MySQL common add, delete, modify and query operations (crud)

️ Preface ️

This article mainly introduces in MySQL Common additions, deletions, modifications and queries in Databases （CRUD）SQL Statement operation .

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MySQL Common operations of adding, deleting, modifying, and querying （CRUD）

1. newly added （create）

grammar ：

insert into  Table name  values ( value , value );

Case study ：
Create a student table to insert ：

create table student(id int ,name varchar(50));
insert into student values (1,' Li Si ');

Bottom line “1 row affected in 5 ms” Indicates that a line has changed .

Specify column insertion ：

insert into student (name) values(' Zhang San ');

Multi line insertion ：

insert into student values (3,' Wang Wu '),(4,' Zhao Liu ');

Insert... Once N Records are better than inserting one point at a time N Insertion is much faster .

2. Inquire about （retrieve）

The query statement is SQL The most core and complex operation in the statement , Next, let's briefly introduce the primary query statement （ Inquire about select Clause no matter how it goes , Will not change the original data of the database ）.
Case study ：

--  Create test scores 
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)
);
--  Insert test data 
insert into exam_result (id,name, chinese, math, english) values 
                                                              (1,' Tang Sanzang ', 67, 98, 56),
                                                              (2,' The Monkey King ', 87.5, 78, 77),
                                                              (3,' Pig Wuneng ', 88, 98.5, 90),
                                                              (4,' Cao mengde ', 82, 84, 67),
                                                              (5,' Liu Xuande ', 55.5, 85, 45),
                                                              (6,' king of Wu in the Three Kingdoms Era ', 70, 73, 78.5),
                                                              (7,' Song Gongming ', 75, 65, 30);

2.1 Full column query

The most basic operation , But it is also dangerous in actual development , If the amount of data is large , Performing a full column query will consume a lot of system resources , It may cause downtime .

select*from  Table name ;

2.2 Specified column query

This is much more efficient than full column queries , When querying, it will be displayed to tell the database which columns to query , The database will return targeted data .

select  Name , Name ...... from  Table name ;

2.3 The query field is an expression

Make some queries at the same time Operation （ Between columns ）
for example , It is expected that the Chinese score in the query results is much higher than the real score ：

Check the total score of each student ：

2.4 Specify alias for query field

By specifying aliases, you can prevent the temporary table column names from being too messy .

2.5 De duplication of query results

For query results , Erase duplicate records .

If you are de duplication for multiple columns , Only when the values of these multiple columns are the same can they be regarded as duplicates

2.6 Sort query results

select  Name ..... from  indicate  order by  Name  asc/desc;

Sort math scores in ascending order asc In ascending order （ The default is ascending. You can leave it blank ）,desc For the descending order

Be careful ：

1. The database record contains NULL The value of will be regarded as the minimum （ It will be regarded as the minimum value in sorting ）

2. Sorting can also be based on aliases or expressions .

3. When specifying multiple column sorting , First sort according to the first column , If the values of the first column are the same, then sort by the results of the second column , The following is the mathematical order , If mathematics is the same, then sort by language .

2.7 Conditions of the query

stay select Add where filter , The query results will keep those that meet the conditions , Filter out the unsatisfied .

select  Name  from  Table name  where  Conditions ;

Comparison operator ：

Operator	explain
>, >=, <, <=	Greater than , Greater than or equal to , Less than , Less than or equal to
=	be equal to ,NULL unsafe , for example NULL = NULL The result is NULL
<=>	be equal to ,NULL Security , for example NULL <=> NULL The result is TRUE(1)
!=, <>	It's not equal to
BETWEEN a0 AND a1	Range match ,[a0, a1], If a0 <= value <= a1, return TRUE(1)
IN (option, …)	If it is option Any one of , return TRUE(1)
IS NULL	yes NULL
IS NOT NULL	No NULL
LIKE	Fuzzy matching .% Denotes any number of （ Include 0 individual ） Any character ;_ Represents any character

Logical operators ：

Operator	explain
AND	More than one condition must be TRUE(1), The result is TRUE(1)
OR	Any one of the conditions is TRUE(1), The result is TRUE(1)
NOT	Condition is TRUE(1), The result is FALSE(0)

notes ：

1. WHERE Conditions can be expressed as , But you can't use aliases .
2. AND Has a higher priority than OR, When used at the same time , You need to use parentheses () The priority part of the package

Case study ：

• The basic query ：
  1. Query the English grades of students who fail in English
  
  2. Query students whose Chinese scores are better than English scores
  
  3. The total query score is 200 Students with the following scores
  
  Note here that aliases cannot be used

• AND And OR：
  1. Query language score is greater than 80 branch , And the English score is greater than 80 Classmate
  
  2. Query language score is greater than 80 branch , Or English scores greater than 80 Classmate
  
  3. Observe AND and OR The priority of the
  and Has a higher priority than or, If you want to break the priority, you need to use parentheses （）
  
  

• Range queries
  1.between …and…
  In the field of programming , Most sections are left closed and right open , Pay attention to SQL Middle is an interval with both left and right closed
  Query Chinese scores in [80, 90] Students and Chinese scores
  
  Use and It can also be realized
  
  2.in
  The query math score is 58 perhaps 59 perhaps 98 perhaps 99 Students and math scores
  
  Use or A similar effect can be achieved
  

• Fuzzy query ：LIKE
  % Match any number of （ Include 0 individual ） character
  _ Match a strict arbitrary character
  
  

• NULL Query for ：IS [NOT] NULL
  And null The relevant operation results are still hard , So we should find the information of students whose Chinese scores are empty , The results cannot be queried in the following way
  
  Yes <=>、is null These two ways to judge whether it is null
  
  

2.8 Paging query

If you find a large number of results , We need to display the page by page in the way shown in the figure above .
stay sql In the use of limit To do paging queries ,
grammar ：

--  Starting subscript is  0
--  from  0  Start , Screening  n  Bar result 
SELECT ... FROM table_name [WHERE ...] [ORDER BY ...] LIMIT n;
--  from  s  Start , Screening  n  Bar result 
SELECT ... FROM table_name [WHERE ...] [ORDER BY ...] LIMIT s, n;
--  from  s  Start , Screening  n  Bar result , More definite than the second usage , It is recommended to use 
SELECT ... FROM table_name [WHERE ...] [ORDER BY ...] LIMIT n OFFSET s;

The first three query results are shown below

From the subscript for 3 Start , Check back three

We mentioned before select*from Dangerous operation , So we can add where Conditions or limit To more secure restrictions .

3. modify （update）

grammar ：

update  Table name  set  Name = value （ Or expressions ）, Name = value （ Or expressions ）..... where  Conditions ;

Here where Restrict those that meet the conditions to modify .
Case study ： Change the math score of Monkey King to 80 branch

update exam_result set math =80 where name=' The Monkey King ';

4. Delete （delete）

grammar ：

delete from  Table name  where  Conditions  [ORDER BY ...] [LIMIT ...];

Delete the exam results of Monkey King

delete from exam_result where name=' The Monkey King ';

Be careful , If not where Conditions , The data of the whole table will be deleted .

5. Map summary

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️ Last words ️

Summing up difficulties , hope uu Don't be stingy with your (＾Ｕ＾)ノ~ＹＯ！！ If there is a problem , Welcome comments and corrections in the comment area