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20180221 Getting started with SQL.md 翻译完毕
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Translating by MjSeven
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Getting started with SQL
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======
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Building a database using SQL is simpler than most people think. In fact, you don't even need to be an experienced programmer to use SQL to create a database. In this article, I'll explain how to create a simple relational database management system (RDMS) using MySQL 5.6. Before I get started, I want to quickly thank [SQL Fiddle][1], which I used to run my script. It provides a useful sandbox for testing simple scripts.
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In this tutorial, I'll build a database that uses the simple schema shown in the entity relationship diagram (ERD) below. The database lists students and the course each is studying. I used two entities (i.e., tables) to keep things simple, with only a single relationship and dependency. The entities are called `dbo_students` and `dbo_courses`.
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The multiplicity of the database is 1-to-many, as each course can contain many students, but each student can study only one course.
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A quick note on terminology:
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1. A table is called an entity.
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2. A field is called an attribute.
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3. A record is called a tuple.
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4. The script used to construct the database is called a schema.
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### Constructing the schema
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To construct the database, use the `CREATE TABLE <table name>` command, then define each field name and data type. This database uses `VARCHAR(n)` (string) and `INT(n)` (integer), where n refers to the number of values that can be stored. For example `INT(2)` could be 01.
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This is the code used to create the two tables:
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```
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CREATE TABLE dbo_students
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(
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student_id INT(2) AUTO_INCREMENT NOT NULL,
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student_name VARCHAR(50),
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course_studied INT(2),
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PRIMARY KEY (student_id)
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);
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CREATE TABLE dbo_courses
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(
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course_id INT(2) AUTO_INCREMENT NOT NULL,
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course_name VARCHAR(30),
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PRIMARY KEY (course_id)
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);
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```
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`NOT NULL` means that the field cannot be empty, and `AUTO_INCREMENT` means that when a new tuple is added, the ID number will be auto-generated with 1 added to the previously stored ID number in order to enforce referential integrity across entities. `PRIMARY KEY` is the unique identifier attribute for each table. This means each tuple has its own distinct identity.
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### Relationships as a constraint
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As it stands, the two tables exist on their own with no connections or relationships. To connect them, a foreign key must be identified. In `dbo_students`, the foreign key is `course_studied`, the source of which is within `dbo_courses`, meaning that the field is referenced. The specific command within SQL is called a `CONSTRAINT`, and this relationship will be added using another command called `ALTER TABLE`, which allows tables to be edited even after the schema has been constructed.
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The following code adds the relationship to the database construction script:
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```
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ALTER TABLE dbo_students
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ADD CONSTRAINT FK_course_studied
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FOREIGN KEY (course_studied) REFERENCES dbo_courses(course_id);
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```
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Using the `CONSTRAINT` command is not actually necessary, but it's good practice because it means the constraint can be named and it makes maintenance easier. Now that the database is complete, it's time to add some data.
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### Adding data to the database
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`INSERT INTO <table name>` is the command used to directly choose which attributes (i.e., fields) data is added to. The entity name is defined first, then the attributes. Underneath this command is the data that will be added to that entity, creating a tuple. If `NOT NULL` has been specified, it means that the attribute cannot be left blank. The following code shows how to add records to the table:
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```
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INSERT INTO dbo_courses(course_id,course_name)
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VALUES(001,'Software Engineering');
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INSERT INTO dbo_courses(course_id,course_name)
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VALUES(002,'Computer Science');
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INSERT INTO dbo_courses(course_id,course_name)
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VALUES(003,'Computing');
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INSERT INTO dbo_students(student_id,student_name,course_studied)
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VALUES(001,'student1',001);
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INSERT INTO dbo_students(student_id,student_name,course_studied)
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VALUES(002,'student2',002);
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INSERT INTO dbo_students(student_id,student_name,course_studied)
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VALUES(003,'student3',002);
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INSERT INTO dbo_students(student_id,student_name,course_studied)
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VALUES(004,'student4',003);
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```
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Now that the database schema is complete and data is added, it's time to run queries on the database.
