7.4 Altering Tables (ADD/DROP/ALTER COLUMN)

Level: 7 | Version: v1.0 | Author: Meptrasoft

Overview & Core Concepts

Once a table exists, you can still change its structure using ALTER TABLE. There are three common alterations:

This topic reuses the employees table from 7.3. If not already created, use the setup script from 7.3.

ALTER TABLE table_name
ADD COLUMN column_name datatype;

Example:

ALTER TABLE employees
ADD COLUMN join_date DATE;

ALTER TABLE table_name
DROP COLUMN column_name;

Example:

ALTER TABLE employees
DROP COLUMN join_date;

Changing a Column's Datatype

ALTER TABLE table_name
ALTER COLUMN column_name TYPE new_datatype;

Example:

ALTER TABLE employees
ALTER COLUMN emp_name TYPE VARCHAR(100);

Key concept in SQL & Database Engineering: Altering Tables (ADD/DROP/ALTER COLUMN).
Understanding Altering Tables (ADD/DROP/ALTER COLUMN) is critical for query optimization, schema integrity, and technical placement rounds.
Always test queries against sample tables to verify edge cases, NULL handling, and expected output.
Follow standard SQL syntax and indexing best practices for production reliability.

What is the core purpose of Altering Tables (ADD/DROP/ALTER COLUMN) in SQL?: It provides a standardized mechanism to manipulate, define, or query relational database records efficiently while enforcing relational algebra rules.
What is a common pitfall when working with Altering Tables (ADD/DROP/ALTER COLUMN)?: Overlooking NULL values, missing indexes on filter/join columns, or writing un-sargable query predicates that cause full table scans.
How is Altering Tables (ADD/DROP/ALTER COLUMN) evaluated in terms of database performance?: The query optimizer generates an execution plan; proper usage ensures minimal I/O disk reads and optimal CPU memory utilization.

Write a query practicing Altering Tables (ADD/DROP/ALTER COLUMN) using the pre-loaded practice tables.
Examine the expected output and verify how NULL values or edge cases behave.

💡 Tip: Test your queries using the in-browser interactive runner above.

*End of Altering Tables (ADD/DROP/ALTER COLUMN)