select DISTINCT vend_id FROM products; DISTINCT去重複的值，必須放在列名前面
select prod_name from products LIMIT 5; 限制結果 LIMIT
LIMIT 4 OFFSET 3 ：從行3開始取4行，== LIMIT 3，4
select acct_key.acct_name from eyou_mail.acct_key; 直接取值數據庫的表內容
select prod_name from products ORDER BY prod_name； ORDER BY 進行排序
select prod_id,prod_price,prod_name from products ORDER BY prod_price,prod_name;
檢索3個列,並按照其中兩個列對結果進行排序------首先按價格，然後按名稱排序
ORDER BY 可以降序(Z到A）順序排序，必須制定關鍵字 DESC。
按照價格以降序排序產品:
select prod_id,prod_price,prod_name From products ORDER BY prod_price DESC;
ORDER BY子句的位置 ： 保證它位於FROM子句之後。如果使用LIMIT，它必須位於ORDER BY之後.
select prod_name,prod_price from products where prod_price = 2.50;
select prod_name,prod_price FROM products Where prod_price BETWEEN 5 AND 10;
select prod_name from products where prod_price IS NULL;
OR與AND不同，它只是mysql檢索匹配任意條件的行
Seletc prod_name,prod_price FROM products where vend_id = 1002 OR vend_id = 1003;
select prod_name,prod_price From products WHER (vend_id = 1002 OR vend_id =1003) AND prod_price >= 10;
select prod_name,prod_price from products where vend_id IN (1002,1003) ORDER BY prod_name;