SQL查詢大全

SQL（結構化查詢語言）是關系型數據庫管理系統中的標準語言，用于管理和查詢數據庫。以下是一些常用的SQL查詢語句：

SELECT語句

SELECT語句用于從數據庫中檢索數據。下面是SELECT語句的基本語法：

	SELECT column1, column2, ...
	FROM table_name;

例子：

	SELECT * FROM customers;

該語句將檢索customers表中的所有列。

WHERE語句

WHERE語句用于指定條件，從而檢索滿足該條件的數據。下面是WHERE語句的基本語法：

	SELECT column1, column2, ...
	FROM table_name
	WHERE condition;

例子：

	SELECT * FROM customers WHERE country='USA';

該語句將檢索customers表中國家為‘USA’的所有客戶。

AND和OR語句

AND和OR語句可以將多個條件組合在一起。下面是AND和OR語句的基本語法：

	SELECT column1, column2, ...
	FROM table_name
	WHERE condition1 AND condition2;
	
	SELECT column1, column2, ...
	FROM table_name
	WHERE condition1 OR condition2;

例子：

	SELECT * FROM customers WHERE country='USA' AND city='New York';
	
	SELECT * FROM customers WHERE country='USA' OR country='Canada';

ORDER BY語句

ORDER BY語句用于對結果進行排序。下面是ORDER BY語句的基本語法：

	SELECT column1, column2, ...
	FROM table_name
	ORDER BY column_name ASC|DESC;

例子：

	SELECT * FROM customers ORDER BY country ASC;
	
	SELECT * FROM customers ORDER BY customer_name DESC;

GROUP BY語句

GROUP BY語句用于對結果進行分組。下面是GROUP BY語句的基本語法：

	SELECT column1, column2, ...
	FROM table_name
	GROUP BY column_name;

例子：

	SELECT country, count(*)
	FROM customers
	GROUP BY country;

該語句將統計每個國家的客戶數。

HAVING語句

HAVING語句用于指定分組后的條件。下面是HAVING語句的基本語法：

	SELECT column1, column2, ...
	FROM table_name
	GROUP BY column_name
	HAVING condition;

例子：

	SELECT country, count(*)
	FROM customers
	GROUP BY country
	HAVING count(*) > 10;

該語句將輸出客戶數大于10的國家。

JOIN語句

JOIN語句用于將兩個表連接在一起。下面是JOIN語句的基本語法：

	SELECT column1, column2, ...
	FROM table1
	JOIN table2
	ON table1.column_name = table2.column_name;

例子：

	SELECT orders.order_id, customers.customer_name
	FROM orders
	JOIN customers
	ON orders.customer_id = customers.customer_id;

該語句將輸出訂單ID和客戶名稱。