In SQL, aliases are used to assign temporary names to tables or columns in a query. Aliases can make the query more readable and concise, especially when dealing with complex queries or when joining multiple tables. The `AS` keyword is typically used to specify an alias.

Here are examples of using aliases in SQL:

1. Alias for a table:

sql
SELECT c.name, c.age
FROM customers AS c

In this example, the `AS c` assigns the alias "c" to the "customers" table. This allows us to refer to the table as "c" in the rest of the query, simplifying the syntax.

2. Alias for a column:

sql
SELECT c.name AS customer_name, c.age AS customer_age
FROM customers AS c

In this example, we assign aliases "customer_name" and "customer_age" to the "name" and "age" columns, respectively. This allows us to refer to the columns with more descriptive names in the result set.

3. Alias for a calculated expression:

sql
SELECT c.name, c.age, c.age * 2 AS double_age
FROM customers AS c

In this example, we calculate the double of the "age" column and assign it the alias "double_age". This allows us to include the calculated expression in the result set with a meaningful name.

Aliases can be used in various parts of a SQL query, including the SELECT clause, FROM clause, WHERE clause, JOINs, and subqueries. They are especially helpful when using self-joins or when there are multiple tables with similar column names.

Note that not all database systems require the use of the `AS` keyword for aliases. Some database systems allow you to directly assign the alias without the `AS` keyword. For example, you can write `FROM customers c` instead of `FROM customers AS c`.

Using aliases can improve the readability and maintainability of your SQL queries, making them easier to understand and work with.