MySQL -- basic operation of database

Prepare test data

To learn SQL Query statement , First of all, we must solve a problem , Data problems .

Here's a test.sql file .

Sign in MySQL, Input source xxx/test.sql Import sql file ,sql The file is actually a script file , There are many lines in it SQL sentence , adopt source Commands can be executed in batches .

After execution , Use show databases; View all databases , I found a new one named test The database of .

 

Use show tables; see test All data tables under the database , Found four tables .

Import the database in another way

--  Create data table 
CREATE TABLE IF NOT EXISTS dept (
	deptno		SMALLINT  PRIMARY KEY,
	dname		VARCHAR(14) ,
	loc			VARCHAR(13) ) ;

--  Insert test data  —— dept
INSERT INTO dept VALUES	(10,'ACCOUNTING','NEW YORK');
INSERT INTO dept VALUES (20,'RESEARCH','DALLAS');
INSERT INTO dept VALUES	(30,'SALES','CHICAGO');
INSERT INTO dept VALUES	(40,'OPERATIONS','BOSTON');

test Database table

Later on SQL At the time of statement , The main use is test Four tables under the database are used as cases , So first of all, you must have a basic understanding of the functions of these tables and the data types of columns .

Departmental table ：dept

 

Relevant English Translation ：

• accounting accounting new york New York

• research researcher dallas Dalas

• sales sales chicago Chicago

• operarions operating boston Boston

Employee list ：emp

 

DECLMAL(7,2) All in all 7 position , There are two digits after the decimal point

SMALLINT, Two bytes ,65535

Relevant English Translation ：

• clerk clerk

• salesman The salesman

• manager The manager

• analyst Laboratory Technician

• president Chairman of the board of directors

Pay scale ：salgrade

 

Wage subsidy table ( Pay slip )：bonus

There is no data in the wage subsidy table

SQL Statement specification

Use SQL Statement please follow the following specifications ：

• SQL Statement is case insensitive . But string constants are case sensitive , It is recommended that the order be capitalized , Table name 、 Database name in lowercase ;

• SQL Statements can be written in one or more lines , Every statement should end with a semicolon ;

• Use spaces and indentation to improve the readability of statements .

• notes ： There are three styles of annotation

  1. A single line comment can be made with “#”

  2. The second way to write a single line note is “-- ”,“--" There is a space between and comments .

  3. Multiline comments can be made with /* */

Basic operation of database

stay MySQL There are many databases , You can use the following command to view all databases ：

SHOW DATABASES;
+--------------------+
| Database           |
+--------------------+
| information_schema |		# It mainly stores some database object information in the system , For example, user table information 、 Column information 、 Permission information 、 Character set information, partition information, etc .
| mysql              |		#MySQL The core database of , Mainly responsible for storing database users 、 User access rights, etc  MySQL  Control and management information that you need to use .
| performance_schema |		# Mainly used to collect database server performance parameters .
| sys                |		#sys  The database mainly provides some views , The data comes from  performation_schema, The main purpose is to make it easier for developers and users to view performance problems .
+--------------------+

These databases can be switched between each other , The basic syntax of switching is as follows ：

USE <dbname>;

You want to know which database you are currently using , Then you can use select database(); Order to see ：

SELECT DATABASE();

+------------+
| DATABASE() |
+------------+
| test       |
+------------+
1 row in set (0.00 sec)

There must be multiple data tables under one database , At this time, you can also directly use the following command to view all data tables ：

SHOW TABLES;

+----------------+
| Tables_in_test |
+----------------+
| bonus          |
| dept           |
| emp            |
| salgrade       |
+----------------+
4 rows in set (0.02 sec)

View the information of a specific table

SELECT * FROM emp;

+-------+--------+-----------+------+------------+------+------+--------+
| empno | ename  | job       | mgr  | hiredate   | sal  | comm | deptno |
+-------+--------+-----------+------+------------+------+------+--------+
|  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800 | NULL |     20 |
|  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600 |  300 |     30 |
|  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250 |  500 |     30 |
|  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975 | NULL |     20 |
|  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 | 1250 | 1400 |     30 |
|  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 | 2850 | NULL |     30 |
|  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 | 2450 | NULL |     10 |
|  7788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 | 3000 | NULL |     20 |
|  7839 | KING   | PRESIDENT | NULL | 1981-11-17 | 5000 | NULL |     10 |
|  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500 |    0 |     30 |
|  7876 | ADAMS  | CLERK     | 7788 | 1987-05-23 | 1100 | NULL |     20 |
|  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950 | NULL |     30 |
|  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000 | NULL |     20 |
|  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300 | NULL |     10 |
+-------+--------+-----------+------+------------+------+------+--------+
14 rows in set (0.01 sec)

If you want to know the table structure of some data tables / Field type , Then you can use DESC command ：

DESC emp;

+----------+-------------+------+-----+---------+-------+
| Field    | Type        | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| empno    | smallint    | NO   | PRI | NULL    |       |
| ename    | varchar(10) | YES  |     | NULL    |       |
| job      | varchar(9)  | YES  |     | NULL    |       |
| mgr      | smallint    | YES  |     | NULL    |       |
| hiredate | date        | YES  |     | NULL    |       |
| sal      | smallint    | YES  |     | NULL    |       |
| comm     | smallint    | YES  |     | NULL    |       |
| deptno   | smallint    | YES  |     | NULL    |       |
+----------+-------------+------+-----+---------+-------+
8 rows in set (0.01 sec)

If you want to create your own database , You need to use the following command ：

CREATE DATABASE <dbname>;

If the database already exists, an error will be reported Can't create database '<dbname>'; database exists, We can first judge whether the database exists , Create... If it doesn't exist , Being ignores ( Only warning without error ).

CREATE DATABASE IF NOT EXISTS  <dbname>;

If you want to delete the database , You can use the following command ：

DROP DATABASE <dbname>;

If the database does not exist, an error will be reported Can't drop database 'ss'; database doesn't exist, When deleting the database, you can determine whether it exists .  

DROP DATABASE IF EXISTS <dbname>;   # If the database exists, delete 

Use DROP DATABASE Be very careful when ordering , After executing the order ,MySQL No prompt confirmation message will be given .DROP DATABASE After deleting the database , All data tables and data stored in the database will also be deleted , And it can't be restored .