Table of contents

basic data type

Integer: optional unsigned modification

intyint 8 bits (-128 - 127)
smallint 16 bits (-32768 - 32767)
mediumint 24 bits (-8388608 - 8388607)
int 32 bits about plus or minus 2.1 billion
bigint 64 bits

Real numbers (with decimal point): approximate calculations using standard floating-point arithmetic

float 4 bytes
double 8 bytes
decimal allows up to 65 digits
Example: decimal(5,2), description: 5 digits length, 2 digits of decimal precision, if the integer part + 2 digits of decimals is too long, an error will be reported, if Only if the decimal part exceeds 2, it will be rounded to two decimal places

string

char : fixed length: msql allocates enough space at one time according to the length of the defined string
Applicable scenarios: shorter strings, and all values are close to the same length

varchar variable length string

Saves space compared to fixed-length types

But ROW_FOMAT=FIXED use fixed length for each row

Applicable scenarios: the maximum length of the string is much larger than the evaluation length, and the column is updated less

Disadvantages: page splitting may occur when frequent modifications and large changes in the length of the string

Do not blindly give excessive length

May encounter maximum length allocation memory issues in temporary tables or sorting

Text、Blob

1. Both are designed to store large data
2. Different from other data, they are stored as independent objects
3. When the value is too large, use the external storage area to store, only use 1-4 bytes for each row to store a pointer

text stores character data : tinytext
smalltext
text
mediumtext
       longtext

Blobs store binary data : tinyblob
                                smallblob
                                    blob
                                    mediumblob
                                    longblob

datetime

datetime
   precision: seconds
   independent of time zone, 8 bytes storage
   range: 1001 to 9999 years

timestamp
   saves the number of seconds since midnight on January 1, 1970,
   occupies 4 bytes of storage space
   . Range: 1970 to 2038
   is related to time zone.
   Default is NOT NULL .
   Usually try to use timestamp
   precision: seconds

date
   yyyy-MM-dd

time
   HH:mm:ss

select identifier

used to associate

as foreign key in other table

Integers are usually the best choice for identity columns

Use the same data type in related tables

Try to avoid strings as identity columns, especially randomly generated strings (such as: uuid) cause both insert and select to be slow

   Inserted values are randomly written to different positions of the index. Inserts are slow and easily lead to page splits and random disk reads.

   Logically adjacent rows are distributed in different places on disk and in memory, and select is slow

   Invalidate mysql query cache

   If you need to store uuid, you should remove "-"

(Insert values are randomly written to different positions in the index, insert is slow, and it is easy to cause page splits, disk random reads
    logically adjacent rows are distributed in different places on disk and memory, slow select
makes mysql query cache invalid .
If you need to store uuid, "-" should be removed)

database command

Create the database:
create database 数据库名

create database if not exists 数据库名 default charset utf8 collate utf8_general_ci;
//默认的数据库编码集：utf8
//collate表示校验规则为utf8_general_ci
	//常用排序类型
		//utf8_general_cs（区分大小写）
		//utf8_genera_ci（不区分大小写）
View all databases:
show databases
Delete the database:
drop database 数据库名
Note: Deleting the database is a dangerous operation, if you want to delete it, it is recommended to back it up first

Create tables and constraints

build table

Command format: create table table name (
               column name 1 data type not null,
                 column name 2 data type,
               column name 3 data type,
               unique(column name 1[, column name 2,..., column name N])
                )

Example:

create table t_student
(
   sid int not null comment '学号',
   sname varchar(60) not null comment '姓名',
   sex tinyint not null default 1 comment '性别：1男， 2女',
   age tinyint not null comment ' 年龄',
   icard varchar(18) not null comment '身份证，唯一约束',
   primary key (sid),
   unique key AK_Key_2 (icard)
) comment '学生信息表';

constraint

Primary key constraint: primary key
add primary key (alter table table name add primary key (primary key name))
delete primary key (alert table table name drop primary key)
non-empty constraint:
 sid int not null comment'学号',
Foreign key constraints:
create table t_score
(
   id int not null comment'记录流水号',
   sid int not null comment'学号',
   cid int not null comment'课程ID',
   score float comment'成绩',
   primary key(id),
   foreign key(sid) references t_student (sid) on delete restrict on update redtrict ,
   unique key ak_key_2(sid, cid)
);
//说明： sid为本表的外键，关联t_student表中的的sid主键，on delete restrict on update redtrict说明在本表有数据的情况下，主表的关联键不能删除或更新。
Add primary key (alert table table name add foreign key (foreign key name) references main table name (primary key name))

