一、增添
规则1：
-
插入的数据应与字段的数据类型相同。比如，将‘abc'插入到id列就不行：
mysql>
create database if not exists bit27_4 default charset=utf8;
Query OK, 1 row affected (0.00 sec)
mysql>
use bit27_4;
Database changed
mysql>
create table goods (id int primary key, goods_name varchar(64) not null default '', price float not null default 0.0);
Query OK, 0 rows affected (0.04 sec)
mysql>
desc goods
->
;
+------------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| goods_name | varchar(64) | NO | | | |
| price | float | NO | | 0 | |
+------------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
mysql>
insert into goods values ('abc','牛排',32.0);
//插入时类型不对应
Query OK, 1 row affected, 1 warning (0.00 sec)
mysql>
select * from goods;
+----+------------+-------+
| id | goods_name | price |
+----+------------+-------+
|
0 | 牛排 | 32 |
+----+------------+-------+
1 row in set (0.00 sec)
规则2：
-
数据的大小应在规定的范围内，例如：不能将一个长度为80的字符串插入到长度为40的列中。（会截断）
mysql>
alter table goods modify goods_name varchar(6) not null default '';
Query OK, 1 row affected (0.01 sec)
Records: 1 Duplicates: 0 Warnings: 0
mysql>
desc goods;
+------------+------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+------------+------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| goods_name | varchar(6) | NO | | | |
| price | float | NO | | 0 | |
+------------+------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
mysql>
insert into goods values (1,'12345678',32.0);
//插入的数据长度超过限制
Query OK, 1 row affected, 1 warning (0.00 sec)
mysql>
select * from goods;
+----+------------+-------+
| id | goods_name | price |
+----+------------+-------+
| 0 | 牛排 | 32 |
| 1 |
123456 | 32 |
+----+------------+-------+
2 rows in set (0.00 sec)
规则3：
-
在values中列出的数据位置必须与被加入的列位置相对应。（数据类型要匹配）
mysql>
insert into goods values (2,32.0,'牛排');
Query OK, 1 row affected, 1 warning (0.00 sec)
mysql>
select * from goods;
+----+------------+-------+
| id | goods_name | price |
+----+------------+-------+
| 0 | 牛排 | 32 |
| 1 | 123456 | 32 |
| 2 | 32.0 |
0 |
+----+------------+-------+
3 rows in set (0.00 sec)
规则4：
-
字符和日期类型应该包含在单引号中。
mysql>
set sql_mode=strict_trans_tables;
Query OK, 0 rows affected (0.00 sec)
mysql>
insert into goods values (3,西瓜,'4.5');
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '??瓜,'4.5')' at line 1
mysql>
insert into goods values (3,'西瓜','4.5');
Query OK, 1 row affected (0.00 sec)
规则5：
-
插入空值，不指定或insert into table values(null)
-
insert into table values(),(),() 一次性添加多条记录
mysql>
create table t1 (name varchar(32));
Query OK, 0 rows affected (0.01 sec)
mysql>
insert into t1 values (null);
Query OK, 1 row affected (0.00 sec)
mysql>
select * from t1;
+------+
| name |
+------+
| NULL |
+------+
1 row in set (0.00 sec)
mysql>
insert into goods values (1,'aa',3.3) , (2,'bb',4.4) , (3,'cc',5.5);
//一次性添加三条记录
Query OK, 3 rows affected (0.00 sec)
-
规则6：
-
如果只给表的某几个字段赋值，则需要制定字段名
-
如果给表中的所有字段添加数据，可以不写前面的字段名称。
mysql>
insert into goods(id,goods_name) values (4,'手机');
Query OK, 1 row affected (0.00 sec)
mysql>
insert into goods values(4,'phone',2.5);
//如果没有给出字段名称，values中必须给出所有的字段值
Query OK, 1 row affected (0.00 sec)
