Summary of Mysql Table Column and Library Addition Deletion Modification and Query Problems

The following is my summary of some basic sql knowledge, mainly for better reference and help other beginners in the future, while recording their own growth, also wrote a little bit

Difficult sql interview questions level questions, good nonsense not to say, see the real questions. . .

#创建数据库
CREATE DATABASE mytest CHARACTER SET gbk
#删除数据库
DROP DATABASE mytest
表的操作
#创建表（create table 表名（columns））
CREATE TABLE students(
id INT PRIMARY KEY,
NAME CHAR(10) NOT NULL,
sex CHAR(4) NOT NULL
);
#删除表(drop table 表名)
DROP TABLE students
#在表格中插入数据（insert into 表名（属性） values(对应的值)）
INSERT INTO students(id,NAME,sex) VALUES(1,"张3","男")
#更新表格数据（update 表名 set 键=值,键=值 where 条件）中间要用“，”隔开，其他的无效  set只需写1个
UPDATE students SET id=2 , NAME="a" WHERE id=1
#删除表中数据（delete from 表名 where 条件）
DELETE FROM students WHERE id=0
#查看表中数据（select 查询的东西 from 表名 where 条件）
SELECT * FROM students WHERE id=1
SELECT * FROM students ORDER BY age DESC(order by升序，order by 列名 desc降序)
列
#添加列（alter table 表名 add 列名 字段类型）
ALTER TABLE students ADD tel CHAR(20)
ALTER TABLE students ADD address CHAR(50) AFTER sex
#删除列（alter table 表名 drop 列名）
ALTER TABLE students DROP address
#修改列属性（alter table 表名 change 需要修改的列名 修改后的列名 新列名字段类型）
ALTER TABLE stu CHANGE telphone tel CHAR(20) DEFAULT "-"
ALTER TABLE students CHANGE tel
ALTER TABLE students RENAME stu
简单函数
SELECT SUM(age) AS "总年龄" FROM students
SELECT AVG(age) AS "平均年龄" FROM students
SELECT MAX(age) AS "最大年龄" FROM students
SELECT COUNT(id) AS "人数" FROM students(统计人数选择主键不然可为空的列会影响结果)
分组
表内容：
2005-05-09 胜
2005-05-09 胜
2005-05-09 负
2005-05-09 负
2005-05-10 胜
2005-05-10 负
2005-05-10 负    　　　　如果要生成下列结果, 该如何写sql语句?
      胜 负
2005-05-09 2 2
2005-05-10 1 2
SELECT DATA AS " ",SUM(result='胜') AS "胜" ,SUM(result='负') AS "负" FROM test1 GROUP BY DATA Case when(case 属性=“” then “” end)
　　　　　　写出由table1.table2得到table3的sql语句 
SELECT t1.部门dep,SUM(CASE WHEN 月份mon='1月份' THEN 业绩yj ELSE NULL END) AS '1月份',
SUM(CASE WHEN 月份mon='2月份' THEN 业绩yj ELSE NULL END) AS '2月份',
SUM(CASE WHEN 月份mon='3月份' THEN 业绩yj ELSE NULL END) AS '3月份' FROM table1 t1 LEFT JOIN table2 t2 ON t1.部门dep=t2.部门dep GROUP BY 部门dep
左连接与右连接(left join 表名 on 连接语句)
Left join与right join区别就是：左连接是以主表为主，显示所有内容，若连接的表没有与它对应的值则不显示或显示为null，右连接同理。
内连接
用1条SQL语句查询出每门课都大于80分的学生姓名(表名为score)
　　Select distinct(去重) name from score where name not in(
　　　　Select name from score where fenshu<=80;
　　);

Union all、union（下分别为t1.t1）
SELECT * FROM t1 UNION ALL SELECT * FROM t2 (不去除重复)===》t3
SELECT * FROM t1 UNION SELECT * FROM t2  （去除重复）=====>t4
（上分别为t3.t4）