oracle delete table field

Oracle database is a commonly used relational database management system that is widely used in enterprise-level software development. In database management, the operation of deleting a table is relatively common, but when deleting certain fields in the table, you need to pay attention to some matters.

1. Modify the table structure

In Oracle database, the command to modify the table structure can use the ALTER TABLE statement, the syntax is as follows:

ALTER TABLE table_name

 ADD (column_name column_datatype [default_expr], 
      column_name column_datatype [default_expr])；

ALTER TABLE table_name

MODIFY (column_name datatype [DEFAULT expr] 
        [inline_contraint] [,] 
        column_name datatype [DEFAULT expr] 
        [inline_contraint] ...);

ALTER TABLE table_name

DROP COLUMN column_name;

The first command is to add a column, the second command is to modify the column type, default value, constraints and other information, and the third command is to add columns. The first command is to delete columns.

It should be noted that if you need to delete a column in the table, using the ALTER TABLE statement will directly remove the column and the constraints related to this column. If the column has FOREIGN KEY or CHECK constraints, you need Manually remove these constraints before deleting the column.

2. Use the CREATE TABLE AS SELECT statement

In addition to using the ALTER TABLE statement to delete certain fields of the table, you can also use the CREATE TABLE AS SELECT statement to delete certain fields of the table. function.

CREATE TABLE table_name_new
AS SELECT column_name1, column_name2, ...
FROM table_name_old;

The above command will create a new table named table_name_new in the database. The fields are specified fields such as column_name1, column_name2, etc., and the data comes from other fields in the table_name_old table except the excluded fields.

For example, in a table named employees, you need to delete the salary field, which can be achieved by using the following command:

CREATE TABLE employees_new
AS SELECT employee_id, last_name, first_name, hire_date , job_id, department_id
FROM employees;

In the new employees_new table, only the employee_id, last_name, first_name, hire_date, job_id and department_id fields are retained, while the excluded salary field is deleted.

It should be noted that when using the CREATE TABLE AS SELECT statement to delete certain fields, you must also pay attention to the constraints of these fields. If the field has FOREIGN KEY or CHECK constraints, you also need to delete the field before deleting it. Remove these constraints manually.

Through the above introduction, we can see that there are two main ways to delete certain fields of a table in the Oracle database. One is to delete through the ALTER TABLE statement, which requires manual removal of relevant constraints; the other is to delete certain fields of the table in the Oracle database. One is to use the CREATE TABLE AS SELECT statement to create a new table that does not contain the fields to be deleted. Either way, care needs to be taken during actual operation to ensure the integrity and correctness of the data.

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