时间：2021-07-01 10:21:17 帮助过：27人阅读
1. 修改表名 alter table 表名 rename 新表名; 2. 增加字段 alter table 表名 add 字段名 数据类型 [完整性约束条件…]; alter table t1 add stu char(10) not null after name; #添加到name字段之后 alter table t1 add sex enum(‘male‘,‘female‘) default ‘male‘ first;#添加到最前面 3. 删除字段 alter table t1 drop sex; 4. 修改字段(增加主键) alter table t1 modify age int(3); alter table t1 modify id int(11) primary key auto_increment; #修改为主键 ? alter table t1 change 旧字段名 新字段名 新数据类型 [完整性约束条件…]; 5. 对已经存在的表增加复合主键 alter table t1 add primary key(ip,port); 6. 删除主键 a. 删除自增约束 alter table t1 modify id int(11) not null; b. 删除主键 alter table t1 drop primary key; 7. 修改存储引擎 alter table it engine=innodb; 8. 增加主键(设置索引) alter table t1 add primary key(id);
create table new_t1 select * from t1; # 复制表结构＋记录,但是key和自增不会复制 alter table new_t1 modify id int(11) primary key auto_increment; #添加主键和自增 #条件为假，查不到任何记录 create table new1_t1 select * from t1 where 1=2; #只复制表结构,但是key和自增不会复制 alter table new_t1 modify id int(11) primary key auto_increment; #添加主键和自增 create table t2 like t1; #只完全复制表结构，不复制记录
drop table t1;
语法
select distinct 查询字段1，查询字段2......... from 表名 where 分组之前的过滤条件 group by 分组条件 having 分组之后的过滤条件 order by 排序字段1 asc,排序字段2 desc limit 5,5;
select id,name from db39.emp where id >= 3 and id <= 6 select * from db39.emp where id between 3 and 6; ? select * from emp where salary = 20000 or salary = 18000 or salary = 17000; select * from emp where salary in (20000,18000,17000); select * from emp where salary not in (20000,18000,17000); select * from emp where id not between 3 and 6; ? 要求：查询员工姓名中包含i字母的员工姓名与其薪资 select name,salary from db39.emp where name like ‘%i%‘ ? 要求：查询员工姓名是由四个字符组成的的员工姓名与其薪资 select name,salary from db39.emp where name like ‘____‘; select name,salary from db39.emp where char_length(name) = 4; ? 要求：查询岗位描述为空的员工名与岗位名 select name,post from db39.emp where post_comment is NULL; select name,post from db39.emp where post_comment is not NULL;
#设置sql_mode为only_full_group_by，意味着以后但凡分组，只能取到分组的依据 mysql> set global sql_mode="strict_trans_tables,only_full_group_by"; ? #聚合函数：每个部门的最高、最低、平均、总工资，计数 select post,max(salary) from emp group by post; select post,min(salary) from emp group by post; select post,avg(salary) from emp group by post; select post,sum(salary) from emp group by post; select post,count(id) from emp group by post;
group_concat (不能做中间结果)、concat 、concat_ws 、as
#group_concat（分组之后使用）：取出分组后，组内定制的详细信息 select post,group_concat(name) from emp group by post; select post,group_concat(name,"_SB") from emp group by post; select post,group_concat(name,": ",salary) from emp group by post; select post,group_concat(name,":",age,":",sex) from emp group by post; +-----------------------------------------+-------------------------------------------------------- | post | group_concat(name,"_SB") | +-----------------------------------------+-------------------------------------------------------- | operation | 程咬铁_SB,程咬铜_SB,程咬银_SB,程咬金_SB,张野_SB | | sale | 格格_SB,星星_SB,丁丁_SB,丫丫_SB,歪歪_SB | |外交大使 | egon_SB | +-----------------------------------------+-------------------------------------------------------- # concat（不分组时用）：自定制取出的结果 select name as 姓名,salary as 薪资 from emp; select concat("NAME: ",name) as 姓名,concat("SAL: ",salary) as 薪资 from emp; +------------------+-----------------+ | 姓名 | 薪资 | +------------------+-----------------+ | NAME: egon | SAL: 7300.33 | | NAME: alex | SAL: 1000000.31 | | NAME: wupeiqi | SAL: 8300.00 | +------------------+-----------------+ # concat_ws (不分组时用)：每个分组结果都用相同的分隔符时使用 select concat_ws(":",name,age,sex,post) as info from emp; +------------------------------------------------------+ | info | +------------------------------------------------------+ | egon:18:male:外交大使 | | 程咬金:18:male:operation | | 程咬银:18:female:operation | | 程咬铜:18:male:operation | | 程咬铁:18:female:operation | +------------------------------------------------------+ # 补充as语法 mysql> select emp.id,emp.name from emp as t1; # 报错 mysql> select t1.id,t1.name from emp as t1; # 查询四则运算 select name,salary*12 as annual_salary from emp;
having的语法格式与where一模一样，只不过having是在分组之后进行的进一步过滤，即where不能用聚合函数，而having是可以用聚合函数的
#统计各部门年龄在30岁以上的员工平均工资，并且保留平均工资大于10000的部门 select post,avg(salary) from emp where age >= 30 group by post having avg(salary) > 10000; +---------+---------------+ | post | avg(salary) | +---------+---------------+ | teacher | 255450.077500 | +---------+---------------+ #强调：having必须在group by后面使用 select * from emp having avg(salary) > 10000;#报错
select distinct post from emp; +-----------------------------------------+ | post | +-----------------------------------------+ | 外交大使 | | teacher | | sale | | operation | +-----------------------------------------+
select * from emp order by salary (asc); #默认升序排 select * from emp order by salary desc; #降序排 select * from emp order by age desc,salary asc; #先按照age降序排，如果相同再按照薪资升序排 ? # 统计各部门年龄在10岁以上的员工平均工资，并且保留平均工资大于1000的部门，然后对平均工资进行排序 select post,avg(salary) from emp where age > 10 group by post having avg(salary) > 1000 order by avg(salary); +-----------------------------------------+---------------+ | post | avg(salary) | +-----------------------------------------+---------------+ | sale | 2600.294000 | |外交大使 | 7300.330000 | | operation | 16800.026000 | | teacher | 151842.901429 | +-----------------------------------------+---------------+
select * from emp limit 3; #从头开始只显示3条信息 select * from emp order by salary desc limit 1; #找到工资最大的一条信息 +----+------+------+-----+------------+---------+--------------+------------+--------+-----------+ | id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id | +----+------+------+-----+------------+---------+--------------+------------+--------+-----------+ | 2 | alex | male | 78 | 2015-03-02 | teacher | NULL | 1000000.31 | 401 | 1 | +----+------+------+-----+------------+---------+--------------+------------+--------+-----------+ # 分页显示 select * from emp limit 0,5;#从0开始显示5条信息(1-5) select * from emp limit 5,5;#从5开始显示5条信息(6-10)
select * from emp where name regexp ‘^jin.*(n|g)$‘; +----+------------+--------+-----+------------+---------+--------------+----------+--------+ | id | name | sex | age | hire_date | post |post_comment| salary | office | depart_id | +----+------------+--------+-----+------------+---------+--------------+----------+--------+ | 6 | jingliyang|female|18| 2011-02-11 | teacher | NULL | 9000.00 | 401 | 1 | | 7 | jinxin |male |18| 1900-03-01 | teacher | NULL | 30000.00 | 401 | 1 | +----+------------+--------+-----+------------+---------+--------------+----------+--------+
python 之 数据库（修改表、复制表、删除表、单表查询）
标签：依据 word class 增加 operation default 完全 sele 数据类型