Delete the primary key (alert table table name drop foreign key constraint name)

Unique constraint: unique key constraint name (field)

Create a unique constraint: alert table table name add unique (column name 1[, column name 2, ..])

create unique index UserNameIndex on 't_user' ('username')

Delete unique constraint: alert table table name drop index unique constraint epitome name

Default value constraint: default

Basic Data Manipulation (CRUD)

data preparation

create database db_t281
use db_t281

-- 1.学生表-t_student
-- sid 学生编号,sname 学生姓名,sage 学生年龄,ssex 学生性别
create table t_student 
(
	sid int not null  auto_increment comment '学号',
	sname varchar(40) not null comment '名称',
	birthday date not null comment '年龄',
	ssex tinyint not null default 1 comment '1男，2女',
	primary key (sid)
);

INSERT INTO t_student VALUES(1, '赵雷' , '1990-01-01' , 1);
INSERT INTO t_student VALUES(2 , '钱电' , '1990-12-21' , 1);
INSERT INTO t_student VALUES(3 , '孙风' , '1990-12-20' , 1);
INSERT INTO t_student VALUES(4 , '李云' , '1990-12-06' , 1);
INSERT INTO t_student VALUES(5 , '周梅' , '1991-12-01' , 2);
INSERT INTO t_student VALUES(6 , '吴兰' , '1992-01-01' , 2);
INSERT INTO t_student VALUES(7 , '郑竹' , '1989-01-01' , 2);
INSERT INTO t_student VALUES(9 , '张三' , '2017-12-20' , 2);
INSERT INTO t_student VALUES(10 , '李四' , '2017-12-25' , 2);
INSERT INTO t_student VALUES(11 , '李四' , '2012-06-06' , 2);
INSERT INTO t_student VALUES(12 , '赵六' , '2013-06-13' , 2);
INSERT INTO t_student VALUES(13 , '孙七' , '2014-06-01' , 2);


-- 2.教师表-t_teacher
-- tid 教师编号,tname 教师名称
CREATE TABLE t_teacher 
(
	tid INT NOT NULL AUTO_INCREMENT COMMENT '教师ID',
	tname VARCHAR(40) NOT NULL COMMENT '教师名称',
	PRIMARY KEY (tid)
);
INSERT INTO t_teacher VALUES(1 , '张五哥');
INSERT INTO t_teacher VALUES(2 , '李卫');
INSERT INTO t_teacher VALUES(3 , '年羹尧');

-- 3.课程表-t_course
-- cid 课程编号,cname 课程名称,tid 教师名称
CREATE TABLE t_course 
(
	cid INT NOT NULL COMMENT '课程ID',
	cname VARCHAR(50) COMMENT '课程名称',
	tid INT COMMENT '教师id',
	PRIMARY KEY (cid)
);
INSERT INTO t_course VALUES(1 , '语文' , 2);
INSERT INTO t_course VALUES(2 , '数学' , 1);
INSERT INTO t_course VALUES(3 , '英语' , 3);

-- 4.成绩表-t_score
-- sid 学生编号,cid 课程编号,score 成绩
CREATE TABLE t_score 
(
	sid INT NOT NULL COMMENT '学号，外键',
	cid INT NOT NULL COMMENT '课程id',
	score decimal(5,2) COMMENT '成绩',
	UNIQUE KEY ak_key_sid_cid (sid, cid)
);
INSERT INTO t_score VALUES(1 , 1 , 80);
INSERT INTO t_score VALUES(1 , 2 , 90);
INSERT INTO t_score VALUES(1 , 3 , 99);
INSERT INTO t_score VALUES(2 , 1 , 70);
INSERT INTO t_score VALUES(2 , 2 , 60);
INSERT INTO t_score VALUES(2 , 3 , 80);
INSERT INTO t_score VALUES(3 , 1 , 80);
INSERT INTO t_score VALUES(3 , 2 , 80);
INSERT INTO t_score VALUES(3 , 3 , 80);
INSERT INTO t_score VALUES(4 , 1 , 50);
INSERT INTO t_score VALUES(4 , 2 , 30);
INSERT INTO t_score VALUES(4 , 3 , 20);
INSERT INTO t_score VALUES(5 , 1 , 76);
INSERT INTO t_score VALUES(5 , 2 , 87);
INSERT INTO t_score VALUES(6 , 1 , 31);
INSERT INTO t_score VALUES(6 , 3 , 34);
INSERT INTO t_score VALUES(7 , 2 , 89);
INSERT INTO t_score VALUES(7 , 3 , 98);

select * from t_student;
select * from t_teacher;
select * from t_course;
select * from t_score;

