Discuss in detail the various methods of Oracle default value setting

Oracle is one of the most popular and powerful relational databases in the world. In Oracle, whenever we insert new data into a table, we often need to assign default values ​​to certain columns in the table. These default values ​​can be specified when the table is created, or added in the ALTER statement. In this article, we will discuss in detail the various methods of Oracle default value setting and their uses.

1. Specify default values ​​when creating the table

In Oracle, we can use the DEFAULT statement to define default values ​​for the columns in the table when creating the table. In the following example, we will create a table named student and define a default value of 'A' for the column named Grade using the DEFAULT keyword:

CREATE TABLE student (
    name VARCHAR2(50),
    age NUMBER,
    Grade VARCHAR2(2) DEFAULT 'A'
);

In this example, when we specify When inserting a new record into the table, if the value of the Grade column is not specified, Oracle will assign it the default value 'A'.

2. Use the ALTER statement to add default values ​​to existing tables

In addition to defining default values ​​when the table is created, we can also use the ALTER statement to define default values ​​for columns in existing tables. . The following example demonstrates how to use the ALTER statement to set a default value for the Grade column in the existing table student:

ALTER TABLE student
    MODIFY Grade DEFAULT 'A';

In this example, if we insert a new record into the student table, if the Grade column is not specified value, Oracle will automatically assign it the default value 'A' instead of NULL.

3. Specify default values ​​for columns in the INSERT statement

We can also use the DEFAULT keyword in the INSERT statement to specify default values ​​for columns. The following example illustrates the use of DEFAULT to insert a new record into a table:

INSERT INTO student(name, age, Grade)
    VALUES('小明', 18, DEFAULT);

In this example, we have used DEFAULT in the INSERT statement to specify a default value for the column named Grade, which will cause Oracle to automatically Assign the default value 'A'.

4. Use sequences to assign default values ​​to columns

In Oracle, you can use sequences to assign default values ​​to columns. A sequence is an object used to generate unique numbers, we can define a sequence and specify a default value for a column using its NEXTVAL function. The following example demonstrates how to use a sequence to specify a default value for a column named ID:

CREATE SEQUENCE student_seq
    START WITH 1
    INCREMENT BY 1;

ALTER TABLE student
    MODIFY ID DEFAULT student_seq.NEXTVAL;

In this example, we create a sequence named student_seq and use it in an ALTER statement as a sequence named The default value for the ID column. Whenever we insert a new record into the student table, if the value of the ID column is not specified, Oracle will call the student_seq.NEXTVAL function to generate a unique value for it and assign it to the ID column.

Conclusion:

In Oracle, we can assign default values ​​to columns in many ways. Defining default values ​​when the table is created is one of the most common ways, and using the ALTER statement to define default values ​​in an existing table extends this flexibility. Using the DEFAULT keyword is also a convenient way to specify a default value for a column in an INSERT statement. Finally, using a sequence to assign default values ​​to columns is an effective way to guarantee data integrity by ensuring that each record has a unique numeric identifier.

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