What are the types of complete positive constraints for oracle databases?

Integrity constraints of Oracle database: NOT NULL: NULL values ​​are prohibited to ensure the integrity of the key fields. UNIQUE: Ensure that the column value or column combination is unique, and is often used in fields such as username and address. PRIMARY KEY: Uniquely identify each record in the table, it is an enhanced version of UNIQUE constraints, and is often used for the primary key of the table. FOREIGN KEY: Establish a parent-child table relationship to ensure that the child table data is consistent with the parent table primary key value, and avoid data silos. CHECK: Restrict column values ​​to meet specific conditions to ensure the rationality of the data, such as age greater than 0 or gender of 'male' or 'female'.

Oracle Database Integrity Constraint: In-depth Analysis and Practice

Many developers are often confused by various integrity constraints when they come into contact with Oracle databases. The purpose of this article is to help you thoroughly understand the integrity constraints of Oracle databases, so that you can be at ease in database design and development, and avoid those crazy pitfalls. After reading this article, you will have an in-depth understanding of NOT NULL, UNIQUE, PRIMARY KEY, FOREIGN KEY, the principles, application scenarios, and potential problems of CHECK constraints, and be able to write more robust and efficient database code.

Let's review the basics first. The integrity constraints of Oracle databases are rules set to ensure the accuracy, consistency and reliability of data. They are enforced at the database level to prevent dirty data from entering the database. This is like building a house. The integrity constraints are foundations and beams and columns, ensuring the stability of the building.

NOT NULL constraint is the simplest constraint, which ensures that NULL values ​​cannot be stored in columns. This is very important in some key fields such as user ID, order number, etc. However, it should be noted that too many NOT NULL constraints may increase the burden of data entry and need to be used with caution according to actual conditions. For example, an optional address field does not necessarily require NOT NULL constraints.

 <code class="sql">CREATE TABLE users ( user_id NUMBER NOT NULL, username VARCHAR2(50) NOT NULL, email VARCHAR2(100) );</code>

The UNIQUE constraint ensures that the values ​​in a column or combination of columns must be unique. This is often used in fields such as username, email address, etc. to ensure the uniqueness of the data. But be aware that the UNIQUE constraint allows NULL values, and if absolute uniqueness is required, the PRIMARY KEY constraint should be used.

 <code class="sql">CREATE TABLE products ( product_id NUMBER PRIMARY KEY, product_name VARCHAR2(100) UNIQUE );</code>

The PRIMARY KEY constraint is an enhanced version of the UNIQUE constraint. It not only ensures that the values ​​in a column or combination of columns are unique, but also serves as the primary key of the table to identify each row record in the table. The primary key must be NOT NULL and UNIQUE. A table can only have one primary key, but can have multiple UNIQUE constraints. When selecting a primary key, factors such as data type, uniqueness, business meaning should be considered. A bad primary key selection may affect the performance of the entire database.

 <code class="sql">CREATE TABLE orders ( order_id NUMBER PRIMARY KEY, customer_id NUMBER, order_date DATE );</code>

FOREIGN KEY constraint is used to establish a relationship between two tables to ensure the consistency of data. The value of the foreign key column must match the primary key value of another table. This is very important in relational databases and can avoid the emergence of data silos. However, be careful that foreign key constraints may reduce the performance of the database and need to be used with caution according to actual conditions. Especially the foreign key correlation between large tables may bring about performance bottlenecks and require careful trade-offs. In addition, cascading operations (ON DELETE CASCADE, ON UPDATE CASCADE) also require caution to avoid accidental data loss.

 <code class="sql">CREATE TABLE order_items ( order_item_id NUMBER PRIMARY KEY, order_id NUMBER, product_id NUMBER, FOREIGN KEY (order_id) REFERENCES orders(order_id), FOREIGN KEY (product_id) REFERENCES products(product_id) );</code>

The CHECK constraint allows you to define a condition that the value of the restriction column must meet this condition. This can be used to ensure the validity of the data, such as age must be greater than 0 and gender must be 'male' or 'female'. However, it should be noted that too many CHECK constraints may affect the performance of the database and need to be used with caution according to actual conditions. Complex CHECK constraints can also be difficult to maintain and understand.

 <code class="sql">CREATE TABLE employees ( employee_id NUMBER PRIMARY KEY, age NUMBER CHECK (age > 0), gender VARCHAR2(10) CHECK (gender IN ('男', '女')) );</code>

In practical applications, it is very important to choose the right integrity constraint type. It is necessary to carefully select and use according to business needs and data characteristics. Remember, too much constraints may degrade the performance of the database, while too few constraints may lead to inconsistent data. A good database design requires finding a balance between performance and data integrity. Only by fully understanding the advantages and disadvantages of various constraints and constantly accumulating experience in practice can we become a true database expert.

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