In today's data-driven era, the importance of databases is self-evident. As a high-performance database management system with completely independent intellectual property rights, Dameng Database has been widely used in the country. This article will introduce in detail the query and operation of Dameng database to help readers better master and use this database.
1. Basic operations of database
- Create a database: Use
create database <database name>Statement to create a new database. For example,create database my_databaseWill create a name calledmy_databasedatabase. - Delete a database: If you need to delete a database, you can use
drop database <database name>Order. But be careful, because this operation will permanently delete the database and all data and objects it contains, such asdrop database test_database。 - Enter the database: When multiple databases exist, you can use
use <database name>Sentences, such asuse my_database, all subsequent operations will be carried out in this database environment.
2. Data table operation
- Query data table: By
show tablesThe command can view all data tables in the current database. - Create a data table: Use
create tableStatement to create a new data table and define the column name, data type, and constraints of the table. For example, create a name calledstudentThe table, includingidandnameTwo fields can be written like this:
create table student(
id int(4) primary key,
name char(20)
);
In the above statement,idThe field is an integer type, with a length of 4, and is defined as a primary key to ensure its uniqueness;nameThe field is character type and has a length of 20.
- View table structure: Can be used
describe <table name>ordesc <table name>To view the detailed structure of the table, including column name, data type, whether it is a primary key, whether it is allowed to be empty, etc. For example,describe studentWill returnstudentStructural information of the table. - Modify the table name: Use
alter table <original table name> rename <new table name>Statement to modify the name of the table. Assume that you want tostudentRename the table tostudent_info, then it can be executedalter table student rename student_info. - Delete table: When a certain data table is no longer needed, you can use
drop table <table name>Command deletes it. For example,drop table student_infoThe name will be deletedstudent_infotable and all the data in it. - Modify table field information: If you want to modify the information of a field in the table, such as data type, constraints, etc., you can use
alter tableSentence. For example,studentin the tableidThe data type of the field is modified toint(20), executablealter table student change id id int(20). - Add table field information: Use
alter table <table name> add <new field name> <data type> <constraint conditions> after <already field name>The statement adds a new field at the specified location. For example, instudent1TableidAdd one after the fieldclassFields, can be written asalter table student1 add class int(4) not null after id. - Delete a table field: By
alter table <table name> drop <field name>Statement to delete a field in the table. For example,alter table student1 drop numberWill come fromstudent1Delete from the tablenumberFields.
3. Data query operation
- Basic query: Use
SELECTStatements to query data. For example, querystudentAll data in the table can be usedSELECT * FROM studentSentence, in which*Indicates querying all columns. If you want to query only specific columns, you can specify the column name, such asSELECT id, name FROM student1. - Conditional query: Pass
WHEREClause to set query conditions and filter out data that meets the conditions. For example, if you query information for students older than 18 years old, you can useSELECT * FROM student WHERE age > 18Sentence. - Sort query: Use
ORDER BYThe clause sorts the query results. For example, sorting students from childhood to adulthood can be usedSELECT * FROM student ORDER BY age ASC; If you want to sort from age to age, useSELECT * FROM student ORDER BY age DESCSentence. - Group query: Use
GROUP BYThe clause groups the data according to the specified columns and can be statistically analyzed in combination with the aggregate function. For example, to check the number of students in each class, you can useSELECT class, COUNT(*) AS student_count FROM student GROUP BY classSentence, in whichCOUNT(*)It is an aggregate function that counts the number of students in each class. - Multi-table query: When you need to get data from multiple tables, you can use multi-table join query. Common connection methods include internal connection, external connection, etc.
- Inner connection query: Use
INNER JOINKeywords, query matching data in two or more tables by specifying connection conditions. For example, if you query information about an employee and his/her department, you can write this:
- Inner connection query: Use
SELECT employees.employee_id, employees.first_name, employees.last_name, departments.department_name
FROM employees
INNER JOIN departments ON employees.department_id = departments.department_id;
```{insert\_element\_16\_}
- **External connection query**：Includes left outer connection、Right outer connection and full outer connection。The left outer join returns all rows in the left table and rows matching the right table；The right outer join returns all rows in the right table and rows matching the left table.；Fully external join returns all rows in both tables，And fill the mismatched rows with NULL 。For example，Query all employees and their departments，Employee information will also be displayed if the employee does not have a department.，You can use the left outer connection：
```sql
SELECT employees.employee_id, employees.first_name, employees.last_name, departments.department_name
FROM employees
LEFT OUTER JOIN departments ON employees.department_id = departments.department_id;
```{insert\_element\_17\_}
### 4. Data insertion, update and delete operations- **Insert data**：use `INSERT INTO` Statement inserts new data record into the table。For example，Towards `student` Insert a student information in the table，Can write this way：
```sql
INSERT INTO student (id, name) VALUES (1, 'Zhang San');
If you want to insert multiple pieces of data, you can use multiple pieces of data.INSERT INTOStatements, or use more efficient batch insertion methods.
- Update data: By
UPDATEStatement to modify existing data in the table. For example,studentIn the tableidThe student's name is 1 changed to 'Li Si', and can be usedUPDATE student SET name = 'Li Si' WHERE id = 1Sentence. - Delete data: Use
DELETE FROMStatement deletes data records in the table. For example, deletestudentIn the tableidStudent information of 1 can be usedDELETE FROM student WHERE id = 1Sentence.
4. Other common operations
- Aggregation functions: Dameng database provides a variety of aggregation functions, such as
COUNTUsed to count row counts,SUMUsed to sum,AVGUsed to find the average value,MAXUsed to find the maximum value,MINUsed to find the minimum value, etc. These aggregate functions can be combined withSELECTUse statements together to conduct statistical analysis of the data. For example, if you look up the highest score in a student's grade table, you can useSELECT MAX(score) FROM score_tablestatement. - Alias: You can specify an alias for table names, column names, or expressions in a query to improve the readability of the query results. For example,
SELECT AS student_id, AS student_name FROM student sIn the statement,studentThe table has an aliass, and foridandnameThe columns have alias specified separatelystudent_idandstudent_name. - Subquery: Subquery refers to nesting another query statement in one query statement. Subqueries can be used as conditions, data sources, or temporary tables to implement more complex query logic. For example, to query information for the youngest student, you can use the following subquery:
SELECT * FROM student WHERE age = (SELECT MIN(age) FROM student);
```{insert\_element\_22\_} The above are just some basic and commonly used contents for query and operation of Dameng database. There are many more advanced functions and features in actual applications waiting for everyone to explore and learn. I hope this article can provide some help and reference for readers when using the Dameng database. You can adjust and supplement the above content according to actual conditions to meet your specific needs.
Summarize
This is the article about the query and operation of Dameng database (DM database). For more related Dameng database query and operation content, please search for my previous articles or continue browsing the related articles below. I hope everyone will support me in the future!