该如何优化下面的sql语句,谢谢
select a.id,a.dept_id,a.user_id,DATE_FORMAT(a.sign_time,'%Y-%m-%d %H:%i:%s') sign_time,c.department_name deptname,b.real_name from wkrj_sign_record a,wkrj_sys_user b,wkrj_sys_department c where a.dept_id=c.department_id and a.user_id=b.user_id ORDER BY sign_time DESC,c.department_order,c.department_id limit 0,20
表结构
CREATE TABLE `wkrj_sign_record` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`dept_id` varchar(36) DEFAULT NULL,
`user_id` int(11) DEFAULT NULL,
`sign_time` datetime DEFAULT NULL,
`state` char(1) DEFAULT '0' COMMENT '0代表未签订1代表正常2代表不正常',
`ok_time` time DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `sign_time` (`sign_time`),
KEY `dept_id` (`dept_id`),
KEY `dept_id_2` (`dept_id`)
) ENGINE=InnoDB AUTO_INCREMENT=69261 DEFAULT CHARSET=utf8;
CREATE TABLE `wkrj_sys_user` (
`user_id` int(11) NOT NULL AUTO_INCREMENT,
`job_id` int(11) DEFAULT NULL,
`roler_id` int(11) DEFAULT NULL,
`department_id` varchar(60) DEFAULT NULL,
`user_name` varchar(60) NOT NULL,
`password` varchar(60) DEFAULT NULL,
`real_name` varchar(30) DEFAULT NULL,
`user_is_enabled` tinyint(4) DEFAULT '1',
`sfz` varchar(20) DEFAULT NULL,
PRIMARY KEY (`user_id`),
KEY `FK_wkrj_sys_department_user` (`department_id`),
KEY `FK_wkrj_sys_roler_user` (`roler_id`),
KEY `FK_wkrj_sys_user_job` (`job_id`),
KEY `user_id` (`user_id`),
) ENGINE=InnoDB AUTO_INCREMENT=473 DEFAULT CHARSET=utf8;
CREATE TABLE `wkrj_sys_department` (
`department_id` varchar(60) NOT NULL,
`department_name` varchar(60) NOT NULL,
`parent_department_id` varchar(60) DEFAULT NULL,
`department_is_leaf` tinyint(1) DEFAULT '0',
`remark` varchar(100) DEFAULT NULL,
`department_order` smallint(10) DEFAULT NULL,
PRIMARY KEY (`department_id`),
KEY `department_order` (`department_order`),
KEY `department_id` (`department_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;