Usage of as in sql

The AS keyword in SQL is used to specify aliases, including tables, columns or derived values: specify aliases for tables to facilitate multiple references. Assign aliases to columns to explicitly reference specific columns. Specify an alias for a derived value, referencing the result of a calculation or operation.

Usage of AS keyword in SQL

In SQL statements, the AS keyword is used for tables, Column or derived value specifies an alias. It mainly has the following uses:

1. Specify an alias for the table

When you need to reference a table multiple times, you can use AS to specify an alias for it. Then use the alias to refer to it. For example:

<code class="sql">SELECT * FROM customers AS c;</code>

2. Specify aliases for columns

Similarly, you can also use AS to specify aliases for columns in query results. This is useful when a query returns multiple columns and specific columns need to be referenced explicitly. For example:

<code class="sql">SELECT customer_id AS ID, customer_name AS Name FROM customers;</code>

3. Specify an alias for the derived value

In SQL, you can use operators and functions to perform calculations and operations on data. You can specify aliases for these derived values ​​by using AS and then reference them in queries. For example:

<code class="sql">SELECT customer_id AS ID, customer_name AS Name,
       DATEDIFF(NOW(), created_at) AS DaysSinceCreation
       FROM customers;</code>

Usage notes:

• The alias following AS must be unique and cannot conflict with other aliases or table names in the query.
• Aliases can consist of letters, numbers, and underscores, but cannot begin with numbers.
• After using an alias, you can use the alias or the original name to reference the item.
• The AS keyword is optional, but its use is highly recommended to improve code readability and maintainability.

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