后端MySQL常用命令-查询/select

时间：2024-01-23 22:55:22

基本查询

SELECT 字段名 from 表名

SELECT id,name,sex,age,weight,birth from ninth_student

条件查询

用where表示查询的条件

SELECT 字段名 from 表名 WHERE 条件

SELECT id,name,age,sex,weight,birth from ninth_student WHERE id BETWEEN 1 and 5

SELECT 字段名 from 表名 WHERE 条件 and/or 条件

SELECT id,name,age,sex,weight,birth from ninth_student WHERE id>5 and sex='男'
SELECT id,name,age,sex,weight,birth from ninth_student WHERE id>5 OR sex='男'

模糊查询/like

利用%

SELECT id,name,age,sex,weight,birth from ninth_student WHERE name LIKE '%亚'
SELECT id,name,age,sex,weight,birth from ninth_student WHERE name LIKE '亚%'
SELECT id,name,age,sex,weight,birth from ninth_student WHERE name LIKE '%亚%'

利用_,一个_代表一个字符，如果前面有两个字，就加两个

SELECT id,name,age,sex,weight,birth from ninth_student WHERE name LIKE '_马'

分页查询/limit

SELECT 字段名 表名 LIMIT (n-1)*每页展示个数,每页展示个数

SELECT id,name,age,sex,weight,birth from ninth_student LIMIT 0,4
SELECT id,name,age,sex,weight,birth from ninth_student LIMIT 4,4

排序

SELECT 字段名 from 表名 ORDER BY age ASC/DESC

SELECT id,name,age,sex,weight,birth from ninth_student ORDER BY age ASC
SELECT id,name,age,sex,weight,birth from ninth_student ORDER BY id DESC

聚合函数

SELECT COUNT(0) from 表名：表示总共有几条数据

SELECT SUM(字段名) from 表名：求和

SELECT AVG(字段名) from 表名：平均数

SELECT MIN(字段名) from 表名：最大

SELECT MAX(字段名) from 表名：最小

SELECT CAST(AVG(字段名) as DECIMAL(10,0)) as AVG from 表名：类型转换

SELECT COUNT(0) from ninth_student
SELECT SUM(age) from ninth_student
SELECT AVG(age) from ninth_student
SELECT MIN(age) from ninth_student
SELECT MAX(age) from ninth_student
SELECT CAST(AVG(age) as DECIMAL(10,0)) as AVG from ninth_student

分组查询

SELECT 字段名,聚合函数 from 表名 GROUP BY 字段名

GROUP BY 字段名：根据...分组

group by 分组字段，select后跟的也是相应的分组字段，他俩是一致的

where是分组前，having是分组后

having后面跟条件的话，也只能跟聚合函数

SELECT sex,COUNT(0),AVG(age) from ninth_student GROUP BY sex
SELECT sex,COUNT(0),AVG(age) from ninth_student WHERE age>22 GROUP BY sex HAVING AVG(age)>20

子查询

把查询到的结果当做另一个查询的条件

SELECT 字段名 from 表1 WHERE 字段名1 = (SELECT 表1_字段名1 from 表 WHERE 字段名='值')

SELECT name from dept WHERE id = (SELECT dept_id from ninth_student WHERE name='小马')

内连接

多表联查，一定要待条件！！！

inner join 表示内连接

on 表和表之间的连接，只能用on

inner join ... on ... 只要看见join就加on

看到,就写where

SELECT 表1.字段名1,表1.字段名1,字段名1,字段名1,字段名1,字段名1,表2.字段名2,表2.字段名2 from 表1 INNER JOIN 表2 ON 表1.字段名1 = 表2.字段名2

SELECT ninth_student.id,ninth_student.name,age,sex,weight,birth,dept.id,dept.name from ninth_student INNER JOIN dept ON ninth_student.id = dept.id
SELECT ninth_student.id,ninth_student.name,age,sex,weight,birth,dept.id,dept.name from ninth_student,dept WHERE ninth_student.dept_id = dept.id
SELECT ninth_student.id,ninth_student.name,age,sex,weight,birth,dept.id,dept.name from ninth_student,dept WHERE ninth_student.id = dept.id

外连接

在实际开发中，左外连接用的比较多

left join查询左边表的所有部分和右边表的交集，就是以左边的表为主

左外是以左边的表为主表，查询的结果只关注左表，不在乎右边

right join查询的是右边表的所有部分和左边表的交集，就是以右边的表为主

SELECT ninth_student.id,ninth_student.name,sex,age,weight,birth,dept.id,dept.name from dept LEFT JOIN ninth_student ON ninth_student.dept_id = dept.id
SELECT ninth_student.id,ninth_student.name,sex,age,weight,birth,dept.id,dept.name from dept RIGHT JOIN ninth_student ON ninth_student.dept_id = dept.id

相关文章