What does Cno mean in mysql

Cno is the default name in MySQL that represents a subquery column alias, used to refer to columns in the subquery. A subquery is a SELECT statement nested in the main query and needs to be assigned a column alias to reference the column. Cno aliases can be used to reference specific columns in a subquery, in ascending order of columns.

The meaning of Cno in MySQL

Cno is a column alias used to represent a subquery in MySQL.

Detailed explanation

A subquery is a SELECT statement nested in the main query. In order to reference a column from a subquery, it needs to be given a column alias. Cno (column number, column number) is the default alias of the column in the subquery.

For example:

<code class="sql">SELECT name,
       (SELECT MAX(salary) FROM employees) AS max_salary
FROM employees;</code>

In this query, the subquery (SELECT MAX(salary) FROM employees) is used to calculate the maximum salary of employees, column alias max_salary represents the columns in the subquery.

Using Cno

Cno can be used to reference specific columns in a subquery. For example:

<code class="sql">SELECT name,
       MAX(Cno) AS max_salary
FROM employees;</code>

In this query, the Cno alias is used to calculate the column number of the row with the maximum salary in the subquery.

Note

• The Cno alias is only valid within the subquery.
• If the subquery has multiple columns, the Cno alias will be incremented in column order.
• Other aliases can be used to override the Cno alias.

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