Add, delete, check and modify MySQL tables

( Note : "- -space+description" can be used in SQL to indicate comment description)

1. New data (Create):

The basic syntax of the new statement:

insert into table_name values();

Let's first create a student table:

-- 创建一张学生表
drop table if exists student; --表示如果有 student 这个表则给删除
create table student (
   id int,
   sn int comment '学号',
   name VARCHAR(20) comment '姓名',
   qq_mail VARCHAR(20) comment 'QQ邮箱'
);

We insert data into this student table:

(1) Single row data + full column insertion:

-- 插入两条记录，value_list 数量必须和定义表的列的数量及顺序一致
insert into student values(100, 10000, '唐三藏', NULL);
insert into student values(101, 10001, '孙悟空', '11111');

(2) Multi-row data + designated column insertion:

-- 插入两条记录，value_list 数量必须和指定列数量及顺序一致
INSERT INTO student (id, sn, name) VALUES
 (102, 20001, '曹孟德'),
 (103, 20002, '孙仲谋');

2. Query data (Retrieve):

Let's first create a test score table and insert data:

-- 创建考试成绩表
drop table if exists exam_result;
create table exam_result (
    id int,
    name varchar(20),
    chinese decimal(3,1),
    math decimal(3,1),
    english decimal(3,1)
);
-- 插入测试数据
insert into exam_result (id,name, chinese, math, english) values
 (1,'唐三藏', 67, 98, 56),
 (2,'孙悟空', 87.5, 78, 77),
 (3,'猪悟能', 88, 98.5, 90),
 (4,'曹孟德', 82, 84, 67),
 (5,'刘玄德', 55.5, 85, 45),
 (6,'孙权', 70, 73, 78.5),
 (7,'宋公明', 75, 65, 30);

(1) Full column query:

Under normal circumstances, we do not use * for full-column queries, because the more columns you query, the larger the amount of data transmitted, and the lower the query efficiency. . .

select * from exam_result;

(2) Specify column query:

-- 指定列的顺序不需要按定义表的顺序来
select id,name,english from exam_result;

(3) The query field is an expression:

-- 表达式不包含字段
select id, name, 10 from exam_result;
-- 表达式包含一个字段
select id, name, english + 10 from exam_result;
-- 表达式包含多个字段
select id, name, chinese + math + english from exam_result;

(4) Alias:

Alias is to give the column name from a nickname

select id,name,chinese+math+english [as]'总分' from exam_result;

We give this chinese + math + english played a named individual score
(alias front as writable not write, if you do not write it in place with enough space)

(5) Deduplication: (distinct)

Remove duplicate lines

select distinct math from exam_result;

(6) Sort: order by

- ASC is ascending (small to large)
- DESC is descending (from largest to smallest)
- defaults to ASC

1. For queries without an ORDER BY clause, the return order is undefined, never rely on this order
2. NULL data is sorted, regarded as smaller than any value, ascending order appears at the top, descending order appears at the bottom

-- 查询同学姓名和 qq_mail，按 qq_mail 排序显示
SELECT name, qq_mail FROM student ORDER BY qq_mail;
SELECT name, qq_mail FROM student  ORDER BY qq_mail desc;

3. Sort using expressions and aliases

-- 查询同学及总分，由高到低
SELECT name, chinese + english + math FROM exam_result 
 ORDER BY chinese + english + math DESC;
 
SELECT name, chinese + english + math total FROM exam_result
 ORDER BY total DESC;

4. Multiple fields can be sorted, and the sorting priority depends on the writing order

-- 查询同学各门成绩，依次按 数学降序，英语升序，语文升序的方式显示
SELECT name, math, english, chinese FROM exam_result
 ORDER BY math DESC, english, chinese;

(7) Conditional query: where

note:

1. WHERE conditions can use expressions, but not aliases.
2. AND has a higher priority than OR. When used at the same time, you need to use parentheses () to wrap the priority execution part

Basic query:

