Sql 简明教程
SQL - SELECT Database, USE Statement
要在 SQL 中使用数据库,我们需要首先选择要使用的数据库。选择数据库后,我们可以对其执行各种操作,例如创建表、插入数据、更新数据和删除数据。
To work with a database in SQL, we need to first select the database we want to work with. After selecting the database, we can perform various operations on it such as creating tables, inserting data, updating data, and deleting data.
The USE DATABASE Statement
SQL USE DATABASE 语句用于从系统中可用的数据库列表中选择一个数据库。选择数据库后,我们可以对其执行各种操作,例如 creating tables 、 inserting data 、更新数据和 deleting data 。
The SQL USE DATABASE statement is used to select a database from a list of databases available in the system. Once a database is selected, we can perform various operations on it such as creating tables, inserting data, updating data, and deleting data.
Syntax
以下是 SQL 中 USE DATABASE 语句的语法:
Following is the syntax of the USE DATABASE statement in SQL −
USE DatabaseName;
此处, DatabaseName 是我们要选择的数据库的名称。数据库名称在 RDBMS 中始终是唯一的。
Here, the DatabaseName is the name of the database that we want to select. The database name is always unique within the RDBMS.
Example
首先,我们将使用以下 SQL CREATE DATABASE 查询创建一个数据库:
First of all we will create a database using the following SQL CREATE DATABASE query −
CREATE DATABASE testDB;
现在,我们可以按如下方式列出所有可用数据库:
Now, we can list all the available databases as follws −
SHOW DATABASES;
输出将显示为:
The output will be displayed as −
Database |
master |
performance_schema |
information_schema |
mysql |
testDB |
Example: Select/Switch Database
以下查询用于将当前数据库选择/切换到 testDB :
Following query is used to select/switch the current database to testDB −
USE testDB;
Database changed
一旦我们完成切换到数据库 testDB ,就可以执行操作,例如创建表和在该表中插入数据,如下所示:
Once we finish switching to the database testDB we can perform operations such as creating a table, and inserting data in that table as shown below −.
CREATE TABLE CALENDAR(MONTHS DATE NOT NULL);
现在,让我们使用 SQL INSERT 语句在 CALENDAR 表中插入一些记录,如下面的查询所示:
Now, let us insert some records in the CALENDAR table using SQL INSERT statements as shown in the query below −
INSERT INTO CALENDAR(MONTHS) VALUES('2023-01-01');
INSERT INTO CALENDAR(MONTHS) VALUES('2023-02-01');
INSERT INTO CALENDAR(MONTHS) VALUES('2023-03-01');
INSERT INTO CALENDAR(MONTHS) VALUES('2023-04-01');
INSERT INTO CALENDAR(MONTHS) VALUES('2023-12-01');
让我们使用 SQL SELECT 语句列出 CALENDAR 表中的所有记录来验证操作,如下所示:
Let’s verify the operation by listing all the records from CALENDAR table using SQL SELECT statement as shown below −
SELECT * FROM CALENDAR;
输出将显示为:
The output will be displayed as −
MONTHS |
2023-01-01 |
2023-02-01 |
2023-03-01 |
2023-04-01 |
2023-12-01 |