规则7：
-
在数据插入的时候，假设主键对应的值已经存在：插入失败！
-
当主键存在冲突的时候(duplicate key)，可以选择性的进行处理:更新操作、替换操作（主键如果没有冲突，就直接插入）。
更新
mysql>
alter table goods modify goods_name varchar(64) not null default '';
Query OK, 5 rows affected (0.00 sec)
Records: 5 Duplicates: 0 Warnings: 0
mysql>
desc goods;
+------------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| goods_name | varchar(64) | NO | | | |
| price | float | NO | | 0 | |
+------------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
mysql>
insert into goods(id,goods_name) values (4,'手机2');
ERROR 1062 (23000): Duplicate entry '4' for key 'PRIMARY'
//插入失败
mysql>
insert into goods(id,goods_name) values (4,'手机2') on duplicate key update goods_name='手机2';
Query OK, 2 rows affected (0.00 sec)
//更新操作，将id=4的数据的指定字段（goods_name）改为手机2
mysql>
select * from goods;
+----+------------+-------+
| id | goods_name | price |
+----+------------+-------+
| 0 | 牛排 | 32 |
| 1 | 123456 | 32 |
| 2 | 32.0 | 0 |
| 3 | 西瓜 | 4.5 |
| 4 | 手机2 | 0 |
+----+------------+-------+
5 rows in set (0.00 sec)
mysql>
insert into goods(id,goods_name,price) values (4,'手机2',4.6) on duplicate key update goods_name='手机2', price=4.6;
Query OK, 2 rows affected (0.00 sec)
mysql>
select * from goods;
+----+------------+-------+
| id | goods_name | price |
+----+------------+-------+
| 0 | 牛排 | 32 |
| 1 | 123456 | 32 |
| 2 | 32.0 | 0 |
| 3 | 西瓜 | 4.5 |
| 4 | 手机2 | 4.6 |
+----+------------+-------+
5 rows in set (0.00 sec)
（替换操作：另一种插入数据的方式）
mysql>
replace into goods values (100,'huawei',999);
//主键如果没有冲突，就直接插入
Query OK, 1 row affected (0.00 sec)
mysql>
select * from goods;
+-----+------------+-------+
| id | goods_name | price |
+-----+------------+-------+
| 0 | 牛排 | 32 |
| 1 | 123456 | 32 |
| 2 | 32.0 | 0 |
| 3 | 西瓜 | 4.5 |
| 4 | 手机2 | 4.6 |
| 100 | huawei | 999 |
+-----+------------+-------+
6 rows in set (0.00 sec)
二、修改
1、update
update使用细节：
-
update 语法可以用新值更新原有表中的各列
-
set子句指示要修改哪些列和要给予哪些值
-
where子句指定应更新哪些行。如果没有where子句，则更新所有行
-
如果需要更新多个字段，可以通过 set 字段1=值1，字段2=值2...
mysql>
update goods set price = 300;
//将所有商品（整表）的价格修改为300
Query OK, 6 rows affected (0.00 sec)
Rows matched: 6 Changed: 6 Warnings: 0
mysql>
select * from goods;
+-----+------------+-------+
| id | goods_name | price |
+-----+------------+-------+
| 0 | 牛排 | 300 |
| 1 | 123456 | 300 |
| 2 | 32.0 | 300 |
| 3 | 西瓜 | 300 |
| 4 | 手机2 | 300 |
| 100 | huawei | 300 |
+-----+------------+-------+
6 rows in set (0.00 sec)
mysql>
update goods set price=1000 where id=100;
//id为100的商品价格修改为1000
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql>
select * from goods;
+-----+------------+-------+
| id | goods_name | price |
+-----+------------+-------+
| 0 | 牛排 | 300 |
| 1 | 123456 | 300 |
| 2 | 32.0 | 300 |
| 3 | 西瓜 | 300 |
| 4 | 手机2 | 300 |
| 100 | huawei | 1000 |
+-----+------------+-------+
6 rows in set (0.00 sec)
mysql>
update goods set price=price+200 where id=101;
//将id为101的产品价格增加200块
Query OK, 1 row affected (0.00 sec)
Rows matched:1 Changed:1 Warnings:0
（修改多列）
mysql>
update goods set goods_name='xiaomi',price=998 where id=2;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql>
select * from goods;
