数据库 之 数据查询（DQL语句）

Posted 2020-10-09

DQL：Data  Query Language,数据查询语言

SELECT

.SELECT col1,col2,... FROMtbl_name[WHERE clause]

[ORDER BY ‘col_name‘[DESC]] [LIMIT [m,]n];

Limit m,n跳过m个，要n个

如以下语句跳过2个，取接下来的三个数值

MariaDB [sunny]> select* from fenshu order by englishlimit2,3;

+----+------+--------+-------+---------+

| id | name | gender |math| english |

+----+------+--------+-------+---------+

| 22 | cd| m|99 |89.00 |

| 10 | tom| NULL| 60.05 |90.00 |

| 16 | lin| NULL| 60.05 |90.00 |

以下语句表示将表fenshu中english这一列分数大于88的id,name,english这三列筛选出来

select id,name,englishfrom fenshu where english > 88;

.字段表示法：

*:所有字段

as：字段别名，col1 ASalias1

亲自as这个关键字可以不写，如name姓名

例子如下

select id as学号,name as姓名,english

as英语from fenshu where english > 88;

也可以给表起别名，一般用于多表查

%是通配符，表示任意字符，下划线通配符，表示支持单个字符

rlike表示支持正则表达式，如果不支持正则表达式，就不能用.*表示所有的值

例子

MariaDB [sunny]> select* from fenshu where name like "ti.*";

Empty set (0.00 sec)

MariaDB [sunny]> select* from fenshu where name rlike "ti.*";

+----+------+--------+------+---------+

| id | name | gender |math | english |

+----+------+--------+------+---------+

| 23 | tim| f|80 |78.00 |

| 26 | tiac | f|80 |78.00 |

+----+------+--------+------+---------+

2 rows in set (0.00 sec)

多表的连接

selectd.id,d.t4,s.math,s.english from datetb d,score s where d.id=s.id;

DQL语句

.WHERE clause:

操作符：

>, <, >=, <=,==, !=

==判断相等，用两个等号来表示

BETWEEN ... AND ...

LIKE:

%：任意长度的任意字符

_：任意单个字符；

RLIKE：正则表达式模式匹配

IS NULL，IS NOT NULL

IN（val1,val2,…）

select * from score t where t.english in(88,90,70) order by t.english desc;

条件逻辑操作：

and，or，not

示例

.DESC students;

.INSERT INTO students

VALUES(1,‘tom‘，‘m‘),(2,‘alice‘,‘f‘);

.INSERT INTOstudents(id,name) VALUES(3,‘jack‘),(4,‘allen‘);

.SELECT * FROM studentsWHERE id < 3;

.SELECT * FROM studentsWHERE gender=‘m‘;

.SELECT * FROM studentsWHERE gender IS NULL;

.SELECT * FROM studentsWHERE gender IS NOT NULL;

.SELECT * FROM studentsORDER BY name DESC LIMIT 2;

.SELECT * FROM studentsORDER BY name DESC LIMIT 1,2;

.SELECT * FROM studentsWHERE id >=2 and id <=4

.SELECT * FROM studentsWHERE BETWEEN 2 AND 4

.SELECT * FROM studentsWHERE name LIKE ‘t%’

.SELECT * FROM studentsWHERE name RLIKE ‘.*[lo].*‘;

.SELECT id stuid,name as stuname FROM students

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