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在MySQL中，別名（Alias）是一個可選的標識符，用于為表或列提供自定義名稱，使用別名可以使查詢語句更易讀、更具可讀性，并且可以簡化復雜的查詢。

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以下是MySQL中使用別名的詳細用法：

1、為表設置別名：

語法：SELECT column_name(s) FROM table_name AS alias_name;

示例：

“`sql

SELECT employee_name, salary

FROM employees AS E;

“`

2、為列設置別名：

語法：SELECT column_name AS alias_name FROM table_name;

示例：

“`sql

SELECT first_name AS ‘姓’, last_name AS ‘名’ FROM employees;

“`

3、在WHERE子句中使用別名：

語法：SELECT column_name(s) FROM table_name AS alias_name WHERE alias_name.column_name = value;

示例：

“`sql

SELECT employee_name, salary

FROM employees AS E

WHERE E.salary > 5000;

“`

4、在JOIN操作中使用別名：

語法：SELECT column_name(s) FROM table1 AS t1 JOIN table2 AS t2 ON t1.column_name = t2.column_name;

示例：

“`sql

SELECT customers.customer_name, orders.order_date

FROM customers AS C

JOIN orders AS O ON C.customer_id = O.customer_id;

“`

5、在GROUP BY子句中使用別名：

語法：SELECT column_name(s), aggregate_function(column_name) FROM table_name AS alias_name GROUP BY alias_name.column_name;

示例：

“`sql

SELECT department_id, COUNT(*) AS num_employees

FROM employees AS E

GROUP BY E.department_id;

“`

6、在ORDER BY子句中使用別名：

語法：SELECT column_name(s) FROM table_name AS alias_name ORDER BY alias_name.column_name ASC|DESC;

示例：

“`sql

SELECT employees.employee_name, employees.salary

FROM employees AS E

ORDER BY E.salary DESC;

“`

7、在聚合函數(shù)中使用別名：

語法：SELECT aggregate_function(column_name) AS alias_name FROM table_name;

示例：

“`sql

SELECT SUM(salary) AS total_salary, AVG(salary) AS average_salary FROM employees;

“`

以上是MySQL中使用別名的一些常見用法，通過使用別名，可以使SQL查詢更加清晰和易于理解，提高代碼的可維護性和可讀性。

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