MySQL学习历程（六）：各种查询

1、基本查询语句

SELECT * FROM tb_name;//返回表tb_name的所有字段
SELECT id, name FROM tb_name;//返回指定的两个字段
SELECT id, name FROM tb_name WHERE name = "simon";//返回满足条件的指定字段

IN 表示一个集合；BETWEEN AND表示两个值之间

SELECT * FROM stu WHERE stu_id IN(8000609050, 8000609051, 8000609053);//stu_id在集合里的返回
SELECT * FROM stu WHERE stu_id NOT IN(8000609050, 8000609051, 8000609053);
SELECT * FROM stu WHERE stu_id IN(8000609050, 8000609051) ORDER BY name;//再按名字升序排序
SELECT * FROM fruits WHERE price BETWEEN 2.0 AND 10.0;//在两个值之间的返回
SELECT * FROM fruits WHERE price NOT BETWEEN 2.0 AND 10.0;//不在两个值之间

LIKE通配符查询，“%”匹配任意长度的字符，“_”匹配单个字符

SELECT * FROM stu WHERE name LIKE "z%";//满足name以z开头
SELECT * FROM stu WHERE name LIKE "z%a";//满足name以z开头，以a结尾
SELECT * FROM stu WHERE name LIKE "%y";//满足name以y结尾
SELECT * FROM stu WHERE name LIKE "a_";//满足name只有两个字符，并且a是第一个

AND、OR组合条件查询

SELECT * FROM tb_name WHERE name IS NULL;//判空的时候用IS NULL
SELECT * FROM tb_name WHERE name IS NOT NULL;
SELECT * FROM tb_name WHERE name IS NOT NULL AND stu_id IN(8000609050, 8000609053);
SELECT * FROM tb_name WHERE name IS NOT NULL OR stu_id IN(8000609050, 8000609053);
SELECT DISTINCT * FROM fruits WHERE price BETWEEN 2.0 AND 10.0;//DISTINCT删除重复结果

ORDER BY查询结果排序，默认升序，ASC关键字是升序，DESC关键字是降序。

SELECT * FROM tb_name ORDER BY price;//默认升序排列
SELECT * FROM tb_name ORDER BY name, price;//先按name升序排序，name相同的再按price升序排序
SELECT * FROM tb_name ORDER BY price DESC; //指定降序
SELECT * FROM tb_name ORDER BY price ASC; //指定升序
SELECT * FROM tb_name ORDER BY name ASC, price DESC;//先按name升序排序，name相同的再按price降序排序

GROUP BY分组查询是对数据按照某个或多个字段进行分组。

SELECT s_id, COUNT(*) AS total FROM fruits GROUP BY s_id;//对s_id进行分组，然后统计每个分组的行数
SELECT s_id, GROUP_CONCAT(f_name) AS names FROM fruits GROUP BY s_id;//对s_id进行分组，显示每个分组里的f_name。
SELECT s_id, GROUP_CONCAT(f_name) AS names
FROM fruits
GROUP BY s_id HAVING COUNT(f_name) > 2;//对s_id进行分组，显示分组f_name数量大于2的分组
SELECT s_id, COUNT(*) AS total FROM fruits GROUP BY s_id WITH ROLLUP;//ROLLUP增加一行，统计记录总和
SELECT * FROM fruits GROUP BY s_id, f_name;//多字段分组，先按s_id分组再按f_name分组

//对num进行分组，把同一分组内的quantity*price的总和大于等于100的分组显示出来。
SELECT num, SUM(quantity * price) AS orderTotal
FROM orderitems
GROUP BY num
HAVING SUM(quantity * price) >= 100;

//再上一条的基础上再加上一个显示排序的功能
SELECT num, SUM(quantity * price) AS orderTotal
FROM orderitems
GROUP BY num
HAVING SUM(quantity * price) >= 100
ORDER BY orderTotal;
//limit限制查询结果的数量
SELECT * FROM fruits LIMIT 5;//显示前两条记录
SELECT * FROM fruits LIMIT 4, 2;//从第五行开始显示，显示两条记录