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### Queries
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Queries follow a set structure using these commands:
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```
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SELECT <attributes>
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FROM <entity>
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WHERE <condition>
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```
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To display all records within the `dbo_courses` entity and display the course code and course name, use an asterisk. This is a wildcard that eliminates the need to type all attribute names. (Its use is not recommended in production databases.) The code for this query is:
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```
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SELECT *
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FROM dbo_courses
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```
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The output of this query shows all tuples in the table, so all available courses can be displayed:
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```
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| course_id | course_name |
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|-----------|----------------------|
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| 1 | Software Engineering |
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| 2 | Computer Science |
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| 3 | Computing |
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```
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In a future article, I'll explain more complicated queries using one of the three types of joins: Inner, Outer, or Cross.
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Here is the completed script:
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```
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CREATE TABLE dbo_students
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(
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student_id INT(2) AUTO_INCREMENT NOT NULL,
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student_name VARCHAR(50),
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course_studied INT(2),
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PRIMARY KEY (student_id)
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);
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CREATE TABLE dbo_courses
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(
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course_id INT(2) AUTO_INCREMENT NOT NULL,
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course_name VARCHAR(30),
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PRIMARY KEY (course_id)
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);
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ALTER TABLE dbo_students
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ADD CONSTRAINT FK_course_studied
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FOREIGN KEY (course_studied) REFERENCES dbo_courses(course_id);
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INSERT INTO dbo_courses(course_id,course_name)
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VALUES(001,'Software Engineering');
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INSERT INTO dbo_courses(course_id,course_name)
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VALUES(002,'Computer Science');
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INSERT INTO dbo_courses(course_id,course_name)
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VALUES(003,'Computing');
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INSERT INTO dbo_students(student_id,student_name,course_studied)
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VALUES(001,'student1',001);
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INSERT INTO dbo_students(student_id,student_name,course_studied)
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VALUES(002,'student2',002);
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INSERT INTO dbo_students(student_id,student_name,course_studied)
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VALUES(003,'student3',002);
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INSERT INTO dbo_students(student_id,student_name,course_studied)
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VALUES(004,'student4',003);
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SELECT *
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FROM dbo_courses
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```
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### Learning more
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SQL isn't difficult; I think it is simpler than programming, and the language is universal to different database systems. Note that `dbo.<entity>` is not a required entity-naming convention; I used it simply because it is the standard in Microsoft SQL Server.
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If you'd like to learn more, the best guide this side of the internet is [W3Schools.com][2]'s comprehensive guide to SQL for all database platforms.
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Please feel free to play around with my database. Also, if you have suggestions or questions, please respond in the comments.