+-----+------------+-------+
| id | goods_name | price |
+-----+------------+-------+
| 0 | 牛排 | 301 |
| 1 | 123456 | 301 |
|
2 | xiaomi | 998 |
| 3 | 西瓜 | 301 |
| 4 | 手机2 | 301 |
| 100 | huawei | 1000 |
+-----+------------+-------+
6 rows in set (0.00 sec)
（限制更新数量）
mysql>
update goods set price = 300 limit 2;
//只更新两行（默认前两行）
Query OK, 2 rows affected (0.00 sec)
Rows matched: 2 Changed: 2 Warnings: 0
mysql>
select * from goods;
+-----+------------+-------+
| id | goods_name | price |
+-----+------------+-------+
| 0 | 牛排 |
300 |
| 1 | 123456 |
300 |
| 2 | xiaomi | 998 |
| 3 | 西瓜 | 301 |
| 4 | 手机2 | 301 |
| 100 | huawei | 1000 |
+-----+------------+-------+
6 rows in set (0.00 sec)
三、删除
1、delete
-
如果不使用where子句，将删除整个表中所有数据
-
delete语句不能删除某一列的值（可以用update置null）
-
使用delete语句仅删除记录，不删除表本身（drop table）
mysql>
delete from goods where id=100;
Query OK, 1 row affected (0.00 sec)
mysql>
select * from goods;
+----+------------+-------+
| id | goods_name | price |
+----+------------+-------+
| 0 | 牛排 | 300 |
| 1 | 123456 | 300 |
| 2 | xiaomi | 998 |
| 3 | 西瓜 | 301 |
| 4 | 手机2 | 301 |
+----+------------+-------+
5 rows in set (0.00 sec)
（在练习删除时，可以复制一份表，避免数据删没了）
mysql>
create table goods2 like goods; //复制表结构
Query OK, 0 rows affected (0.01 sec)
mysql>
desc goods2;
+------------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| goods_name | varchar(64) | NO | | | |
| price | float | NO | | 0 | |
+------------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
mysql>
insert into goods2 select * from goods;
//因为两个表的表结构（字段名、数据类型、表的约束）完全相同，所以可以直接移植过去。
Query OK, 5 rows affected (0.00 sec)
Records: 5 Duplicates: 0 Warnings: 0
mysql>
select * from goods2;
+----+------------+-------+
| id | goods_name | price |
+----+------------+-------+
| 0 | 牛排 | 300 |
| 1 | 123456 | 300 |
| 2 | xiaomi | 998 |
| 3 | 西瓜 | 301 |
| 4 | 手机2 | 301 |
+----+------------+-------+
5 rows in set (0.00 sec)
2、truncate
mysql>
delete from goods;
//删除表中所有数据，给出几条数据受影响
Query OK, 5 rows affected (0.00 sec)
mysql>
select * from goods;
Empty set (0.00 sec)
mysql>
truncate goods2;
//快速（所以清空时用这个） 不返回多少行数据受影响 都不会清除表结构
Query OK, 0 rows affected (0.00 sec)
mysql>
select * from goods2;
Empty set (0.00 sec)
3、delete和truncate的区别：
-
效果一样，truncate速度快；
-
delete可以带where条件，删除更加灵活；
-
delete可以返回被删除的记录数，而truncate返回0
-
推荐使用delete进行删除，推荐使用truncate进行清空。
四、查询
mysql>
source /home/du/student_data.sql
//把student_data.sql文件粘贴到linux系统的/home/du下，然后在虚拟机上运行该语句，可导入文件内容
Query OK, 0 rows affected, 1 warning (0.00 sec)
Query OK, 1 row affected (0.00 sec)
Database changed
Query OK, 0 rows affected, 1 warning (0.00 sec)
Query OK, 0 rows affected (0.02 sec)
Query OK, 1 row affected (0.00 sec)
Query OK, 1 row affected (0.00 sec)
Query OK, 1 row affected (0.00 sec)
Query OK, 1 row affected (0.00 sec)
Query OK, 1 row affected (0.00 sec)
Query OK, 1 row affected (0.00 sec)
Query OK, 1 row affected (0.00 sec)
+----+--------------+---------+---------+------+
| id | name | chinese | english | math |
+----+--------------+---------+---------+------+
| 1 | 李涛 | 89.0 | 78.0 | 90.0 |
| 2 | 唐僧 | 67.0 | 98.0 | 56.0 |
| 3 | 孙悟空 | 87.0 | 78.0 | 77.0 |
