How to Create A Table In Oracle?

To create a table in Oracle, you will need to use the SQL command CREATE TABLE followed by the name of the table you want to create. You will then need to specify the column names and data types for each column in the table. You can also specify constraints such as primary keys, foreign keys, and check constraints when creating the table. Once you have defined all the columns and constraints, you can execute the SQL statement to create the table in your Oracle database.

How to create a table with default values in Oracle?

To create a table with default values in Oracle, you can include the DEFAULT constraint in the column definition. Here is an example of how you can create a table with default values for some columns:

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CREATE TABLE my_table (
    id NUMBER PRIMARY KEY,
    name VARCHAR2(50) DEFAULT 'John Doe',
    age NUMBER DEFAULT 30,
    created_date DATE DEFAULT SYSDATE
);

In this example:

• The "name" column will have a default value of 'John Doe' if no value is provided during insertion.
• The "age" column will have a default value of 30 if no value is provided during insertion.
• The "created_date" column will have a default value of the current date and time (using the SYSDATE function) if no value is provided during insertion.

You can modify the DEFAULT value for a column using the ALTER TABLE statement:

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ALTER TABLE my_table
MODIFY name VARCHAR2(50) DEFAULT 'Jane Doe';

This will change the default value for the "name" column to 'Jane Doe'.

How to create a table with indexes in Oracle?

To create a table with indexes in Oracle, you can use the following SQL syntax:

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CREATE TABLE table_name (
    column1 datatype,
    column2 datatype,
    column3 datatype,
    PRIMARY KEY (column1),
    INDEX index_name (column2)
);

In this syntax:

• table_name is the name of the table you want to create.
• column1, column2, and column3 are the names of the columns in the table, along with their data types.
• PRIMARY KEY (column1) specifies that column1 is the primary key of the table.
• INDEX index_name (column2) creates an index named index_name on column2.

You can also create unique indexes by using the UNIQUE keyword before the INDEX keyword:

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CREATE TABLE table_name (
    column1 datatype,
    column2 datatype,
    column3 datatype,
    PRIMARY KEY (column1),
    UNIQUE INDEX index_name (column2)
);

After creating the table with indexes, you can insert data into the table and the index will be automatically updated to reflect the new data.

How to create a table with default values for columns in Oracle?

To create a table with default values for columns in Oracle, you can use the following SQL statement:

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CREATE TABLE table_name (
  column1 datatype DEFAULT default_value,
  column2 datatype DEFAULT default_value,
  ...
);

Here is an example of creating a table with default values for columns:

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CREATE TABLE employees (
  employee_id NUMBER PRIMARY KEY,
  first_name VARCHAR2(50) DEFAULT 'John',
  last_name VARCHAR2(50) DEFAULT 'Doe',
  hire_date DATE DEFAULT SYSDATE
);

In this example, the employees table is created with default values for the first_name, last_name, and hire_date columns. The default value for first_name and last_name is 'John' and 'Doe' respectively, while the default value for hire_date is the current date (SYSDATE function).

What is the difference between composite keys and primary keys in Oracle tables?

A primary key is a column or a set of columns that uniquely identifies each row in a table. It enforces the uniqueness of each row and ensures that there are no duplicate values in the key column(s). Only one primary key can be defined in a table.

On the other hand, a composite key is a key that consists of two or more columns that together uniquely identify each row in a table. Unlike a primary key, a composite key does not have any specific constraints associated with it, such as ensuring uniqueness or enforcing not null values.

In summary, the main difference between a primary key and a composite key in Oracle tables is that a primary key enforces uniqueness and not null constraints, while a composite key is simply a combination of two or more columns used to uniquely identify rows in a table.

What is the use of unique constraints in Oracle tables?

Unique constraints in Oracle tables are used to ensure that a column, or a combination of columns, contain unique values within a table. This means that no two rows can have the same value for the column(s) specified in the unique constraint.

Unique constraints are used to enforce data integrity and prevent duplicate entries in a table, ensuring that each row in the table is uniquely identifiable. They are often used on columns that should contain unique values, such as primary keys or any other columns that should not have duplicates.

By applying unique constraints, it helps maintain the quality of data in the table and prevents inconsistencies or errors that can arise from having duplicate values. They also help improve query performance by allowing the database to quickly find and access specific rows based on unique values.