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[数据库]mariadb的select语句


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;