SELECT `table`.`id`, `table`.`text`, `table`.`idm`, @i := @i + 1 AS `count` 
FROM `table`, (SELECT @i:=0) AS `var` WHERE `table`.`idm` = 5 
ORDER BY `table`.`id`;

UPDATE `table` 
INNER JOIN 
    (SELECT `table`.`id`, @i := @i + 1 AS `count` 
    FROM `table`, (SELECT @i:=0) AS `var` 
    WHERE `table`.`idm` = 5 
    ORDER BY `table`.`id`) AS `temp` 
USING(`id`) 
SET `table`.`count` = `temp`.`count`;

UPDATE `table` SET `count` = `count` - 1 
WHERE `id` > 123 AND `idm` = 5;

INSERT INTO `table` (`поле1`, `поле2`, `count`) 
    SELECT 'значение поля1', 'значение поля2', MAX(`count`) + 1 
    FROM `table` 
    WHERE `idm` = 'чему оно там равно';

SELECT IF(COUNT(`idm`) > 0, COUNT(`idm`), 1) AS `count`
FROM `table` 
WHERE `что-то` = `чему-то` AND ... OR ... и т.д.;

INSERT INTO table (id,somefield,count) 
    SELECT NULL, '$somefieldvar',IF(MAX(`count`), MAX(`count`)+1, 1)
    FROM table 
    WHERE idm = '123'

INSERT INTO table (id,somefield,count) 
    SELECT NULL, '$somefieldvar',IF(`count`, `count`+1, 1)
    FROM table 
    WHERE idm = '123' ORDER BY idm desc LIMIT 1