//查询某张表所有数据
select * from temp;

//查询指定列和条件的数据
//查询name和age这两列，age等于22的数据
select name,age from temp where age = 22;

//as对列重命名
//as可以省略不写，如果重命名的列名出现特殊字符，如单引号，那就需要用双引号引在外面
select name as ‘名称‘ from temp;

//给table去别名
select t.name Name from temp as t;

//where条件查询
>、>=、<、<=、=、<>都可以出现在where语句中
select from t where a > 2 or a>=3 or a<5 or a<=6 or a=7 or a<>0;

//and 并且
//查询名称等于Jack并且年龄大于20的
select * from temp where age > 20 and name = ‘jack‘;

//or或者
--满足一个条件即可
select * from temp where name = ‘jack‘ or name = ‘jackson‘;

//between v and v2
--大于等于v且小于等于v2
select * from temp where age between 20 and 25;

//in 查询
--可以多个条件，类似于or
--查询id 在括号中出现的数据
select *from temp where id in (1, 2, 3);

//like模糊查询
--查询name以j开头的
select * from temp where name like ‘j%‘;
--查询name包含k的
select * from temp where name like ‘%k%‘;
--escape转义,指定\为转义字符，上面的就可以查询name中包含“_”的数据
select * from temp where name like ‘\_%‘ escape ‘\‘;

//is null、is not null
--查询为null的数据
select * from temp where name is null;
--查询不为null 的数据
select * from temp where name is not null;

//order by
--排序，升序(desc)、降序(asc)
--默认升序
select * from temp order by id;
select * from temp order by id asc;
--多列组合
select * from temp order by id, age;

//not
select * from temp where not (age > 20);
select * from temp where id not in(1, 2);

//distinct去掉重复数据
select distinct id from temp;
//多列将是组合的重复数据
select distinct id, age from temp;

//查询常量
select 5+2;
select concat(‘a‘, ‘bbb‘);

//concat函数，字符串连接
//concat和null进行连接，会导致连接后的数据成为null
select concat(name, ‘-eco‘) from temp;

//对查询的数据进行运算操作
select age +2, age / 2, age - 2, age * 2 from temp where age - 2 > 22;