Mysql 简明教程

MySQL − Queries

MySQL 是一个开放源码的关系管理系统 (RDBMS)，允许我们存储和管理数据或信息。MySQL 中的查询是用于从数据库表中检索或操作数据的命令。

MySQL is an open-source relational management system (RDBMS) that allows us to store and manage data or information. The queries in MySQL are commands that are used to retrieve or manipulate the data from a database table.

以下是在 MySQL 中常用的命令：SELECT、UPDATE、DELETE、INSERT INTO、CREATE TABLE、ALTER TABLE、DROP TABLE、CREATE DATABASE、ALTER DATABASE、CREATE INDEX、DROP INDEX 等。

Following are the commonly used commands in MySQL: SELECT, UPDATE, DELETE, INSERT INTO, CREATE TABLE, ALTER TABLE, DROP TABLE, CREATE DATABASE, ALTER DATABASE, CREATE INDEX, DROP INDEX, etc.

Note: 这些关键字不区分大小写。例如， create table 与 CREATE TABLE 相同。

Note: These keywords are not case-sensitive. For instance, create table is the same as CREATE TABLE.

MySQL Create Database

MySQL 中的 create database 查询可用于在 MySQL 服务器中创建一个数据库。

The create database query in MySQL can be used to create a database in the MySQL server.

Syntax

以下为此查询的语法：

Following is the syntax for the query −

CREATE DATABASE databasename;

Example

在下面的查询中，我们正在创建一个名为 tutorials 的数据库。

In the following query, we are creating a database named tutorials.

CREATE DATABASE tutorials;

MySQL Use Database

MySQL use database 查询用于选择创建一个数据库以执行创建、插入、更新表或视图等操作。

The MySQL use database query is used to select a database to perform operations such as creating, inserting, updating tables or views, etc.

Syntax

以下为此查询的语法：

Following is the syntax for the query −

USE database_name;

Example

下面的查询选择名为 tutorials 的数据库：

The following query selects a database named tutorials −

USE tutorials;

MySQL Create Query

MySQL create 查询可用于创建数据库、表、索引、视图等。

The MySQL create query can be used to create databases, tables, indexes, views, etc.

Syntax

以下为此查询的语法：

Following is the syntax for the query −

CREATE [table table_name |index index_name | view view_name];

Example

这里，我们正在使用以下 CREATE 查询创建一个名为 STUDENTS 的表：

Here, we are creating a table named STUDENTS using the following CREATE query −

CREATE TABLE CUSTOMERS (
   ID int,
   NAME varchar(20),
   AGE int,
   PRIMARY KEY (ID)
);

MySQL Insert Query

MySQL insert 查询可用于插入特定表中的记录。

The MySQL insert query can be used to insert records within a specified table.

Syntax

以下为此查询的语法：

Following is the syntax for the query −

INSERT INTO table_name (column1, column2, column3, ...)
VALUES (value1, value2, value3, ...);

Example

在以下查询中，我们将一些记录插入一个名为 CUSTOMERS 的表中：

In the following query, we are inserting some records into a table named CUSTOMERS −

INSERT INTO CUSTOMERS (ID, NAME, AGE) VALUES (1, "Nikhilesh", 28);
INSERT INTO STUDENTS (ID, NAME, AGE) VALUES (2, "Akhil", 23);
INSERT INTO STUDENTS (ID, NAME, AGE) VALUES (3, "Sushil", 35);

MySQL Update Query

MySQL update query 可用于修改特定表中的现有记录。

The MySQL update query can be used to modify the existing records in a specified table.

Syntax

以下为此查询的语法：

Following is the syntax for the query −

UPDATE table_name
SET column1 = value1, column2 = value2, ...
WHERE condition;

Example

UPDATE CUSTOMERS SET NAME = "Nikhil" WHERE ID = 1;

MySQL Alter Query

MySQL 中的 ALTER query 可用于添加、删除或修改现有表中的列。

The ALTER query in MySQL can be used to add, delete, or modify columns in an existing table.

Syntax

以下为此查询的语法：

Following is the syntax for the query −

ALTER TABLE table_name
[ADD|DROP] column_name datatype;

Example

在这里，我们尝试向现有的 CUSTOMERS 表中添加一个名为 ADDRESS 的列。

Here, we are trying to add a column named ADDRESS to the existing CUSTOMERS table.

ALTER TABLE CUSTOMERS
ADD COLUMN ADDRESS varchar(50);

MySQL Delete Query

MySQL 中的 Delete query 可用于删除特定表中的现有记录。

The Delete query in MySQL can be used to delete existing records in a specified table.

Syntax

以下为此查询的语法：

Following is the syntax for the query −

DELETE FROM table_name WHERE condition;

Example

在以下查询中，我们从 CUSTOMERS 表中删除了一个记录，其中 ID 等于 3。

In the following query, we are deleting a record from CUSTOMERS table where the ID is equal to 3.

DELETE FROM CUSTOMERS WHERE ID = 3;

MySQL Truncate Table Query

MySQL truncate table 查询可用于删除所有记录，但不删除表本身。

The MySQL truncate table query can be used to remove all the records but not the table itself.

Syntax

以下为此查询的语法：

Following is the syntax for the query −

TRUNCATE [TABLE] table_name;

Example

在以下查询中，我们使用 truncate table 查询从 CUSTOMERS 表中删除所有记录：

In the following query, we are removing all the records from the CUSTOMERS table using the truncate table query −

TRUNCATE TABLE CUSTOMERS;

MySQL Drop Query

MySQL drop query 用于删除数据库中的现有表。

The MySQL drop query is used to delete an existing table in a database.

Syntax

以下为此查询的语法：

Following is the syntax for the query −

DROP TABLE table_name;

Example

在这里，我们尝试使用 drop table 查询删除名为 CUSTOMERS 的表。

Here, we are trying to delete the table named CUSTOMERS using the drop table query.

DROP TABLE CUSTOMERS;