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/18/2/getting-started-sql
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作者：[Aaron Cocker][a]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]:https://opensource.com/users/aaroncocker
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[1]:http://sqlfiddle.com
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[2]:https://www.w3schools.com/sql/default.asp
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239
translated/tech/20180221 Getting started with SQL.md
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239
translated/tech/20180221 Getting started with SQL.md
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开始使用 SQL
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======
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使用 SQL 构建数据库比大多数人想象得要简单。实际上，你甚至不需要成为一个有经验的程序员来使用 SQL 创建数据库。在本文中，我将解释如何使用 MySQL 5.6 来创建简单的关系型数据库管理系统（RDMS）。在开始之前，我想快速地感谢 [SQL Fiddle][1]，这是我用来运行脚本的工具。它提供了一个用于测试简单脚本的有用的沙箱。
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在本教程中，我将构建一个使用下面实体关系图（ERD）中显示的简单架构的数据库。数据库列出了学生和正在学习的课程。为了保持简单，我使用了两个实体（即表），只有一种关系和依赖关系。这两个实体称为 `dbo_students` 和 `dbo_courses`。
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数据库的多样性是一对多的，因为每门课程可以包含很多学生，但每个学生只能学习一门课程。
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关于术语的快速说明：
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1. 一张表称为一个实体。
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2. 一个字段称为一个属性。
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3. 一条记录称为一个元组。
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4. 用于构建数据库的脚本称为架构。
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### 构建架构
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要构建数据库，使用 `CREATE TABLE <表名>` 命令，然后定义每个字段的名称和数据类型。数据库使用 `VARCHAR(n)` （字符串）和 `INT(n)` （整数），其中 n 表示可以存储的值的长度。例如 `INT(2)` 可能是 01。
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这是用于创建两个表的代码：
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```
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CREATE TABLE dbo_students
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(
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student_id INT(2) AUTO_INCREMENT NOT NULL,
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student_name VARCHAR(50),
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course_studied INT(2),
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PRIMARY KEY (student_id)
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);
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CREATE TABLE dbo_courses
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(
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course_id INT(2) AUTO_INCREMENT NOT NULL,
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course_name VARCHAR(30),
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PRIMARY KEY (course_id)
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);
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```
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`NOT NULL` 意味着字段不能为空，`AUTO_INCREMENT` 意味着当一个新的元组被添加时，ID 号将自动生成，并将 1 添加到先前存储的 ID 号，来强化各实体之间的完整参照性。 `PRIMARY KEY` 是每个表的惟一标识符属性。这意味着每个元组都有自己的不同的标识。
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### 关系作为一种约束
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就目前来看，这两张表格是独立存在的，没有任何联系或关系。要连接它们，必须标识一个外键。在 `dbo_students` 中，外键是 `course_studied`，其来源在 `dbo_courses` 中，意味着该字段被引用。SQL 中的特定命令为 `CONSTRAINT`，并且将使用另一个名为 `ALTER TABLE` 的命令添加这种关系，这样即使在架构构建完毕后，也可以编辑表。
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以下代码将关系添加到数据库构造脚本中：
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```
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ALTER TABLE dbo_students
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ADD CONSTRAINT FK_course_studied
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FOREIGN KEY (course_studied) REFERENCES dbo_courses(course_id);
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```
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使用 `CONSTRAINT` 命令实际上并不是必要的，但这是一个好习惯，因为它意味着约束可以被命名并且使维护更容易。现在数据库已经完成了，是时候添加一些数据了。
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### 将数据添加到数据库
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`INSERT INTO <表名>`是用于直接选择将数据添加到哪些属性（即字段）的命令。首先声明实体名称，然后声明属性。命令下边是添加到实体的数据，从而创建一个元组。如果指定了 `NOT NULL`，这表示该属性不能留空。以下代码将展示如何向表中添加记录：
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```
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INSERT INTO dbo_courses(course_id,course_name)
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VALUES(001,'Software Engineering');
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INSERT INTO dbo_courses(course_id,course_name)
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|
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VALUES(002,'Computer Science');
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|
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INSERT INTO dbo_courses(course_id,course_name)
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VALUES(003,'Computing');
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INSERT INTO dbo_students(student_id,student_name,course_studied)
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VALUES(001,'student1',001);
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INSERT INTO dbo_students(student_id,student_name,course_studied)
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VALUES(002,'student2',002);
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INSERT INTO dbo_students(student_id,student_name,course_studied)