2、使用集合函数进行查询

函数名	功能
AVG()	返回平均值
COUNT()	计算行数
MAX()	返回最大值
MIN()	返回最小值
SUM()	求和函数

SELECT COUNT(email) AS email_num FROM student;
SELECT num, SUM(quantity) AS items_total FROM orderitems GROUP BY o_num;
SELECT s_id, AVG(price) AS avg_price FROM fruits GROUP BY s_id;
SELECT s_id, MAX(f_price) AS max_price FROM fruits GROUP BY s_id;
SELECT s_id, MIN(f_price) AS max_price FROM fruits GROUP BY s_id;

3、连接查询

3.1、内连接

内连接是将两个表中相同列的值也相同的行保留下来。

对于一个问题，两个表有相同的字段s_id，找出两个表中s_id相同的记录。有以下两种查询方法：

//非内连接形式
SELECT suppliers.s_id, s_name, f_name, f_price
FROM fruits, suppliers
WHERE fruits.s_id = suppliers.s_id;
//内连接形式
SELECT suppliers.s_id, s_name, f_name, f_price
FROM fruits INNER JOIN suppliers
ON fruits.s_id = suppliers.s_id;

非内连接形式效率低。

3.2、外连接

内连接是返回查询结果中仅符合查询条件和连接条件的行。外连接不进包含符合条件的行也包含不符合连接条件的行。

1. LEFT OUTER JOIN(左外连接)：左表中的所有记录和右表中连接字段相等的记录。

2. RIGHT OUTER JOIN(右外连接)：右表中的所有记录和右表中连接字段相等的记录。

SELECT customers.c_id, orders.o_num
FROM customers LEFT OUTER JOIN orders
ON customers.c_id = orders.c_id;

SELECT customers.c_id, orders.o_num
FROM customers RIGHT OUTER JOIN orders
ON customers.c_id = orders.c_id;

3.3、复合连接查询

复合连接查询就是在内连接或外连接的基础上再加上过滤条件。

//再加一个条件
SELECT suppliers.s_id, s_name, f_name, f_price
FROM fruits INNER JOIN suppliers
ON fruits.s_id = suppliers.s_id AND f_price > 23;
//加上排序条件
SELECT customers.c_id, orders.o_num
FROM customers LEFT OUTER JOIN orders
ON customers.c_id = orders.c_id
ORDER BY customers.c_id;

4、嵌套查询

一个查询语句内部嵌套另一个查询语句。常见的操作符有ANY(SOME),ALL,IN,EXISTS。

//ANY：num1大于子查询里面的任意一行
SELECT num1 FROM tb1 WHERE  num1 > ANY (SELECT num2 FROM tb2);
//ALL：num1大于子查询里面的所有行
SELECT num1 FROM tb1 WHERE  num1 > ALL (SELECT num2 FROM tb2);
//EXISTS：因为WHERE后面是个表达式，EXISTS意思是子查询有行返回则是true，子查询没有结果返回则返回false
SELECT num1 FROM tb1 WHERE  EXISTS (SELECT num2 FROM tb2);
//IN：num1在子查询返回的结果里则为真，另外还有NOT IN。
SELECT num1 FROM tb1 WHERE  num1 IN (SELECT num2 FROM tb2);

5、合并查询结果

使用UNION关键字将多个SELECT查询结果合并成单个结果集。合并时，两个表对应的列和数据类型必须严格相同。UNION会在合并时删除重复记录，UNION ALL不会删除重复记录并且效率较高。

SELECT ... FROM ...
UNION [ALL]
SELECT ... FROM ...

6、为表和字段取别名

取个别名好偷懒，下次用着方便。

//表名 AS 表别名
SELECT c.c_id, o.o_num
FROM customers AS c OUTER JOIN orders AS o
ON c.c_id = o.c_id
//列名 AS 列别名
SELECT f1.f_name AS fruit_name, f1.f_price AS fruit_price
FROM fruits AS f1
WHERE f1.f_price < 8;

7、使用正则表达式查询

正则表达式可以找个时间集中学习一下，使用方法就是使用正则表达式作为匹配条件，将满足条件的返回。

SELECT * FROM fruits WHERE f_name REGEXP '^p';//满足f_name以p开头的返回
SELECT * FROM fruits WHERE f_name REGEXP 'p$';//满足f_name以p结尾的返回