What are the integrity constraints of oracle database tables?

The integrity constraints of Oracle databases can ensure data accuracy, including: NOT NULL: null values ​​are prohibited; UNIQUE: guarantee uniqueness, allowing a single NULL value; PRIMARY KEY: primary key constraint, strengthening UNIQUE, and prohibiting NULL values; FOREIGN KEY: maintaining relationships between tables, foreign keys refer to primary table primary keys; CHECK: limit column values ​​according to conditions.

Integrity constraints of Oracle database tables: in-depth analysis and practical experience

Many developers are often confused about the integrity constraints of tables when they come into contact with Oracle databases. The purpose of this article is to explain in an easy-to-understand manner the mechanisms, application scenarios of various integrity constraints in Oracle databases, as well as some easy-to-understand pitfalls and best practices. After reading this article, you will be able to use these constraints more skillfully to build a more robust and reliable database system.

Let’s start with the basics. The integrity constraints of Oracle databases are essentially to ensure the accuracy, consistency and reliability of data. They are like the "guards" of the database. When data insertion, update, delete and other operations, they strictly check the legality of the data to prevent the generation of dirty data.

NOT NULL constraint: This constraint is the most basic, it ensures that a certain column cannot contain null values ​​(NULL). This is very useful in many scenarios. For example, fields such as user name, product name, etc. should be filled in forcibly. It seems simple, but in actual applications, you need to carefully consider whether it is really necessary to force NOT NULL. If a field allows missing values, forcibly adding NOT NULL will increase the complexity of the application and even affect the efficiency of data entry.

UNIQUE constraint: This constraint ensures that the values ​​of a column or a group of columns must be unique throughout the table. For example, fields such as user's account number and product ID usually require setting UNIQUE constraints to prevent duplicate data. It should be noted that the UNIQUE constraint allows NULL values, but only one NULL value can appear in the same column. When designing a database, be careful to select the columns of UNIQUE constraints to avoid unnecessary constraints.

PRIMARY KEY constraint: The primary key constraint is an enhanced version of UNIQUE constraint. It also has the characteristics of UNIQUE constraints and does not allow NULL values. The primary key is a unique identifier for records in a table and is crucial in database design. A table can only have one primary key, but the primary key can be composed of multiple columns (composite primary key). When selecting a primary key, you need to consider the uniqueness, stability and efficiency of the data. Try to choose short and concise data types as primary keys, such as NUMBER(10) or VARCHAR2(32), and avoid using too long strings as primary keys, which will affect query efficiency.

FOREIGN KEY Constraint: Foreign key constraints are one of the most important integrity constraints in relational databases, and are used to maintain relationships between tables. Foreign Key Constraint Specifies that a column (foreign key) in one table must refer to the primary key (primary table) in another table. For example, the Customer ID column in the order table can be used as a foreign key to refer to the Customer ID column in the Customer table. This ensures that the customer ID in the order table must exist in the customer table, avoiding the occurrence of "orphan records". The implementation of foreign key constraints depends on the cascading operations of the database, such as ON DELETE CASCADE, which can ensure that when the main table record is deleted, the relevant foreign key record is also deleted. But be careful when using cascading operations, as it can lead to unexpected data loss. In complex database systems, the design of foreign key constraints requires careful planning to avoid problems such as circular references.

CHECK constraint: This constraint is used to restrict the value of a column that must meet the specified conditions. For example, a CHECK constraint can be set to ensure that the value of the age field must be greater than or equal to 0. CHECK constraints are very flexible and can customize various complex constraints according to business needs. But excessive use of CHECK constraints may affect the performance of the database, so the pros and cons need to be weighed.

Code Example (PL/SQL):

 <code class="sql">-- 创建一个简单的客户表CREATE TABLE Customers ( CustomerID NUMBER(10) PRIMARY KEY, FirstName VARCHAR2(50) NOT NULL, LastName VARCHAR2(50) NOT NULL, Email VARCHAR2(100) UNIQUE ); -- 创建一个订单表，包含外键约束CREATE TABLE Orders ( OrderID NUMBER(10) PRIMARY KEY, CustomerID NUMBER(10) NOT NULL, OrderDate DATE, CONSTRAINT FK_Orders_Customers FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID) ON DELETE CASCADE ); -- 添加一个CHECK约束，限制订单日期必须小于当前日期ALTER TABLE Orders ADD CONSTRAINT CK_OrderDate CHECK (OrderDate </code>

This code shows how to create and modify tables using various integrity constraints. Remember, in actual applications, appropriate constraints should be selected according to specific business needs and sufficient testing should be carried out to ensure the stability and reliability of the database. Don't blindly add constraints, weigh the benefits and potential performance impact of constraints. Only by fully understanding the mechanisms and potential problems of constraints can we avoid unnecessary troubles in database design and maintenance.

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