CREATE DATABASE a;
USE a;
SHOW CREATE DATABASE a;
SHOW DATABASES;
CREATE TABLE students(
    num INT,
    sname VARCHAR(20),
    sex VARCHAR(20)
)
    
DROP DATABASE a;
INSERT INTO students(sname) VARCHAR('张三');
INSERT INTO students(num,sname,sex)
VALUES(1,'张三','男');
INSERT INTO students VALUES(2,'李四','女');
UPDATE students SET sex='男' WHERE sname = '李四';
DESC students;
ALTER TABLE students ADD age INT;
ALTER TABLE students ADD birthday DATE AFTER age;
ALTER TABLE students ADD sdate TIMESTAMP FIRST;
ALTER TABLE students CHANGE sdate dates TIMESTAMP;
ALTER TABLE students DROP;
DROP TABLE students;
UPDATE students SET num='1001' WHERE sname='张三';
UPDATE students SET num='1002' WHERE sname='李四';
DELETE FROM students WHERE sex='男';
DELETE FROM students;
DROP TABLE students;
TRUNCATE TABLE students;

SHOW DATABASES;
CREATE DATABASE db1;
CREATE DATABASE db1 CHARSET GBK;
ALTER DATABASE db1 CHARSET UTF8;
SHOW CREATE DATABASE db1;
DROP DATABASE a;
USE db1;
CREATE TABLE student(
    num INT,
    sname VARCHAR(20)    
);
DESC student;
ALTER TABLE student RENAME students;
ALTER TABLE students ADD age INT;
ALTER TABLE students ADD sex VARCHAR(10) AFTER ;
ALTER TABLE students ADD tid INT FIRST;
ALTER TABLE students CHANGE sex sex INT;
ALTER TABLE students DROP tid;
DROP TABLE students;


USE db1;
CREATE TABLE student(
    num INT,
    sname VARCHAR(20),
    age INT
);
INSERT INTO student(num,sname)
VALUES(1001,'张三');
INSERT INTO student VALUES(1002,'李四',20);
INSERT INTO student VALUES(1003,'王五',21),(1004,'赵六',20);
UPDATE student SET age=22 WHERE sname='张三';
DELETE FROM student WHERE num=1004;
DELETE FROM student;
TRUNCATE TABLE student;
ALTER TABLE student ADD birthday DATE;
ALTER TABLE student ADD sdate TIMESTAMP FIRST;
DROP TABLE student;

CREATE TABLE student(
    tid INT PRIMARY KEY AUTO_INCREMENT,
    uname VARCHAR(20) UNIQUE,
    sex VARCHAR(20) NOT NULL,
    age INT DEFAULT 0
);
INSERT INTO student VALUES(NULL,'张三','男',20);
DELETE FROM student WHERE tid=2;
INSERT INTO student VALUES(NULL,'李四','男',21);
SELECT * FROM student;
SELECT uname,sex FROM student;
SELECT * FROM student WHERE uname='张三';
SELECT * FROM student WHERE age=20 AND sex='男';
SELECT * FROM student WHERE age BETWEEN 20 AND 21 OR sex='女';
SELECT * FROM student WHERE age >20;
SELECT * FROM student WHERE age<>21;