-- 查询英语不及格的同学及英语成绩 ( < 60 )
SELECT name, english FROM exam_result WHERE english < 60;
-- 查询语文成绩好于英语成绩的同学
SELECT name, chinese, english FROM exam_result WHERE chinese > english;
-- 查询总分在 200 分以下的同学
SELECT name, chinese + math + english '总分' FROM exam_result
 WHERE chinese + math + english < 200;

and 与 or

-- 查询语文成绩大于80分，且英语成绩大于80分的同学
SELECT * FROM exam_result WHERE chinese > 80 and english > 80;
-- 查询语文成绩大于80分，或英语成绩大于80分的同学
SELECT * FROM exam_result WHERE chinese > 80 or english > 80;
-- 观察AND 和 OR 的优先级：
SELECT * FROM exam_result WHERE chinese > 80 or math>70 and english > 70;
SELECT * FROM exam_result WHERE (chinese > 80 or math>70) and english > 70;

Range query:
1. between… and…

-- 查询语文成绩在 [80,90] 分的同学及语文成绩
select name,chinese from exam_result where chinese between 80 and 90;
-- 使用 and 也可以实现
select name,chinese from exam_result where chinese >= 80 and chinese <= 90;

2、in

-- 查询数学成绩是 58 或者 59 或者 98 或者 99 分的同学及数学成绩
SELECT name, math FROM exam_result WHERE math IN (58, 59, 98, 99);
-- 使用 OR 也可以实现
SELECT name, math FROM exam_result WHERE math = 58 OR math = 59 OR math
= 98 OR math = 99;

Fuzzy query: like

-- % 匹配任意多个（包括 0 个）字符
SELECT name FROM exam_result WHERE name LIKE '孙%';-- 匹配到孙悟空、孙权
-- _ 匹配严格的一个任意字符
SELECT name FROM exam_result WHERE name LIKE '孙_';-- 匹配到孙权

null query: is [not] null

-- 查询 qq_mail 已知的同学姓名
SELECT name, qq_mail FROM student WHERE qq_mail IS NOT NULL;
-- 查询 qq_mail 未知的同学姓名
SELECT name, qq_mail FROM student WHERE qq_mail IS NULL;

(8) Paging query: limit

Basic format:

-- 起始下标为 0
-- 从 0 开始，筛选 n 条结果
SELECT ... FROM table_name [WHERE ...] [ORDER BY ...] LIMIT n;
-- 从 s 开始，筛选 n 条结果
SELECT ... FROM table_name [WHERE ...] [ORDER BY ...] LIMIT s, n;
-- 从 s 开始，筛选 n 条结果，比第二种用法更明确，建议使用
SELECT ... FROM table_name [WHERE ...] [ORDER BY ...] LIMIT n OFFSET s;

Give a chestnut:

-- 第 1 页
SELECT id, name, math, english, chinese FROM exam_result ORDER BY id LIMIT 3
OFFSET 0;
-- 第 2 页
SELECT id, name, math, english, chinese FROM exam_result ORDER BY id LIMIT 3
OFFSET 3;
-- 第 3 页，如果结果不足 3 个，不会有影响
SELECT id, name, math, english, chinese FROM exam_result ORDER BY id LIMIT 3
OFFSET 6;

Three, modify the data (Update):

-- 将孙悟空同学的数学成绩变更为 80 分
UPDATE exam_result SET math = 80 WHERE name = '孙悟空';
-- 将曹孟德同学的数学成绩变更为 60 分，语文成绩变更为 70 分
UPDATE exam_result SET math = 60, chinese = 70 WHERE name = '曹孟德';
-- 将总成绩倒数前三的 3 位同学的数学成绩加上 30 分
UPDATE exam_result SET math = math + 30 ORDER BY chinese + math + english LIMIT
3;
-- 将所有同学的语文成绩更新为原来的 2 倍
UPDATE exam_result SET chinese = chinese * 2;

Fourth, delete data (Delete):

-- 删除孙悟空同学的考试成绩
DELETE FROM exam_result WHERE name = '孙悟空';
-- 删除整张表数据
-- 准备测试表
DROP TABLE IF EXISTS for_delete;
CREATE TABLE for_delete (
 id INT,
 name VARCHAR(20)
);
-- 插入测试数据
INSERT INTO for_delete (name) VALUES ('A'), ('B'), ('C');
-- 删除整表数据
DELETE FROM for_delete;