Mysql database composition
Server: Mainly store data, receive SQL statements sent by users, and return execution results to the client
Client: send the sql statement to be executed by the user, and display the execution result returned by the server
Command line database connection method
type of data
number type
string type
time and date type
constraint
Command line operation database, table
| view all databases | show database; |
| use database |
use database name; |
| View the currently used database | select database(); |
| create database | create database database name charset=utf8; |
| delete database | drop database database name; |
| View all tables in the database | show tables; |
| View the structure of the database table | desc table name;(desc =describe) |
| View the creation statement of the data table | show create table table name; |
Database table operations
1. Create a database table
grammar:
create table table name (
field name 1 type constraint,
Field name 2 type constraint
... ... ...
)
example:
Create a student table with field requirements: name (length 10), age, height (2 decimal places)
CREATE table students(
id int UNSIGNED PRIMARY KEY auto_increment,
name VARCHAR(20),
height decimal(5,2)
)2. Delete the database table
drop table tablename (this command will permanently delete the table and all its data)
drop table if exists students (Check if the table exists before dropping the table. If the table exists, the table will be dropped; if the table does not exist, no error will be raised)
Data addition, deletion, modification and query operations
Increase (insert):
1.1 Add a row of data
insert into table name values(...)
example:
insert into students values(0,'jingbeng',12.66);
//Primary key self-growth can be replaced by 0 or NULL
Another way of writing:
insert into students(name) values('nnn')
1.2 Adding multiple rows of data
Method 1: Write multiple insert statements, separated by " ;"
insert into students(name) value ('jingbeng1');
insert into students(name) value ('jingbeng3');
insert into students values(0,'jingbeng2',23,167.56)
Method 2: Insert multiple pieces of data through one insert statement, and separate the data with ","
Format 1: insert into table name values (...), (...) ...
Example: insert into students values(0,'Arthur 3',23,167.56),(0,'Arthur 4',23,167.56)
Format 2: insert into table name (field name 1,...) value (value 1,...), (value 1,...)...
Example: insert into students(name) value ('Old Master Q 5'),('Old Master Q 6')
2. Check (select):
1.1 Simple query
select * from table name;
Example: select * from students;
1.2 Query some field data
select field 1, field 2... from table name;
例子:select name,sex,age from students;
1.3 alias
Table alias: select alias. field 1, alias. field 2... from table name as alias;
例子:select s.name,s.age from students as s;
Field alias: select field 1 as alias 1, field 2 as alias from table name
例子:select name as n,age as a from students;
1.4 Deduplication of field content
select distinct field from table name;
1.5 Conditional query (where)
select field 1, field 2... from table name where condition;
例: select * from students where id=1;
Description: where supports multiple operators for conditional processing
comparison operation
logic operation
fuzzy query
range query
Empty judgment
1.5.1 Conditional query - comparison operator
Example 1: Query Xiao Qiao's age
select age from students where name='小乔’
Example 2: Query students under the age of 20
select * from students where age<20
Example 3: Query students whose hometown is not in Beijing
select * from students where hometown!='北京'
1.5.2 Conditional query - logical operators
Example 1: Query female students whose age is less than 20
select * from students where age<20 and sex='女'
Example 2: Query female students or students in class '1'
select * from students where sex='女' or class='1班
Example 3: Query non-Tianjin students
select * from students where not hometown='天津'
1.5.3 Condition query - fuzzy query
Keyword: like
% : match any number of characters
- : match any character
Example 1: Query students with the surname Sun
select * from students where name like '孙%'
Example 2: Query students whose surname is Sun and whose first name is one character
select * from students where name like '孙_'
Example 3: Query students whose names end with 'Joe'
select * from students where name like '%乔'
Example 4: Query students whose names contain '白'
select * from students where name like '%白%'
1.5.4 Condition query - range query
in means in a non-contiguous range
Example: Query students whose hometown is Beijing, Shanghai or Guangdong
select * from students where hometown in('Beijing','Shanghai','Guangdong')
between ... and ... means in a continuous range
Example: Query students whose age is 18 to 20
select * from students where age between 18 and 20
1.5.5 Conditional query - empty judgment
Note : Mysql empty means null, which is different from '' (empty).
Judgment is empty : is null
Example: Query students who have not filled in their ID cards
select * from students where card is null
Judgment is not empty : is not null
Example: Query students who have filled in their ID cards
select * from students where card is not null
1.6 Sorting (desc needs to be added in descending order)
1.7 Aggregate functions
The use of aggregate functions is for the convenience of data statistics
Aggregate functions cannot be used in where:
count(): Query the total number of records
max (field name): query the maximum value
min (field name): query the minimum value
sum (field name): sum
avg (field name): find the average
1.8 grouping (group by)
Syntax: select field 1, field 2, aggregate function ... from table name group by field 1, field 2...
1.9 Screening after grouping (having...)
Syntax : select field 1, field 2, aggregation ... from table name group by field 1, field 2, field 3... having conditions
Comparison between having and where:
where is to filter the data of the table specified after from, which belongs to the filtering of original data.
Having is to filter the results of group by .
Aggregate functions can be used in the condition after having, but not after where .
1.10 Pagination - get some data
Syntax: select * from table name limit start count
start indicates the starting position, and the index starts from 0 by default
count means to get count pieces of data from the start position
Example: Get the information of rows 2-4 of the student table
select * from students limit 1,3;
1.10.1 Page display
Paging query: select * from students limit (n-1)*m,m
Description: n indicates which page of data is displayed; m indicates how many pieces of data are displayed on each page
1.11 Connection query
Inner join (inner join...on...)
grammar:
select * from table 1 inner join table 2 on table 1. column = table 2. column;
select * from table 1, table 2 where table 1. column = table 2, column;
Left join (left join...on...)
Definition: (right join and vice versa)
- A left join selects all rows from the left table (the table on the left) and includes matching rows from the right table (the table on the right) that meet the join condition.
- If a row in the left table does not have a matching row in the right table, the result set will show that the value of the corresponding column in the right table is NULL.
- All rows from the left table will be included in the result set, while rows from the right table that do not meet the join criteria will be excluded
Syntax: select * from Table 1 left join Table 2 on Table 1. Column = Table 2. Column
right join
ps: In the left and right joins, whoever is left (right) and which longest table is placed on the left (right) side
1.14 Subqueries
act as a condition
act as a data source
change (update)
Syntax: update table name set field name 1= new value 1, field name 2= new value 2...where condition
delete
Syntax: delete from table name where condition;