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VALUES(003,'student3',002);
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INSERT INTO dbo_students(student_id,student_name,course_studied)
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VALUES(004,'student4',003);
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```
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现在数据库架构已经完成并添加了数据，现在是时候在数据库上运行查询了。
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### 查询
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查询遵循使用以下命令的集合结构：
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```
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SELECT <attributes>
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FROM <entity>
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WHERE <condition>
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```
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要显示 `dbo_courses` 实体内的所有记录并显示课程代码和课程名称，请使用 * 。 这是一个通配符，它消除了键入所有属性名称的需要。（在生产数据库中不建议使用它。）此处查询的代码是：
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```
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SELECT *
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FROM dbo_courses
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```
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此处查询的输出显示表中的所有元组，因此可显示所有可用课程：
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```
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| course_id | course_name |
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|-----------|----------------------|
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| 1 | Software Engineering |
|
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|
||||
| 2 | Computer Science |
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|
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| 3 | Computing |
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```
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在以后的文章中，我将使用三种类型的连接之一来解释更复杂的查询：Inner,Outer 或 Cross。
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这是完整的脚本：
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||||
```
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CREATE TABLE dbo_students
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|
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(
|
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|
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student_id INT(2) AUTO_INCREMENT NOT NULL,
|
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|
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student_name VARCHAR(50),
|
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|
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course_studied INT(2),
|
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|
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PRIMARY KEY (student_id)
|
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|
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);
|
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|
||||
|
||||
|
||||
CREATE TABLE dbo_courses
|
||||
|
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(
|
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|
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course_id INT(2) AUTO_INCREMENT NOT NULL,
|
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|
||||
course_name VARCHAR(30),
|
||||
|
||||
PRIMARY KEY (course_id)
|
||||
|
||||
);
|
||||
|
||||
|
||||
|
||||
ALTER TABLE dbo_students
|
||||
|
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ADD CONSTRAINT FK_course_studied
|
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|
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FOREIGN KEY (course_studied) REFERENCES dbo_courses(course_id);
|
||||
|
||||
|
||||
|
||||
INSERT INTO dbo_courses(course_id,course_name)
|
||||
|
||||
VALUES(001,'Software Engineering');
|
||||
|
||||
INSERT INTO dbo_courses(course_id,course_name)
|
||||
|
||||
VALUES(002,'Computer Science');
|
||||
|
||||
INSERT INTO dbo_courses(course_id,course_name)
|
||||
|
||||
VALUES(003,'Computing');
|
||||
|
||||
|
||||
|
||||
INSERT INTO dbo_students(student_id,student_name,course_studied)
|
||||
|
||||
VALUES(001,'student1',001);
|
||||
|
||||
INSERT INTO dbo_students(student_id,student_name,course_studied)
|
||||
|
||||
VALUES(002,'student2',002);
|
||||
|
||||
INSERT INTO dbo_students(student_id,student_name,course_studied)
|
||||
|
||||
VALUES(003,'student3',002);
|
||||
|
||||
INSERT INTO dbo_students(student_id,student_name,course_studied)
|
||||
|
||||
VALUES(004,'student4',003);
|
||||
|
||||
|
||||
|
||||
SELECT *
|
||||
|
||||
FROM dbo_courses
|
||||
```
|
||||
|
||||
### 学习更多
|
||||
|
||||
SQL 并不困难；我认为它比编程简单，并且该语言对于不同的数据库系统是通用的。 请注意，`dbo.<实体>` (译者注：文章中使用的是 `dbo_<实体>`) 不是必需的实体命名约定；我之所以使用，仅仅是因为它是 Microsoft SQL Server 中的标准。
|
||||
|
||||
如果你想了解更多，在网络上这方面的最佳指南是 [W3Schools.com][2] 中对所有数据库平台的 SQL 综合指南。
|
||||
|
||||
请随意使用我的数据库。另外，如果你有任何建议或疑问，请在评论中回复。(译注：请点击原文地址进行评论回应)
|
||||
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
via: [https://opensource.com/article/18/2/getting-started-sql](https://opensource.com/article/18/2/getting-started-sql)
|
||||
|
||||
作者：[Aaron Cocker][a]
|
||||
译者：[MjSeven](https://github.com/MjSeven)
|
||||
校对：[校对者ID](https://github.com/校对者ID)
|
||||
|
||||
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
|
||||
|
||||
[a]:https://opensource.com/users/aaroncocker
|
||||
[1]:http://sqlfiddle.com
|
||||
[2]:https://www.w3schools.com/sql/default.asp
|
||||
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