Questions and Answers
What is the main purpose of the DROP TABLE statement?
Which statement would be used to delete all rows from the author table without removing the table itself?
What does the IMMEDIATE keyword specify when truncating a table?
Which SQL statement is specifically used to change the structure of an existing table?
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What happens to the data when you execute a DROP TABLE statement on a table with existing data?
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Which clause is used with the alter table statement to change the data type of an existing column?
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What is the consequence of altering a column's data type to one that is incompatible with existing data?
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Which command is used to remove an existing column from a table?
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In the example, how is the new telephone number column defined in the author table?
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What is the primary purpose of the alter table statement?
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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.
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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.