| 4 | 老妖婆 | 88.0 | 98.0 | 90.0 |
| 5 | 红孩儿 | 82.0 | 84.0 | 67.0 |
| 6 | 如来佛祖 | 55.0 | 85.0 | 45.0 |
| 7 | 菩萨 | 75.0 | 65.0 | 30.0 |
+----+--------------+---------+---------+------+
7 rows in set (0.00 sec)
（指定列查询）
mysql>
select id,name,math from student;
+----+--------------+------+
| id | name | math |
+----+--------------+------+
| 1 | 李涛 | 90.0 |
| 2 | 唐僧 | 56.0 |
| 3 | 孙悟空 | 77.0 |
| 4 | 老妖婆 | 90.0 |
| 5 | 红孩儿 | 67.0 |
| 6 | 如来佛祖 | 45.0 |
| 7 | 菩萨 | 30.0 |
+----+--------------+------+
7 rows in set (0.00 sec)
（去除重复行查询）
mysql>
select math from student;
+------+
| math |
+------+
| 90.0 |
| 56.0 |
| 77.0 |
| 90.0 |
| 67.0 |
| 45.0 |
| 30.0 |
+------+
7 rows in set (0.00 sec)
mysql>
select distinct math from student;
+------+
| math |
+------+
| 90.0 |
| 56.0 |
| 77.0 |
| 67.0 |
| 45.0 |
| 30.0 |
+------+
6 rows in set (0.00 sec)
（列运算[类型为可运算类型]）
mysql>
select id,name,chinese+english+math from student;
+----+--------------+----------------------+
| id | name | chinese+english+math |
+----+--------------+----------------------+
| 1 | 李涛 | 257.0 |
| 2 | 唐僧 | 221.0 |
| 3 | 孙悟空 | 242.0 |
| 4 | 老妖婆 | 276.0 |
| 5 | 红孩儿 | 233.0 |
| 6 | 如来佛祖 | 185.0 |
| 7 | 菩萨 | 170.0 |
+----+--------------+----------------------+
7 rows in set (0.00 sec)
（设置列别名）
mysql>
select id,name,chinese+english+math as total_score from student;
//as可省略，中间用空格分开即可
+----+--------------+-------------+
| id | name | total_score |
+----+--------------+-------------+
| 1 | 李涛 | 257.0 |
| 2 | 唐僧 | 221.0 |
| 3 | 孙悟空 | 242.0 |
| 4 | 老妖婆 | 276.0 |
| 5 | 红孩儿 | 233.0 |
| 6 | 如来佛祖 | 185.0 |
| 7 | 菩萨 | 170.0 |
+----+--------------+-------------+
7 rows in set (0.00 sec)
mysql>
select id '编号' ,name '姓名', chinese+english+math '总分' from student;
//省略了as
+--------+--------------+--------+
| 编号 | 姓名 | 总分 |
+--------+--------------+--------+
| 1 | 李涛 | 257.0 |
| 2 | 唐僧 | 221.0 |
| 3 | 孙悟空 | 242.0 |
| 4 | 老妖婆 | 276.0 |
| 5 | 红孩儿 | 233.0 |
| 6 | 如来佛祖 | 185.0 |
| 7 | 菩萨 | 170.0 |
+--------+--------------+--------+
7 rows in set (0.00 sec)
mysql>
select id '编号' ,name '姓名', chinese+english+math+10 '总分' from student;
+--------+--------------+--------+
| 编号 | 姓名 | 总分 |
+--------+--------------+--------+
| 1 | 李涛 | 267.0 |
| 2 | 唐僧 | 231.0 |
| 3 | 孙悟空 | 252.0 |
| 4 | 老妖婆 | 286.0 |
| 5 | 红孩儿 | 243.0 |
| 6 | 如来佛祖 | 195.0 |
| 7 | 菩萨 | 180.0 |
+--------+--------------+--------+
7 rows in set (0.00 sec)
mysql>
select id '编号' ,name '姓名', (chinese+english+math)*1.6 '总分' from student;
+--------+--------------+--------+
| 编号 | 姓名 | 总分 |
+--------+--------------+--------+
| 1 | 李涛 | 411.2 |
| 2 | 唐僧 | 353.6 |
| 3 | 孙悟空 | 387.2 |
| 4 | 老妖婆 | 441.6 |
| 5 | 红孩儿 | 372.8 |
| 6 | 如来佛祖 | 296.0 |
| 7 | 菩萨 | 272.0 |
+--------+--------------+--------+
7 rows in set (0.00 sec)
mysql>
select id '编号' ,name '姓名', (chinese+english+math)*1.6 '总分' from student where name like '唐%';
//%表任意
+--------+--------+--------+
| 编号 | 姓名 | 总分 |
+--------+--------+--------+
| 2 | 唐僧 | 353.6 |
+--------+--------+--------+
1 row in set (0.00 sec)
（查询总分大于200分的同学的信息）
mysql>
select id,name,(chinese+english+math) as total from student where total>200;
