按条件查询：
select username,password from users where id=1;
in 查询
select * from users where id in (1,2,3);
select * from users where id not in (1,2,3);
between and 查询
select * from users where id between 1 and 10;
select * from users where id not between 1 and 10;
like 查询
select * from users where username like "%m%"
and 查询
select * from users where id=1 and username='xxx';
or 查询
select * from users where id=1 or username='xxx';
当 and 与 or 一起使用时：and的优先级高于or,先运算and再运算or。
聚合查询
count() 返回行数；
select count(*) from users;
select count(id) from users;
分组查询
GROUP BY
mysql> SELECT * FROM users GROUP BY password;
+----+----------+----------+
| id | username | password |
+----+----------+----------+
| 3 | moon1 | 123456 |
| 1 | moon | 456789 |
+----+----------+----------+
2 rows in set (0.01 sec)
mysql> SELECT * FROM users GROUP BY username;
+----+----------+----------+
| id | username | password |
+----+----------+----------+
| 2 | alex1 | 456789 |
| 1 | moon | 456789 |
| 3 | moon1 | 123456 |
+----+----------+----------+
3 rows in set (0.01 sec)
limit 查询
select * from users limit 2,5;
子查询
where型子查询 把内层查询结果当作外层查询的比较条件
from型子查询 把内层查询结果当作外层查询的比较条件
exists型子查询 把外层查询结果拿到内层，看内层的查询是否成立
union 联合查询 两次查询的列数必须一致
内连接
inner join on：结合两个表中的记录，返回关联字段相符的记录，也就是两个表的交集部分。
select * from a_user as a inner join b_class on a.class_id=b.user_id;
左链接
left join on：左表的记录都会显示出来，而右表只会显示符合搜索条件的记录。右表记录不足的地方均为NULL。
select * from a_user as a left join b_class as b on a.class_id=b.user_id;
右链接
right join on：右表的记录都会显示出来，而左表只会显示符合搜索条件的记录。左表记录不足的地方均为NULL。
select * from a_user as a right join b_class as b on a.class_id=b.user_id;
跨库查询:information_schema
schemata表：提供了当前mysql中所有数据库的信息
tables表：提供了关于数据库中的表的信息
columns表：提供了表中的列信息
and 1=2 union select 1,2,schema_name from information_schema.schemata limit 0,1
select 1,2,table_name from information_schema.tables where table_schema='' limit 0,1
select 1,2,column_name from information_schema.columns where table_name='' limit 0,1
