show variables like '%pack%';
set global max_allowed_packet=1073741824;

SELECT MAX(HISTNUM) FROM CLIENTS

SELECT MAX(CAST(HISTNUM AS INT)) FROM CLIENTS

conversion error from string ""
GDSCODE = 335544334
SQLCODE = -413

SELECT HISTNUM FROM CLIENTS ORDER BY CHAR_LENGTH(HISTNUM) DESC, HISTNUM DESC ROWS 1

SELECT HISTNUM FROM CLIENTS ORDER BY CHAR_LENGTH(HISTNUM) DESC, HISTNUM DESC ROWS 1 TO 1

SELECT FIRST 1 SKIP 0 HISTNUM FROM CLIENTS ORDER BY CHAR_LENGTH(HISTNUM) DESC, HISTNUM DESC

SELECT HISTNUM FROM CLIENTS WHERE CHAR_LENGTH(HISTNUM)<7 ORDER BY CHAR_LENGTH(HISTNUM) DESC, HISTNUM DESC ROWS 1

SELECT FIRST 1 SKIP 0 HISTNUM FROM CLIENTS WHERE CHAR_LENGTH(HISTNUM)<7 ORDER BY CHAR_LENGTH(HISTNUM) DESC, HISTNUM DESC

CREATE TABLE `photos_temp` AS (SELECT MIN(`id`) AS `id` FROM `photos` GROUP BY `photo`);
ALTER TABLE `photos_temp` ADD PRIMARY KEY(`id`);
DELETE FROM `photos` WHERE `id` NOT IN (SELECT `id` FROM `photos_temp`);
DROP TABLE `photos_temp`;

SELECT TABLE_NAME FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_SCHEMA = 'db_name' AND ENGINE = 'MyISAM';

ALTER TABLE `table_name` ENGINE=InnoDB;

SELECT * FROM `catalog` WHERE `uid` IN (SELECT `uid` FROM `catalog` GROUP BY `uid` HAVING COUNT(`uid`)>1) ORDER BY `uid` ASC, `id` DESC;

SELECT * FROM `catalog` WHERE `uid` IN (SELECT `uid` FROM `catalog` GROUP BY `uid` HAVING COUNT(`uid`)>1) GROUP BY `uid` ORDER BY `uid` ASC, `id` DESC;