ERROR 1054 (42S22): Unknown column 'total' in 'where clause'
mysql>
select id,name,(chinese+english+math) as total from student where (chinese+english+math)>200;
//where后不能用别名
+----+-----------+-------+
| id | name | total |
+----+-----------+-------+
| 1 | 李涛 | 257.0 |
| 2 | 唐僧 | 221.0 |
| 3 | 孙悟空 | 242.0 |
| 4 | 老妖婆 | 276.0 |
| 5 | 红孩儿 | 233.0 |
+----+-----------+-------+
5 rows in set (0.00 sec)
（一些练习）
mysql>
select * from student;
+----+--------------+---------+---------+------+
| id | name | chinese | english | math |
+----+--------------+---------+---------+------+
| 1 | 李涛 | 89.0 | 78.0 | 90.0 |
| 2 | 唐僧 | 67.0 | 98.0 | 56.0 |
| 3 | 孙悟空 | 87.0 | 78.0 | 77.0 |
| 4 | 老妖婆 | 88.0 | 98.0 | 90.0 |
| 5 | 红孩儿 | 82.0 | 84.0 | 67.0 |
| 6 | 如来佛祖 | 55.0 | 85.0 | 45.0 |
| 7 | 菩萨 | 75.0 | 65.0 | 30.0 |
+----+--------------+---------+---------+------+
7 rows in set (0.00 sec)
mysql>
select * from student where id>10 and name like '李%';
Empty set (0.00 sec)
mysql>
select * from student where id>10 or name like '李%';
+----+--------+---------+---------+------+
| id | name | chinese | english | math |
+----+--------+---------+---------+------+
| 1 | 李涛 | 89.0 | 78.0 | 90.0 |
+----+--------+---------+---------+------+
1 row in set (0.00 sec)
mysql>
select id,name,(chinese+english+math) as total from student where (chinese+english+math)>200 and math<chinese and name like '唐%';
+----+--------+-------+
| id | name | total |
+----+--------+-------+
| 2 | 唐僧 | 221.0 |
+----+--------+-------+
1 row in set (0.00 sec)
mysql>
select id,name,english from student where english>=80 and english<=90;
+----+--------------+---------+
| id | name | english |
+----+--------------+---------+
| 5 | 红孩儿 | 84.0 |
| 6 | 如来佛祖 | 85.0 |
+----+--------------+---------+
2 rows in set (0.00 sec)
mysql>
select id,name,english from student where english between 80 and 90;
//between and两边都是闭区间
+----+--------------+---------+
| id | name | english |
+----+--------------+---------+
| 5 | 红孩儿 | 84.0 |
| 6 | 如来佛祖 | 85.0 |
+----+--------------+---------+
2 rows in set (0.00 sec)
mysql>
select id,name,math from student where math=89 or math=90 or math=91;
+----+-----------+------+
| id | name | math |
+----+-----------+------+
| 1 | 李涛 | 90.0 |
| 4 | 老妖婆 | 90.0 |
+----+-----------+------+
2 rows in set (0.00 sec)
mysql>
select id,name,math from student where math in(89,90,91);
//与上面的语句效果一样，更方便
+----+-----------+------+
| id | name | math |
+----+-----------+------+
| 1 | 李涛 | 90.0 |
| 4 | 老妖婆 | 90.0 |
+----+-----------+------+
2 rows in set (0.00 sec)
（排序[默认升序] order by 语句一般放在select的尾部）
mysql>
select id,name,math from student order by math asc;
//asc即升序，默认也是升序
+----+--------------+------+
| id | name | math |
+----+--------------+------+
| 7 | 菩萨 | 30.0 |
| 6 | 如来佛祖 | 45.0 |
| 2 | 唐僧 | 56.0 |
| 5 | 红孩儿 | 67.0 |
| 3 | 孙悟空 | 77.0 |
| 1 | 李涛 | 90.0 |
| 4 | 老妖婆 | 90.0 |
+----+--------------+------+
7 rows in set (0.00 sec)
mysql>
select id,name,(chinese+english+math) as total from student order by (chinese+english+math) desc;
//desc降序
+----+--------------+-------+
| id | name | total |
