Getting started database days2

• Reference books 《MySql From entry to mastery 》
• Learning video 【 Han Shunping said MySQL】 Zero basis a week to learn MySQL -sql mysql course mysql video mysql introduction _ Bili, Bili _bilibili
• Mengxin beginner level , If there is any mistake, please give advice , Not very grateful

Catalog

insert Statement notes

 update Statement notes

 Delect Use of statements

Select Use of statements

Use expressions to operate on the columns of the query

​ edit  Where Operators commonly used in statements

​ edit   Use order by Clause sort query results

---

insert Statement notes

• The inserted data should be of the same data type as the field For example 'abc' Add to int The type will be wrong
• The length of the column should be within the specified range , for example ： You can't change a length to 80 The string of is added to a length of 40 In the column of
• stay values The data positions listed in must correspond to the arrangement positions of the added columns
• Character and date data are contained in single quotation marks
• Columns can be inserted with null values 【 The premise is that the field can be empty 】,insert into table value(null)
• If you are adding data to all fields in the table , You can not write the previous field name
• Use of default values , When a field value is not given , If there is a default value, it will be added , Otherwise, the report will be wrong . If a column is not specified not null , When adding data , There is no given value , Will default to null . If we want to specify the default value of a column , You can specify... When creating a table

The demonstration is as follows ：

Create the table first s1

CREATE TABLE s1(
	id INT,
	`name` VARCHAR(10),
	price DOUBLE);
	--  Add data 
	INSERT INTO s1 (id,`name`,price)
		VALUES(10,' Huawei mobile phones ',3999);
	INSERT INTO s1 (id,`name`,price)
		VALUES(10,' Apple mobile phone ',3999);
	SELECT * FROM s1;

  When the added value corresponds to id As a 'abc' Times wrong

When added name The value exceeds the specified 10 There are errors in characters

 

  notes ： If in price DOUBLE Then write it down NOT NULL, Then this cannot be empty

  Add data to all fields , You can omit the field name

 

Here it is price After setting the default value 100, Even if you don't write, it will be assigned automatically 100 

 update Statement notes

 UPDATE        Table name         

                SET   col_name1=expr1[,col_name2=expr2 ...]

                [WHERE where_definition]

  Modify the table to table t99 To demonstrate

 1. Change all employee salaries to 5000 element .[ If you don't have where  Conditions , Will modify all records , Therefore, we need to be cautious ]

UPDATE t99 SET salary =5000
SELECT * FROM t99

 2. Name is Goblins   The salary of the employee is revised to 3000 element

UPDATE t99
	SET salary =3000
	WHERE user_name =' Goblins '

 3. take Old monster     My salary is increased on the original basis 1000 element

 4. Modify multiple columns at the same time

 

 Delect Use of statements

  The grammar is as follows :

delete from  Table name

                [WHERE  where_definition]

Delete all records in the table

 Delete sentence Can't delete The value of a column ( You can use update Set to null perhaps ' ')

•   Use delete Statement only delete records , Don't delete the table itself , If you want to delete a table , Use drop table sentence .drop  table Table name ;

Select Use of statements

Basic grammar ：

SELECT  [DISTINCT] *  | {column1, column2 . column3..}

                        FROM    Table name

Create a student table

Look up the names of all the students and their English scores  

SELECT `name`,english FROM student;

  De duplication of Chinese scores during query

SELECT DISTINCT chinese FROM student

  If in chinese Write in front name etc. , Only when the data corresponding to the corresponding column is repeated can the duplication be removed

Use expressions to operate on the columns of the query

 SELECT         *  |{column | expression ,column2| expression, .. }

                        FROM Table name ;

example ： Count the total score of each student

Use aliases to indicate student scores

 Where Operators commonly used in statements

  Query all English scores greater than 80 People who

  Query math is greater than 80 branch also id Greater than 4 People who

Query people whose English scores are greater than Chinese scores

Total query score greater than 200 branch , And the math score is less than the Chinese score  , A student surnamed Zhu

  Query English scores in 80-90 Between the students

  Query English scores in 80,90,70 Of the students

  Check all students surnamed Liu

  Use order by Clause sort query results

 SELECT column1 ,column2 ,column3..

                        FROM table;

                        order by column asc|desc,  ...

• Order by Specify the columns to arrange , The sorted column can be either the column name in the table , It can also be select The column name specified after the statement
• Asc Ascending [ Default ],Desc Descending
• ORDER BY Clause should be in SELECT End of statement

  example ： Sort the math grades and output them ( Descending )

  Sort the total score from high to low and output ( Descending )

  Will all ‘ Bucket ’ The total score of students with surnames is sorted from low to high and then output ( Ascending )

  The above is today's video notes , In case of infringement, please contact me to delete .

                God helps those who help themselves , Come on with Xiao Wu ！