How do you use aliases in SQL?

In SQL, aliases are temporary names assigned to tables or columns within a query to improve readability, simplify complex expressions, or resolve naming conflicts. They are created using the AS keyword (which is optional in some databases) and exist only for the duration of the query. Aliases help make code cleaner and more maintainable, especially when dealing with long table names, calculated fields, or joins between multiple tables.

Column Aliases Column aliases rename output columns in query results. This is useful for calculated fields, aggregated values, or columns with unclear names. For example, SELECT first_name + ' ' + last_name AS full_name FROM employees; creates a full_name alias for the concatenated result. Without the alias, the column might appear as an unnamed expression in the output. Similarly, SELECT COUNT(*) AS total_orders FROM orders; assigns a clear name to the count result. Aliases can also simplify referencing columns in clauses like ORDER BY or GROUP BY (e.g., ORDER BY total_orders DESC). Note that some databases restrict alias usage in the WHERE clause due to SQL’s execution order.

Table Aliases Table aliases shorten table names for easier referencing, especially in joins. For instance, FROM customers AS c JOIN orders AS o ON c.id = o.customer_id allows using c and o instead of full table names throughout the query. This is critical in self-joins, where the same table is used twice. Consider a scenario where an employees table has a manager_id column referencing its own rows:

SELECT e.name AS employee, m.name AS manager
FROM employees e
LEFT JOIN employees m ON e.manager_id = m.id;

Here, e and m distinguish between the employee and manager instances of the same table. Table aliases are also required when using subqueries in FROM clauses (e.g., FROM (SELECT ...) AS subquery).

Best Practices and Caveats Use aliases consistently and meaningfully—short names like a, b are acceptable for simple queries, but descriptive names (e.g., prod for products) improve readability in complex joins. Avoid using reserved words or existing table/column names as aliases. Be aware that aliases are scoped to the query and cannot be reused in nested queries or across separate statements. When working with aggregate functions or expressions, aliases ensure results are labeled clearly for applications or reports consuming the data. Always test aliases in your specific database, as syntax nuances (like quotation requirements for spaces) may vary slightly between systems like MySQL, PostgreSQL, and SQL Server.