+----+--------------+-------+
| 4 | 老妖婆 | 276.0 |
| 1 | 李涛 | 257.0 |
| 3 | 孙悟空 | 242.0 |
| 5 | 红孩儿 | 233.0 |
| 2 | 唐僧 | 221.0 |
| 6 | 如来佛祖 | 185.0 |
| 7 | 菩萨 | 170.0 |
+----+--------------+-------+
7 rows in set (0.00 sec)
mysql>
select id,name,(chinese+english+math) as total from student order by total desc;
//查询的时候可以用total，where后面不可以
+----+--------------+-------+
| id | name | total |
+----+--------------+-------+
| 4 | 老妖婆 | 276.0 |
| 1 | 李涛 | 257.0 |
| 3 | 孙悟空 | 242.0 |
| 5 | 红孩儿 | 233.0 |
| 2 | 唐僧 | 221.0 |
| 6 | 如来佛祖 | 185.0 |
| 7 | 菩萨 | 170.0 |
+----+--------------+-------+
7 rows in set (0.00 sec)
mysql>
select id,name,(chinese+english+math) as total from student order by total,name desc;
//total采用了默认的升序，先排总分，总分相同时再排名字？？？？
+----+--------------+-------+
| id | name | total |
+----+--------------+-------+
| 7 | 菩萨 | 170.0 |
| 6 | 如来佛祖 | 185.0 |
| 2 | 唐僧 | 221.0 |
| 5 | 红孩儿 | 233.0 |
| 3 | 孙悟空 | 242.0 |
| 1 | 李涛 | 257.0 |
| 4 | 老妖婆 | 276.0 |
+----+--------------+-------+
7 rows in set (0.00 sec)
（返回某一列的行总数）
mysql>
select count(*) from student;
//返回整表的行数
+----------+
| count(*) |
+----------+
| 8 |
+----------+
1 row in set (0.00 sec)
mysql>
select count(id) from student;
+-----------+
| count(id) |
+-----------+
| 8 |
+-----------+
1 row in set (0.00 sec)
mysql>
select count(name) from student;
+-------------+
| count(name) |
+-------------+
| 8 |
+-------------+
1 row in set (0.00 sec)
mysql>
select * from student;
+----+--------------+---------+---------+------+
| id | name | chinese | english | math |
+----+--------------+---------+---------+------+
| 1 | 李涛 | 89.0 | 78.0 | 90.0 |
| 2 | 唐僧 | 67.0 | 98.0 | 56.0 |
| 3 | 孙悟空 | 87.0 | 78.0 | 77.0 |
| 4 | 老妖婆 | 88.0 | 98.0 | 90.0 |
| 5 | 红孩儿 | 82.0 | 84.0 | 67.0 |
| 6 | 如来佛祖 | 55.0 | 85.0 | 45.0 |
| 7 | 菩萨 | 75.0 | 65.0 | 30.0 |
| 8 | 李雅 | 79.0 | 31.0 | 56.0 |
+----+--------------+---------+---------+------+
8 rows in set (0.00 sec)
mysql>
select count(math) from student where math>90;
+-------------+
| count(math) |
+-------------+
| 0 |
+-------------+
1 row in set (0.00 sec)
注意：【count(列名)会排除为null的情况】
（求和）
mysql>
select sum(math) from student;
+-----------+
| sum(math) |
+-----------+
| 511.0 |
+-----------+
1 row in set (0.00 sec)
mysql>
select sum(math) as m_total , sum(chinese) as c_total , sum(english) as e_total from student;
+---------+---------+---------+
| m_total | c_total | e_total |
+---------+---------+---------+
| 511.0 | 622.0 | 617.0 |
+---------+---------+---------+
1 row in set (0.00 sec)
（求平均值）
mysql>
select sum(chinese) / count(chinese) from student;
+-------------------------------+
| sum(chinese) / count(chinese) |
+-------------------------------+
| 77.75000 |
+-------------------------------+
1 row in set (0.00 sec)
mysql>
select avg(chinese) from student;
+--------------+
| avg(chinese) |
+--------------+
| 77.75000 |
+--------------+
1 row in set (0.00 sec)
（求最大/最小值）
mysql>
select id,name,max(math) from student;
//错误，不可带其他列，max和min都只能算自己这一列的。这里之后再求min时出错
+----+--------+-----------+
| id | name | max(math) |
+----+--------+-----------+
| 1 | 李涛 | 90.0 |
