select DISTINCT vend_id FROM products; DISTINCT去重复的值，必须放在列名前面
select prod_name from products LIMIT 5; 限制结果 LIMIT
LIMIT 4 OFFSET 3 ：从行3开始取4行，== LIMIT 3，4
select acct_key.acct_name from eyou_mail.acct_key; 直接取值数据库的表内容
select prod_name from products ORDER BY prod_name； ORDER BY 进行排序
select prod_id,prod_price,prod_name from products ORDER BY prod_price,prod_name;
检索3个列,并按照其中两个列对结果进行排序------首先按价格，然后按名称排序
ORDER BY 可以降序(Z到A）顺序排序，必须制定关键字 DESC。
按照价格以降序排序产品:
select prod_id,prod_price,prod_name From products ORDER BY prod_price DESC;
ORDER BY子句的位置 ： 保证它位于FROM子句之后。如果使用LIMIT，它必须位于ORDER BY之后.
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与AND不同，它只是mysql检索匹配任意条件的行
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;