QT database learning notes (I) basic concepts of database

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One 、 Concept

● database DB（DataBase）

It's the warehouse where data is stored , Its characteristic is that the data is organized according to the data model , Is highly structured , It can be shared by multiple users and has certain security .

● Database management system DBMS（DataBase Management System）

It is a database management system software located between the user application and the operating system , Its main function is to organize 、 Storing and managing data , Efficiently access and maintain data , frequently-used DBMS Yes Oracle、Microsoft SQL Server and MySQL etc. .

● What is? SQL Language ？

     Structured query language （Structured Query Language,SQL） Is a standard language for relational database operations ,

● SQL The composition of the three parts

1)、 Data definition language （Data Description Language,DDL）, Data definition language DDL Used to create 、 Delete 、 Modify various objects in the database ----- surface 、 View 、 Indexes 、 A synonym for 、 Clustering, etc

Create database ：  Create database 
Alter database ：  modify the database 
Create table ：    Create new table 
Alter table ：     change （ change ） Database table 
Drop table ：      Delete table 
Create index ：    Create index （ The search button ）
Drop index :       Delete index 

2)、 Data manipulation language （Data Manipulation Language,DML）, Used to manipulate various objects in the database , Retrieve and modify data

Select ：		 Getting data from a database table 
Update ：		 Update the data in the database 
Delete ：		 Delete data from the database table 
Insert into ：	 Insert data into the database 

3)、 Data control language （Data Control Language,DCL） For safety management , Determine which users can view or modify data in the database

① to grant authorization

GRANT：   to grant authorization 

② Roll back

ROLLBACK [WORK] TO [SAVEPOINT]：   Go back to a certain point .

SQL>ROLLBACK;    					 The rollback command returns the database state to the last committed state .

③ Submit

COMMIT [WORK]： Submit .

●  Explicitly commit , use COMMIT The commit done directly by the command is an explicit commit .
SQL>COMMIT;

●  Implicitly submit , use SQL The commit done indirectly by the command is an implicit commit .
ALTER,AUDIT,COMMENT,CONNECT,CREATE,DISCONNECT,DROP,EXIT,GRANT,NOAUDIT,QUIT,REVOKE,RENAME.

●  Automatic submission , If the AUTOCOMMIT Set to ON, Then insert 、 modify 、 After the delete statement is executed ,  The system will automatically submit , This is auto submit .
SQL>SET AUTOCOMMIT ON;

● Database table

A database usually contains one or more tables . Each table has a name identifier , Table contains records with data （ That's ok ）, as follows ：

● QtSql Module hierarchy

● Qt5 Database driven

Qt SQL The module uses database driver to communicate with different database interfaces . because Qt Of SQL The interface of the model is database independent , So all the database specific code is included in these drivers .Qt The database driver supported now is shown in the figure below .

Two 、 Data manipulation language DML（ Add or delete check change ）

1. increase （INSERT）

● INSERT grammar

grammar 1：

INSERT INTO target [IN externaldatabase] (fields_list)		
{
   DEFAULT VALUES|VALUES（DEFAULT|expression_list）}		    

grammar 2：

INSERT INTO target [IN externaldatabase] fields_list
{
   SELECT…|EXECUTE…}

● example

insert into teacher values(”,’allen’,' Dalian No.1 Middle School ’,'1976-10-10′);
insert into student values(1,' Zhang San ',21)

2. Delete （DELETE）

● DELETE grammar

DELETE FROM table_names
[WHERE…]

● example

① from students Delete the name in the table as “ Luo Liang ” The record of ：

DELETE FROM students WHERE name = ' Luo Liang '

② from students In the table to delete ID by “3” The record of ：

delete from students where id = 3

3. check （SELECT ）

● complete SELECT grammar

SELECT [DISTINCT] [ Alias .] Field name or expression  [AS  Column headings ]   /*  Specify the columns or rows to select and their qualifications  */				
FROM  table_source                                 /* FROM Clause , Specify a table or view  */
[ WHERE  search_condition ]                	       /* WHERE Clause , Specify query criteria  */
[ GROUP BY group_by_expression ]                   /* GROUP BY Clause , Specify grouping expression  */
[ ORDER BY order_expression [ ASC | DESC ]]        /* ORDER BY Clause , Specify sort expression and order  */

● example

① Inquire about Student database . Inquire about students Name and total credits of each student in the table .

USE Student SELECT name,totalscore FROM students

② Query all records in the table . Inquire about students All the information of each student in the table , Most used

SELECT * FROM students

③ Conditions of the query . Inquire about students The total credits in the table are greater than or equal to 120 The situation of my classmates .

SELECT * FROM students WHERE totalscore >=  120

④ Conditions of the query & Get the Department name and total score . Inquire about students The total credits in the table are greater than or equal to 120 The situation of my classmates .

SELECT department,name,totalscore FROM students WHERE totalscore >=  120

⑤ Multiple criteria query . Inquire about students The system in the table is “ Computer ” And the total credits are greater than or equal to 120 The situation of my classmates .

SELECT * FROM students WHERE department=' Computer ' AND totalscore >= 120

⑥ Use LIKE Predicate for pattern matching . Inquire about students Last name in the table “ king ” And the situation of single name students .

SELECT * FROM students WHERE name LIKE ' king _'

4. Change （UPDATE）

● UPDATE grammar

UPDATE table_name
SET Field_1=expression_1[,Field_2=expression_2…]
[FROM table1_name|view1_name[,table2_name|view2_name…]]
[WHERE…]

among ,Field Is the field that needs to be updated ,expression Represents the new value expression of the field to be updated .

● example

① Increase the total score of computer students 10：

UPDATE students 
SET totalscore = totalscore + 10 
WHERE department = ' Computer '

② take ID by 2 The name of the student is changed to “ Xiao Ming ”：

update students 
SET name = ' Xiao Ming ' 
WHERE id = 2

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Reference resources ： QT/C++ Foundation and improvement series QT Intermediate series of video courses