mysql-3 检索数据（1）

SELECT 语句

SELECT检索表数据，必须至少给出两条信息--------想选择什么，以及从什么地方选择。

检索一个列 

SELECT prod_name FROM products;

上述语句利用SELECT语句从products表中检索一个名为prod_name的列

检索多个列

SELECT prod_id,prod_name,prod_price FROM products;

检索所有的列

SELECT  * FROM products;

检索不同的行

SELECT vend_id FROM products;

select返回了14行，但是表中只有4个供应商，因为products表中列出了14个产品，如何检索有不同值的列表

SELECT DISTINCT vend_id FROM products;

限制结果

SELECT prod_name FROM products LIMIT 5;

 SELECT prod_name FROM products LIMIT 5 , 5;

从第五行开始输出5行

第一个数位开始位置，第二个数为检索的行数

SELECT prod_name FROM products LIMIT 4 OFFSET 3;

从第三行开始，输出四行

使用完全限定输出表名

SELECT products.prod_name FROM products;

排序检索数据

排序数据

SELECT prod_name FROM products ORDER BY prod_name;

mysql对prod_name列以字母顺序排序

按多个排序

SELECT prod_id,prod_price,prod_name FROM products ORDER BY prod_price , prod_name;

首先按照价格然后按照名称排序     作用，当价格相同的时候，会再按照名称来排序。

如果prod_price列中的数据所有值都是唯一的，则不会按照prod_name来排序

制定排序方向

ORDER BY默认是按照升序的方式排列，如果进行降序排列，就必须标出 DESC;

SELECT prod_id , prod_price ,prod_name FROM products ORDER BY prod_price DESC;

SELECT prod_id , prod_price ,prod_name FROM products ORDER BY prod_price DESC, prod_name;

先按照价格从高到底排序，价格相同的再按照名称升序排列。

与DESC相反的是ASC，ASC没有多大作用，因为默认就是ASC升序排列。

SELECT prod_price FROM products ORDER BY prod_price DESC LIMIT 2;

找出两个最贵的物品的值

过滤数据

使用where子句

SELECT prod_name ,prod_price FROM products WHERE prod_price = 2.50;

查询价格等于2.50的商品名字和价格

检查单个值

SELECT prod_name ,prod_price FROM products WHERE prod_name = 'fuses';

SELECT prod_name,prod_price FROM products WHERE prod_price <10;

 SELECT vend_id , prod_name FROM products WHERE vend_id <> 1003;

查询供应商不是1003制造的所有产品

 SELECT prod_name , prod_price FROM products WHERE prod_price BETWEEN 5 AND 10;

查询价格在5和之间的商品名称和价格

数据过滤

组合where子句

SELECT prod_id , prod_price ,prod_name FROM products WHERE vend_id =1003 AND prod_price <=10;

SELECT prod_id , prod_price ,prod_name FROM products WHERE vend_id =1002 OR vend_id = 1003;

计算次序

SELECT prod_name , prod_price FROM products WHERE vend_id = 1002 OR vend_id = 1003 AND prod_price >=10;

从上面的结果来看，有两个值的结果小于10，显然返回的结果未按照预期的进行过滤，为什么会这样呢，原因是在于计算的次序，SQL在处理OR操作符前，优先处理AND操作符，当SQL看到上述where子句时，它理解为由供应商1003制造的任何价格都为10美元以上，包含10美元，或者由供应商1002提供的产品，不管价格。换句话说，由于AND的计算次序中优先级更高，操作符被错误的组合了。正确的操作方法：

SELECT prod_name , prod_price FROM products WHERE (vend_id = 1002 OR vend_id = 1003) AND prod_price >=10;

IN 操作符

 IN操作符来制定条件范围，范围中的每个条件都可以进行匹配。

SELECT prod_name , prod_price FROM products WHERE vend_id IN (1002,1003) ORDER BY prod_name;

IN和OR的用法有点类似，上面也可以这样写：

SELECT prod_name , prod_price FROM products WHERE vend_id=1002 OR vend_id=1003 ORDER BY prod_name;

NOT操作符

where子句中的not操作符的用法只有一个，那就是否定后它之后所有的任何条件

SELECT prod_name , prod_price FROM products WHERE vend_id NOT IN (1002,1003) ORDER BY prod_name;

上面用到的所有查询命令

SELECT prod_name FROM products;
SELECT prod_id,prod_name,prod_price FROM products;
SELECT * FROM products;
SELECT vend_id FROM products;
SELECT DISTINCT vend_id FROM products;
SELECT prod_name FROM products LIMIT 5;
SELECT prod_name FROM products LIMIT 5 , 5;
SELECT prod_name FROM products LIMIT 1 , 1;
SELECT prod_name FROM products LIMIT 4 OFFSET 3;
SELECT products.prod_name FROM products;
SELECT prod_name FROM products ORDER BY prod_name;
SELECT prod_id,prod_price,prod_name FROM products ORDER BY prod_price , prod_name;
SELECT prod_id , prod_price ,prod_name FROM products ORDER BY prod_price DESC;
SELECT prod_id , prod_price ,prod_name FROM products ORDER BY prod_price DESC, prod_name;
SELECT prod_price FROM products ORDER BY prod_price DESC LIMIT 2;
SELECT prod_name ,prod_price FROM products WHERE prod_price = 2.50;
SELECT prod_name ,prod_price FROM products WHERE prod_name = 'fuses';
SELECT prod_name,prod_price FROM products WHERE prod_price <10;
SELECT vend_id , prod_name FROM products WHERE vend_id <> 1003;
SELECT prod_name , prod_price FROM products WHERE prod_price BETWEEN 5 AND 10;
SELECT prod_id , prod_price ,prod_name FROM products WHERE vend_id =1003 AND prod_price <=10;
SELECT prod_id , prod_price ,prod_name FROM products WHERE vend_id =1002 OR vend_id = 1003;
SELECT prod_name , prod_price FROM products WHERE vend_id = 1002 OR vend_id = 1003 AND prod_price >=10;
SELECT prod_name , prod_price FROM products WHERE (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;
SELECT prod_name , prod_price FROM products WHERE vend_id=1002 OR vend_id=1003 ORDER BY prod_name;
SELECT prod_name , prod_price FROM products WHERE vend_id NOT IN (1002,1003) ORDER BY prod_name;

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