The data table is as follows:

t_student student table t_teacher teacher table

t_course curriculum t_score score table

1) Query the information and course scores of students whose grades in the "1" course are higher than those in the "2" course

 SELECT stu.sid,stu.sname,stu.ssex,c1.cid, c1.score, c2.cid, c2.score
   FROM t_student stu
        INNER JOIN  (SELECT t1.sid, t1.cid, t1.score FROM t_score t1 WHERE t1.cid = 1 ) c1 ON stu.sid = c1.sid
        INNER JOIN  (SELECT t2.sid, t2.cid, t2.score FROM t_score t2 WHERE t2.cid = 2) c2 ON stu.sid = c2.sid 
 WHERE c1.score > c2.score

2) Query the information of students who take both "1" courses and "2" courses

//方法一
SELECT stu.sid,stu.sname,stu.ssex,c1.cid, c1.score, c2.cid, c2.score
   FROM t_student stu
        INNER JOIN  (SELECT t1.sid, t1.cid, t1.score FROM t_score t1 WHERE t1.cid = 1 ) c1 ON stu.sid = c1.sid
        INNER JOIN  (SELECT t2.sid, t2.cid, t2.score FROM t_score t2 WHERE t2.cid = 2) c2 ON stu.sid = c2.sid

//方法二
SELECT stu.`sid`,stu.`sname`, stu.`ssex`, tmp.c1num, tmp.c2num FROM t_student stu INNER JOIN 
   (
     SELECT t.`sid`, 
            SUM(CASE WHEN t.cid = 1 THEN t.`score` ELSE 0 END) c1num, 
            SUM(CASE WHEN t.cid = 2 THEN t.`score` ELSE 0 END) c2num FROM t_score t GROUP BY t.`sid`
   ) tmp ON stu.sid = tmp.sid AND tmp.c1num > 0 AND tmp.c2num > 0;

3) Inquire about elective "1" courses but not elective "2" courses

SELECT stu.* FROM t_student stu 
  WHERE stu.sid IN(SELECT t1.sid FROM t_score t1 WHERE t1.cid = 1)
	AND stu.sid NOT IN (SELECT t1.sid FROM t_score t1 WHERE t1.cid = 2)

SELECT stu.`sid`,stu.`sname`, stu.`ssex`, tmp.c1num, tmp.c2num FROM t_student stu INNER JOIN 
   (
     SELECT t.`sid`, 
            SUM(CASE WHEN t.cid = 1 THEN t.`score` ELSE 0 END) c1num, 
            SUM(CASE WHEN t.cid = 2 THEN t.`score` ELSE 0 END) c2num FROM t_score t GROUP BY t.`sid`
   ) tmp ON stu.sid = tmp.sid AND tmp.c1num > 0 AND tmp.c2num = 0;

4) Query the case where "1" course does not exist but "2" course exists

 SELECT t1.sid,t1.cid,t1.score 
    FROM t_score t1 
  WHERE t1.cid = 2 AND t1.sid NOT IN (SELECT t2.sid FROM t_score t2 WHERE t2.cid = 1);

Query the highest score, lowest score and average score of each subject:
1) Display column: course ID, course name, highest score, lowest score, average score, number of electives, pass rate, average rate, excellent rate
2) The excellent rate is >=60, medium: 70-80, excellent: 80-90, excellent: >=90
3) The query results are required to be sorted in descending order by number of people.

SELECT t2.cid '课程ID', 
       t2.cname '课程名称',
       MAX(t1.score) '最高分',
       MIN(t1.score) '最低分',
       ROUND(AVG(t1.score), 2) '平均分',
       COUNT(t1.sid) '选修人数',
       ROUND(SUM(CASE WHEN t1.score >= 60 THEN 1 ELSE 0 END) / COUNT(t1.sid), 2) '及格率',
       ROUND(SUM(CASE WHEN t1.score >=70 AND t1.score < 80 THEN 1 ELSE 0 END)/COUNT(t1.sid),2) '中等率',
       ROUND(SUM(CASE WHEN t1.score >=80 AND t1.score < 90 THEN 1 ELSE 0 END)/COUNT(t1.sid),2) '优良率',
       ROUND(SUM(CASE WHEN t1.score >= 90 THEN 1 ELSE 0 END)/COUNT(t1.sid), 2) '优秀率'
 FROM t_score t1
      INNER JOIN t_course t2 ON t1.cid = t2.cid
  GROUP BY t2.cid, t2.cname
  ORDER BY COUNT(t1.sid) DESC, t2.cid ASC;

Mysql basic data types and CRUD