mariadb的查询流程图
select语句的从句分析顺序：from(过滤表)-->where(过滤行)-->group by(分组)-->having(分组过滤)-->order by(排序)--
>select(选取字段)-->limit(查询限制)-->最终结果
DISTINCT: 数据去重
SQL_CACHE: 显式指定存储查询结果于缓存之中
SQL_NO_CACHE: 显式查询结果不予缓存
show global variables like '%query%';
query_cache_type | ON 表示缓存开启
query_cache_size | 0 表示缓存空间大小，如果为0则不缓存
query_cache_type的值为'DEMAND'时，显式指定SQL_CACHE的SELECT语句才会缓存；其它均不予缓存
缓存并不会缓存所有查询结果，例如select now()；就不会缓存
WHERE子句：指明过滤条件以实现“选择”的功能
算术操作符：+, -, *, /, %
比较操作符：=, !=, <>, <=>, >, >=, <, <=
BETWEEN min_num AND max_num
IN (element1, element2, ...)
IS NULL
IS NOT NULL
LIKE:
%: 任意长度的任意字符
_：任意单个字符
RLIKE
逻辑操作符： NOT,AND,OR
GROUP：根据指定的条件把查询结果进行“分组”以用于做“聚合”运算：avg(), max(), min(), count(), sum()
HAVING: 对分组聚合运算后的结果指定过滤条件
ORDER BY: 根据指定的字段对查询结果进行排序：升序ASC 降序：DESC
LIMIT [[offset,]row_count]：对查询的结果进行输出行数数量限制
例如：
select name,age from students where age/2=11;
select name,age from students where age+30>50;
select distinct gender from students;
select name as stuname from students;
select name,classid from students where classid is null;
select avg(age),gender from students group by gender;
select avg(age) as ages,gender from students group by gender having ages>20;
select count(stuid) as NO,classid from students group by classid;
select count(stuid) as NO,classid from students group by classid having NO>2;
select name,age from students order by age limit 10,10; (第一个表示偏移10个，第二个表示取10个)
多表查询：
交叉连接：笛卡尔乘积(最消耗资源的一种查询) 例如：select * from students,teachers; 如果students有20行，teachers也有20行，则显示400行
内连接：
等值连接：让表之间的字段以“等值”建立连接关系；
不等值连接
自然连接
自连接
外连接：
左外连接 例如：FROM tb1 LEFT JOIN tb2 ON tb1.col=tb2.col
右外连接 例如：FROM tb1 RIGHT JOIN tb2 ON tb1.col=tb2.col
等值连接：
select s.name as studentname,t.name as teachername from students as s,teachers as t where s.teacherid=t.tid;(这个够复杂)
select s.name,c.class from students as s,classes as c where s.classid=c.classid;
左外连接：
select s.name,c.class from students as s left join classes as c on s.classid=c.classid;
以左表作为基准
select s.name,c.class from students as s right join classes as c on s.classid=c.classid;
以右表为基准
子查询：在查询语句嵌套着查询语句,基于某语句的查询结果再次进行的查询 (mariadb对子查询优化不够，建议一般情况不使用)
select name,age from students where age>(select avg(age) from students);
select name,age from students where age in (select age from teachers);
select s.ages,s.classid from (select avg(age) as ages,classid from students where classid is not null group by classid) as s where s.ages>30;
联合查询：UNION
select name,age from students union select name,age from teachers;
