SQL Aliases

An alias is a temporary name, scoped to one query, for a column or a table. Column aliases were already covered with AS back in the SELECT lesson, this lesson briefly recaps that and introduces table aliases, which don't matter much yet but become essential the moment a query involves more than one table.

Learning Objectives

• Recap renaming a column's result with AS.
• Alias a table, with and without the AS keyword.
• Understand why table aliases matter once a query references more than one table.

Column Aliases: A Recap

Covered in full in the SELECT lesson: AS renames a column in the result only, without touching the actual table. The identifier-quoting differences for a multi-word alias (backticks, double quotes, or brackets depending on the database) were covered there too.

SELECT salary AS annual_salary
FROM employees;

Table Aliases

A table alias gives a table a short temporary name, e for employees here, that can then prefix column names: e.first_name instead of employees.first_name. On a single-table query this just saves typing, its real value shows up once a query joins two or more tables, covered in the next module, where column names alone can become ambiguous between tables.

SELECT e.first_name, e.salary
FROM employees AS e;

SELECT e.first_name, e.salary
FROM employees e;

Try It Yourself

Write a query that selects every employee's first_name and salary, using e as a table alias for employees, written in a way that works on all six databases in this course.

SELECT e.first_name, e.salary
FROM employees e;

-- Omitting AS here works on every database, including Oracle.
-- Including AS also works, except on Oracle specifically.

Frequently Asked Questions

Common Beginner Mistakes

Best Practices

• Omit AS for table aliases to keep a query portable across all six databases in this course, including Oracle.
• Keep AS for column aliases, it's accepted everywhere and makes the alias's intent explicit to a reader.
• Pick short but recognizable table aliases, especially once a query involves more than one or two tables.

Interview Questions

Summary

This lesson recapped column aliases with AS, introduced table aliases and why they matter once joins are involved, and covered Oracle's specific restriction: no AS before a table alias, even though AS is otherwise standard and portable for column aliases.

That completes Module 2, Operators and Filtering: combining conditions with AND/OR/NOT, pattern matching with LIKE and wildcards, matching a list with IN, matching a range with BETWEEN, and naming columns and tables with aliases.

What's Next?

Module 3, SQL Functions, is next: aggregate functions in depth, string functions, numeric functions, date functions, and the CASE expression.