查

查看帮助文档目录列表（命令行）：

help contents;

查看帮助文档具体内容：

help 目录内容;

查看数据库：

show databases

查看数据库中包含的所有表：

show tables

查看指定表结构：

describe bill
desc bill

查看默认存储引擎：

show variables like `storage_engine%`;

查询表数据：

select 字段名|表达式|函数|常量 from `表名` where ..... order by .... group by ..... limit ...

常量列：

select `studentNo` as '学号','常量列数据' as '别名' from .....

子查询：

SELECT * FROM `student` WHERE 字段 比较运算符(子查询语句)

SELECT * FROM `student` WHERE 字段 INT(子查询语句)
SELECT * FROM `student` WHERE 字段 NOT IN(子查询语句)
SELECT * FROM `student` WHERE EXISTS(子查询语句)
SELECT * FROM `student` WHERE NOT EXISTS(子查询语句)

分组查询：

select * from `student` group by `字段1`,`字段2`.......

分组筛选查询：

SELECT * FROM `student` GROUP BY `字段1`,`字段2`....... HAVING 限制条件如：studentNo>2

内连接查询：

select cus.`cusName`,cus.`cusSex`,cus.cusIdNo,cus.`cusPhone`,
chec.`roomId`,chec.`checkInDate`,chec.`checkOutDate`,roomt.`price`,
chec.`deposit`,chec.`checkInId` from `checkIn` chec 
join `room` r on r.`roomId`=chec.`roomId` 
join `roomType` roomt on r.`typeId`=roomt.`typeId` 
join `customer` cus on cus.`cusId`=chec.`cusId` 
where r.`roomId`=? and r.`status`=? and chec.`billstate`=?

左外连接查询（结果包含左表所有行，若右表没有匹配则填上NULL）：

select cus.`cusName`,cus.`cusSex`,cus.cusIdNo,cus.`cusPhone`,
chec.`roomId`,chec.`checkInDate`,chec.`checkOutDate`,roomt.`price`,
chec.`deposit`,chec.`checkInId` from `checkIn` chec 
left join `room` r on r.`roomId`=chec.`roomId` 
left join `roomType` roomt on r.`typeId`=roomt.`typeId` 
left join `customer` cus on cus.`cusId`=chec.`cusId` 
where r.`roomId`=? and r.`status`=? and chec.`billstate`=?

右外连接查询（结果包含右表所有行，若左表没有匹配则填上NULL）：

select cus.`cusName`,cus.`cusSex`,cus.cusIdNo,cus.`cusPhone`,
chec.`roomId`,chec.`checkInDate`,chec.`checkOutDate`,roomt.`price`,
chec.`deposit`,chec.`checkInId` from `checkIn` chec 
right join `room` r on r.`roomId`=chec.`roomId` 
right join `roomType` roomt on r.`typeId`=roomt.`typeId` 
right join `customer` cus on cus.`cusId`=chec.`cusId` 
where r.`roomId`=? and r.`status`=? and chec.`billstate`=?

查看索引：

SHOW INDEX 索引名 FROM 表名  列较多的情况后面加\G