SQL Alter Table Statement

Questions and Answers

What is the main purpose of the DROP TABLE statement?

To delete all rows from a table

To change the structure of an existing table

To rename an existing table

To remove a table and its data from the database (correct)

Which statement would be used to delete all rows from the author table without removing the table itself?

DROP TABLE author;

TRUNCATE TABLE author IMMEDIATE; (correct)

DELETE FROM author;

REMOVE ALL FROM author;

What does the IMMEDIATE keyword specify when truncating a table?

The operation can be undone

The statement will be processed immediately and cannot be undone (correct)

The table will be dropped after rows are deleted

The truncation will happen after other operations

Which SQL statement is specifically used to change the structure of an existing table?

ALTER TABLE Signup and view all the answers

What happens to the data when you execute a DROP TABLE statement on a table with existing data?

The data is deleted along with the table. Signup and view all the answers

Which clause is used with the alter table statement to change the data type of an existing column?

MODIFY Signup and view all the answers

What is the consequence of altering a column's data type to one that is incompatible with existing data?

An error message will be displayed, and the statement will not run. Signup and view all the answers

Which command is used to remove an existing column from a table?

ALTER TABLE with DROP COLUMN Signup and view all the answers

In the example, how is the new telephone number column defined in the author table?

As a bigint type Signup and view all the answers

What is the primary purpose of the alter table statement?

To change the structure of an existing table. Signup and view all the answers

Study Notes

Alter Table Statement

Utilized to add or remove columns, modify data types, and manage keys and constraints in a database table.
Follows the syntax: ALTER TABLE table_name; without parentheses for parameters.
Each row in the statement indicates a single change to be applied to the table.

Adding a Column

To add a new column, use the syntax: ALTER TABLE table_name ADD COLUMN column_name data_type;
Example: ALTER TABLE author ADD COLUMN telephone_number BIGINT; which stores up to 19 digits.

Modifying a Column

Changing a column's data type is done using the MODIFY clause: ALTER TABLE table_name MODIFY column_name new_data_type;
For example: ALTER TABLE author MODIFY telephone_number CHAR(20); allows for inclusion of special characters.

Data Type Compatibility

Altering a column to a new data type can lead to issues if existing data is incompatible.
Example: Converting a column from CHAR to a numeric data type fails if non-numeric data is present, triggering an error.

Dropping a Column

To remove a column from a table, use: ALTER TABLE table_name DROP COLUMN column_name;
Example: ALTER TABLE author DROP COLUMN telephone_number; deletes the specified column.

Dropping a Table

The DROP TABLE statement completely removes a table along with its data using the syntax: DROP TABLE table_name;
Example: DROP TABLE author; deletes the author table and its contents.

Deleting Data

Use the DELETE statement to remove data, but it's more efficient to employ: TRUNCATE TABLE table_name IMMEDIATE;
Example: TRUNCATE TABLE author IMMEDIATE; quickly deletes all rows from the author table without removing the structure.

Summary of Commands

ALTER TABLE adjusts the structure (add/modify/drop columns).
DROP TABLE deletes an entire table and its data.
TRUNCATE TABLE clears all records in a table while retaining its schema.

Description

Test your knowledge on the SQL ALTER TABLE statement, which is essential for modifying existing database tables. This quiz will cover how to add or remove columns, change data types, and manage keys and constraints. Prepare to enhance your understanding of SQL commands and table alterations.