+----+--------+-----------+
1 row in set (0.00 sec)
mysql>
select max(math) from student;
+-----------+
| max(math) |
+-----------+
| 90.0 |
+-----------+
1 row in set (0.00 sec)
mysql>
select min(math) from student;
+-----------+
| min(math) |
+-----------+
| 30.0 |
+-----------+
1 row in set (0.00 sec)
分组语句
先利用 mysql> source /home/du/scott_data.sql 导入表
mysql>
desc dept;
+--------+--------------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------+--------------------------+------+-----+---------+-------+
| deptno | int(2) unsigned zerofill | NO | | NULL | |
| dname | varchar(14) | YES | | NULL | |
| loc | varchar(13) | YES | | NULL | |
+--------+--------------------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
mysql>
desc emp;
+----------+--------------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+--------------------------+------+-----+---------+-------+
| empno | int(6) unsigned zerofill | NO | | NULL | |
| ename | varchar(10) | YES | | NULL | |
| job | varchar(9) | YES | | NULL | |
| mgr | int(4) unsigned zerofill | YES | | NULL | |
| hiredate | datetime | YES | | NULL | |
| sal | decimal(7,2) | YES | | NULL | |
| comm | decimal(7,2) | YES | | NULL | |
| deptno | int(2) unsigned zerofill | YES | | NULL | |
+----------+--------------------------+------+-----+---------+-------+
8 rows in set (0.00 sec)
mysql>
desc salgrade;
+-------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| grade | int(11) | YES | | NULL | |
| losal | int(11) | YES | | NULL | |
| hisal | int(11) | YES | | NULL | |
+-------+---------+------+-----+---------+-------+
3 rows in set (0.00 sec)
mysql>
select deptno,avg(sal),max(sal) from emp group by deptno;
//以部门编号进行分组查询，即查询不同部门的部门编号、平均工资、最高工资
+--------+-------------+----------+
| deptno | avg(sal) | max(sal) |
+--------+-------------+----------+
| 10 | 2916.666667 | 5000.00 |
| 20 | 2175.000000 | 3000.00 |
| 30 | 1566.666667 | 2850.00 |
+--------+-------------+----------+
3 rows in set (0.00 sec)
mysql>
select deptno,job,avg(sal),max(sal) from emp group by deptno,job; //每个部门不同职位的平均工资、最大工资
+--------+-----------+-------------+----------+
| deptno | job | avg(sal) | max(sal) |
+--------+-----------+-------------+----------+
| 10 | CLERK | 1300.000000 | 1300.00 |
| 10 | MANAGER | 2450.000000 | 2450.00 |
| 10 | PRESIDENT | 5000.000000 | 5000.00 |
| 20 | ANALYST | 3000.000000 | 3000.00 |
| 20 | CLERK | 950.000000 | 1100.00 |
| 20 | MANAGER | 2975.000000 | 2975.00 |
| 30 | CLERK | 950.000000 | 950.00 |
| 30 | MANAGER | 2850.000000 | 2850.00 |
| 30 | SALESMAN | 1400.000000 | 1600.00 |
+--------+-----------+-------------+----------+
mysql>
select deptno,avg(sal) from emp group by deptno;
+--------+-------------+
| deptno | avg(sal) |
+--------+-------------+
| 10 | 2916.666667 |
| 20 | 2175.000000 |
| 30 | 1566.666667 |
+--------+-------------+
3 rows in set (0.00 sec)
mysql>
select deptno,avg(sal) from emp group by deptno having avg(sal)<2000;
//这里不用where，用having
+--------+-------------+
| deptno | avg(sal) |
+--------+-------------+
| 30 | 1566.666667 |
+--------+-------------+
1 row in set (0.00 sec)