sql语句-单表查询

一：单表查询

CREATE TABLE `Score`(
    `s_id` VARCHAR(20),
    `c_id`  VARCHAR(20),
    `s_score` INT(3),
    PRIMARY KEY(`s_id`,`c_id`)
);


insert into Score values('01' , '01' , 80);
insert into Score values('01' , '02' , 90);
insert into Score values('01' , '03' , 99);
insert into Score values('02' , '01' , 70);
insert into Score values('02' , '02' , 60);
insert into Score values('02' , '03' , 80);
insert into Score values('03' , '01' , 80);
insert into Score values('03' , '02' , 80);
insert into Score values('03' , '03' , 80);
insert into Score values('04' , '01' , 50);
insert into Score values('04' , '02' , 30);
insert into Score values('04' , '03' , 20);
insert into Score values('05' , '01' , 76);
insert into Score values('05' , '02' , 87);
insert into Score values('06' , '01' , 31);
insert into Score values('06' , '03' , 34);
insert into Score values('07' , '02' , 89);
insert into Score values('07' , '03' , 98);

 

CREATE TABLE `Student`(
    `s_id` VARCHAR(20),
    `s_name` VARCHAR(20) NOT NULL DEFAULT '',
    `s_birth` VARCHAR(20) NOT NULL DEFAULT '',
    `s_sex` VARCHAR(10) NOT NULL DEFAULT '',
    PRIMARY KEY(`s_id`)
);


insert into Student values('01' , '赵雷' , '1990-01-01' , '男');
insert into Student values('02' , '钱电' , '1990-12-21' , '男');
insert into Student values('03' , '孙风' , '1990-05-20' , '男');
insert into Student values('04' , '李云' , '1990-08-06' , '男');
insert into Student values('05' , '周梅' , '1991-12-01' , '女');
insert into Student values('06' , '吴兰' , '1992-03-01' , '女');
insert into Student values('07' , '郑竹' , '1989-07-01' , '女');
insert into Student values('08' , '王菊' , '1990-01-20' , '女');

 

1.查询条件

比较

等于
SELECT * FROM score WHERE s_score=80         
大于
SELECT * FROM score WHERE s_score>80
小于
SELECT * FROM score WHERE s_score<80
小于等于
SELECT * FROM score WHERE s_score<=80
大于等于
SELECT * FROM score WHERE s_score>=80
不等于
SELECT * FROM score WHERE s_score!=80
不等于
SELECT * FROM score WHERE s_score<>80
不小于
SELECT * FROM score WHERE s_score!<80
不大于
SELECT * FROM score WHERE s_score!>80

范围

在60-80之间的记录
SELECT * FROM score WHERE s_score  BETWEEN 60 AND 80
不在60-80之间的记录
SELECT * FROM score WHERE s_score NOT BETWEEN 60 AND 80

in的使用

成绩只要是80的记录
SELECT * FROM score WHERE s_score IN(80)
成绩不是80的其他所有记录
SELECT * FROM score WHERE s_score NOT IN(80)
如果是中文就要用单引号，记得名字要写全不要只写一个云或李
SELECT * FROM student WHERE s_name  IN('李云')
姓名不是云的所有记录（因为姓名都是两个字的）
SELECT * FROM student WHERE s_name NOT IN('云')
多组的话可以这样查(成绩是80和60的会被查出来，14表中没有就没有了，sql不会出错的)
SELECT * FROM score WHERE s_score IN(80,60,14)

like的使用

名字中最后一个字是云的记录
SELECT * FROM student WHERE s_name LIKE '%云' 
名字中最先一个字是李的记录
SELECT * FROM student WHERE s_name LIKE '李%'
名字中包含一个字是雷的记录
SELECT * FROM student WHERE s_name LIKE '%雷%'
名字中不包含一个字是云的记录
SELECT * FROM student WHERE s_name NOT LIKE '%云%'
名字中类似第二个字是云的且只有两个字的记录
SELECT * FROM student WHERE s_name LIKE '_云' 

空值

学生表中名字这个字段是null的记录
SELECT * FROM student WHERE s_name IS NULL
学生表中名字这个字段不是null的记录
SELECT * FROM student WHERE s_name IS NOT NULL

and与or

id大于3且性别是男的
SELECT * FROM student WHERE s_id>3 AND s_sex='男'
要么是id大于3的，要么性别是男，两者符合一个就行，都符合当然也行
SELECT * FROM student WHERE s_id>3 OR s_sex='男'

 order by

通常是用做对查询结果按照一个或多个属性进行升序、降序

按照成绩（s_score）升序，成绩相同的按照id(s_id)升序
SELECT * FROM score ORDER BY s_score,s_id ASC

降序是  DESC

 distinct   与  group by 两者都有去重的能力

去重后的成绩
SELECT DISTINCT s_score FROM score

分组（分组后是去重的）后的成绩与其对应的id
SELECT s_score,s_id FROM score GROUP BY s_score

结论：distinct只适用于查询不重复记录的条数，例如select coun(distinct s_score) from score  代表不重复的成绩记录有多少条

SELECT DISTINCT s_score，s_id FROM score  这样会变成成绩与id都相同的才会去重

SELECT s_id,DISTINCT s_score FROM score  而这样mysql会报错

只有group by适合查询一整条不重复记录的数据

 

聚集函数

count  统计条数    select count(*) from score;select count(distinct s_score) from score;   distinct不写的话默认是all

sum   计算某一列的总和（必须是数值型）select sum(s_score) from score

avg   计算某一列的平均值（必须是数值型）select avg(s_score) from score

max  求某一列的最大值   select max(s_score) from score

min   求某一列的最小值   select min(s_score) from score

值得一提的是，计算所有老师的平均工资可以这样  select avg(salary) from salary

如果要计算每个院系的老师的平均工资就要分组 select dept_name,avg(salary) from salary group by dept_name

也就是说（假设） ，把100个老师（共五个院系，每个院系20个老师）分为五组，每组20人，对每组的20人计算平均值，以不同院系区分

如果，select dept_name,id,avg(salary) from salary group by dept_name   中间加个id这样是不可以的，因为id没有被分组除非

select dept_name,id,avg(salary) from salary group by dept_name,id      这样才行

另外group by后面可以接having语句，having可以用聚集函数，where不可以

区别在于，having是分组之后才生效的，where是未分组时生效的