CREATE TABLE IF NOT EXISTS `mq_info` ( 	`id` INT(12) UNSIGNED AUTO_INCREMENT COMMENT 'mq信息数据库自增id',	`topic` VARCHAR ( 255 ) NOT NULL COMMENT 'mq_topic名称',   	`message` longtext NOT NULL COMMENT 'mq_message内容', 	`markinfo` VARCHAR ( 255 ) COMMENT '该MQ的标识字段', 	PRIMARY KEY ( `id` ) ) ENGINE = INNODB DEFAULT CHARSET = utf8;

select * from mq_info;

select * from mq_info limit 20;

insert into mq_info ( topic,message,markinfo) values('mq_test','{"testId":165,"operation":3}','testInfo001')

//将id=1的数据的topic都改为'mqUpdate_test'update mq_info set topic='mqUpdate_test' where id='1'

//将id=1或topic='mq_test'的数据的topic都改为'mqUpdate_test'，// markinfo都改为'testInfo_update'update mq_info set topic='mqUpdate_test',markinfo='testInfo_update' where id='1' or topic='mq_test'

delete from mq_info where id = 1

delete from mq_info

//删除mq_info表中的markinfo字段alter table mq_info drop column markinfo

//修改mq_info表中的message字段类型为varchar类型alter table mq_info modify column message varchar(255);

//修改mq_info表中的markinfo字段的备注信息为'自定义该MQ的唯一标识'alter table mq_info modify markinfo varchar(255) comment '自定义该MQ的唯一标识'

//把mq_info表中的markinfo字段名改为description_Info，//类型是varchar(255)alter table mq_info change markinfo description_Info varchar(255)

//向mq_info表新增insert_time字段，字段类型为datetime，//备注信息为'MQ落库时间'alter table mq_info add insert_time datetime comment 'MQ落库时间'

alter table mq_info rename to cart_mq_info

//将insert_time字段在表中的排列顺序//修改为显示在create_time字段后面alter table mq_info modify insert_time tinyint(4) after create_time//“FIRST”为可选参数，指的是将字段1修改为表的第一个字段，//“AFTER 字段名2”是将字段1插入到字段2的后面ALTER TABLE MODLFY 字段名1 数据类型 FIRST|AFTER 字段名2

//复制mq_info表结构生成一张新表order_mq_infocreate table order_mq_info like mq_info

//查询mq_info表中topic字段包含test的所有数据SELECT * FROM mq_info where topic like '%test%';//优化写法：locate（‘substr’,str,pos）, 提升查询效率SELECT * FROM mq_info where locate('test', topic)>0

//查询mq_info表中topic等于'order_info'，并且markinfo不等于'test'的所有数据select * from mq_info where topic = 'order_info' and (markinfo != 'test' or markinfo is null)//注意加上条件markinfo is null，不然Markinfo为null的数据不会被查出来//注意()的作用，加括号是为了提高优先级先执行括号内的查询条件//在这个基础上再进行topic = 'order_info'的查询

//查询每个老师带的学生的平均年龄（group by语法）//由于是查每个老师，因此要用TeacherID作为分组条件select TeacherID, avg(Age) as AGE from student group by TeacherID

//查询每个老师带的学生的平均年龄且平均年龄大于12（group by + having语法）//条件是带函数的，就必须用having，而不能用whereselect TeacherID, avg(Age) as AGEfrom student group by TeacherID having avg(Age) > 12

//teacher表中没有存学生姓名，只存了学生id//需要将teacher表中create_time大于等于2022-10-28 00:00:00的所有数据以及学生的姓名查出来select teacher.*,student.studentName,from teacher_manage replay LEFT JOIN student_manage student on teacher.student_id=student.id where teacher.create_time >= '2022-10-28 00:00:00';