
`CONCAT()`	Return concatenated string
`ELT()`	Return string at index number
`EXPORT_SET()`	Return a string such that for every bit set in the value bits, you get an on string and for every unset bit, you get an off string
`FIELD()`	Return the index (position) of the first argument in the subsequent arguments
`FIND_IN_SET()`	Return the index position of the first argument within the second argument
`FORMAT()`	Return a number formatted to specified number of decimal places
`HEX()`	Return a hexadecimal representation of a decimal or string value
`INSERT()`	Insert a substring at the specified position up to the specified number of characters
`INSTR()`	Return the index of the first occurrence of substring
`LCASE()`	Synonym for LOWER()
`LEFT()`	Return the leftmost number of characters as specified
`LENGTH()`	Return the length of a string in bytes
`LIKE`	Simple pattern matching
`LOAD_FILE()`	Load the named file
`LOCATE()`	Return the position of the first occurrence of substring
`LOWER()`	Return the argument in lowercase
`LPAD()`	Return the string argument, left-padded with the specified string
`LTRIM()`	Remove leading spaces
`MAKE_SET()`	Return a set of comma-separated strings that have the corresponding bit in bits set
`MATCH`	Perform full-text search
`MID()`	Return a substring starting from the specified position
`NOT LIKE`	Negation of simple pattern matching
`NOT REGEXP`	Negation of REGEXP
`OCTET_LENGTH()`	A synonym for LENGTH()
`ORD()`	Return character code for leftmost character of the argument
`POSITION()`	A synonym for LOCATE()
`QUOTE()`	Escape the argument for use in an SQL statement
`REGEXP`	Pattern matching using regular expressions
`REPEAT()`	Repeat a string the specified number of times
`REPLACE()`	Replace occurrences of a specified string
`REVERSE()`	Reverse the characters in a string
`RIGHT()`	Return the specified rightmost number of characters
`RLIKE`	Synonym for REGEXP
`RPAD()`	Append string the specified number of times
`RTRIM()`	Remove trailing spaces
`SOUNDEX()`	Return a soundex string
`SOUNDS LIKE`(v4.1.0)	Compare sounds
`SPACE()`	Return a string of the specified number of spaces
`STRCMP()`	Compare two strings
`SUBSTR()`	Return the substring as specified
`SUBSTRING_INDEX()`	Return a substring from a string before the specified number of occurrences of the delimiter
`SUBSTRING()`	Return the substring as specified
`TRIM()`	Remove leading and trailing spaces
`UCASE()`	Synonym for UPPER()
`UNHEX()`(v4.1.2)	Convert each pair of hexadecimal digits to a character
`UPPER()`	Convert to uppercase