DQL language (1)

DQL language

DQL data query language

SELECT syntax

Query all student information (all columns, low efficiency)

select * from student;

Query specified column (student number, name) is not case sensitive

SELECT studentno,studentname FROM student;

Take the alias keyword as, you can also use spaces

SELECT studentno AS 学号,studentname AS 姓名 FROM student;

• You can also use as to alias the table, which is used to distinguish between multiple table queries

SELECT studentno AS 学号,studentname AS 姓名 FROM student AS s;

• distinct removes duplicates, the default is all
• Expressions can be used in select queries

Query that meets the conditions

select studentno,studentresult from result
where studentresult between 95 and 100;

Comparison operator

• Wildcard used for like joining:% (0 to any number of characters) _ (one character)

  select StudentNo,StudentName
  from student
  where studentname like '李%';
  

• The escape symbol \ can I use my own escape symbol

  

• in matches the same character in multiple values

• null can not be used =, and null must be written is or is not

  • Note: If the default is non-empty, after deleting the content, it is not null, but ='' to match

  

Link query

• Inner join. inner joinQuery the intersection of the result sets in the two tables

• SELECT s.`StudentNo`,`StudentName`,`SubjectNo`,`StudentResult`
  FROM `student`  s
  INNER JOIN `result`  r
  ON r.`StudentNo`=s.`StudentNo`
  

• Outer join outer join

  • Left outer connector left jointo the left table as a reference, the right table one by one, the matching is not on, the process returns records left table, right table to null fill

  • Right outer connection right joinis opposite to above

    

    ​

• Self-connection

CREATE TABLE categorg(
category_id INT(11) AUTO_INCREMENT PRIMARY KEY,
category_name VARCHAR(10),
pid INT(10)
);


INSERT INTO `categorg`(`category_name`,pid)
VALUES('软件开发',0),('美术设计',0),
('数据库继承',1),('ps基础',2),('色彩搭配',2),
('php基础',1),('一起学java',1);

select
  a.`category_name` as '父栏目',
  b.`category_name` as '子栏目'
from
  `categorg` a
  join `categorg` b
  on b.`pid`=a.`category_id`
where a.pid = 0 ;

​

• Non-equivalent join

等值连接
SELECT s.`StudentNo`,`StudentName`,`SubjectNo`,`StudentResult`
FROM `student`  s,`result`  r
WHERE r.`StudentNo`=s.`StudentNo`;

Little knowledge

round() rounds
rand() random numbers

Use in for subquery

SELECT * FROM student WHERE `StudentNo` IN (SELECT ROUND(RAND()*2+1011));

Cartesian collection, very memory consuming, not recommended

case field when value then operation else operation end;

In many cases, the case field when value then operation when value then operation else operation end;

Full connection union: union and deduplication union all: union without deduplication