MySQL学习笔记（六）

一、分组数据

　　数据分组

1 mysql> SELECT COUNT(*) AS num_prods

2     -> FROM products

3     -> WHERE vend_id = 1003;

4 +-----------+

5 | num_prods |

6 +-----------+

7 |         7 |

8 +-----------+

9 1 row in set (0.03 sec)

　　创建分组

 1 mysql> SELECT vend_id, COUNT(*) AS num_prods

 2     -> FROM products

 3     -> GROUP BY vend_id;

 4 +---------+-----------+

 5 | vend_id | num_prods |

 6 +---------+-----------+

 7 |    1001 |         3 |

 8 |    1002 |         2 |

 9 |    1003 |         7 |

10 |    1005 |         2 |

11 +---------+-----------+

12 4 rows in set (0.03 sec)

***GROUP BY 子句可以包含任意数目的列。这使得能对分组进行嵌套，为数据分组提供更细致的控制。

***如果在GROUP BY子句中嵌套了分组，数据将在最后规定的分组上进行汇总。换句话说，在建立分组时，指定的所有列都一起计算。

***GROUP BY子句中列出的每个列都必须是检索列或有效的表达式（但不能是聚集函数）。如果在SELECT中使用表达式，则必须在GROUP BY子句中指定相同的表达式。不能使用别名。

***除聚集计算语句外，SELECT语句中的每个列都必须在GROUP BY子句中给出。

***如果分组列中具有NULL值，则NULL将作为一个分组返回。如果列中有多行NULL值，它们将分为一组。

***GROUP BY子句必须出现在WHERE子句之后，ORDER BY子句之前。

使用WITH ROLLUP关键字，可以得到每个分组以及每个分组汇总级别的值

 1 mysql> SELECT vend_id, COUNT(*) AS num_prods

 2     -> FROM products

 3     -> GROUP BY vend_id WITH ROLLUP;

 4 +---------+-----------+

 5 | vend_id | num_prods |

 6 +---------+-----------+

 7 |    1001 |         3 |

 8 |    1002 |         2 |

 9 |    1003 |         7 |

10 |    1005 |         2 |

11 |    NULL |        14 |

12 +---------+-----------+

13 5 rows in set (0.00 sec)

　　过滤分组

 1 mysql> SELECT cust_id, COUNT(*) AS orders

 2     -> FROM orders

 3     -> GROUP BY cust_id

 4     -> HAVING COUNT(*) >= 2;

 5 +---------+--------+

 6 | cust_id | orders |

 7 +---------+--------+

 8 |   10001 |      2 |

 9 +---------+--------+

10 1 row in set (0.06 sec)

***HAVING支持所有WHERE操作符 （WHERE过滤行，HAVING过滤分组）

WHERE在数据分组前进行过滤，HAVING在数据分组后进行过滤。

 1 mysql> SELECT vend_id, COUNT(*) AS num_prods

 2     -> FROM products

 3     -> WHERE prod_price >= 10

 4     -> GROUP BY vend_id

 5     -> HAVING COUNT(*) >= 2;

 6 +---------+-----------+

 7 | vend_id | num_prods |

 8 +---------+-----------+

 9 |    1003 |         4 |

10 |    1005 |         2 |

11 +---------+-----------+

12 2 rows in set (0.00 sec)

 1 mysql> SELECT vend_id, COUNT(*) AS num_prods

 2     -> FROM products

 3     -> GROUP BY vend_id

 4     -> HAVING COUNT(*) >= 2;

 5 +---------+-----------+

 6 | vend_id | num_prods |

 7 +---------+-----------+

 8 |    1001 |         3 |

 9 |    1002 |         2 |

10 |    1003 |         7 |

11 |    1005 |         2 |

12 +---------+-----------+

13 4 rows in set (0.00 sec)

　　分组和排序

***一般在使用GROUP BY子句时，应该也给出ORDER BY子句。这是保证数据正确排序的唯一方法。

 1 mysql> SELECT order_num, SUM(quantity*item_price) AS ordertotal

 2     -> FROM orderitems

 3     -> GROUP BY order_num

 4     -> HAVING SUM(quantity*item_price) >= 50;

 5 +-----------+------------+

 6 | order_num | ordertotal |

 7 +-----------+------------+

 8 |     20005 |     149.87 |

 9 |     20006 |      55.00 |

10 |     20007 |    1000.00 |

11 |     20008 |     125.00 |

12 +-----------+------------+

13 4 rows in set (0.00 sec)

为按总计订单价格排序输出，需要添加ORDER BY子句

 1 mysql> SELECT order_num, SUM(quantity*item_price) AS ordertotal

 2     -> FROM orderitems

 3     -> GROUP BY order_num

 4     -> HAVING SUM(quantity*item_price) >= 50

 5     -> ORDER BY ordertotal;

 6 +-----------+------------+

 7 | order_num | ordertotal |

 8 +-----------+------------+

 9 |     20006 |      55.00 |

10 |     20008 |     125.00 |

11 |     20005 |     149.87 |

12 |     20007 |    1000.00 |

13 +-----------+------------+

14 4 rows in set (0.00 sec)

SELECT子句及其顺序

子句	说明	是否必须使用
SELECT	要返回的列或表达式	是
FROM	从中检索数据的表	仅在从表选择数据时使
WHERE	行级过滤	否
GROUP BY	分组说明	仅在按组计算时使用
HAVING	组级过滤	否
ORDER BY	输出排序顺序	否
LIMIT	要检索的行数	否

 

 

 

 

 

 

 

 

 

 

 

二、使用子查询

　　利用子查询进行过滤

　　子查询(subquery)就是嵌套在其他查询中的查询。

 1 mysql> SELECT cust_id

 2     -> FROM orders

 3     -> WHERE order_num IN (SELECT order_num

 4     ->                     FROM orderitems

 5     ->                     WHERE prod_id = 'TNT2');

 6 +---------+

 7 | cust_id |

 8 +---------+

 9 |   10001 |

10 |   10004 |

11 +---------+

12 2 rows in set (0.00 sec)

　　子查询总是从内向外处理。在处理上面的SELECT语句时，MySQL实际上执行了两个操作。

首先，它执行下面的查询：

 SELECT order_num FROM orderitems WHERE prod_id = 'TNT2';

+-----------+

| order_num |

+-----------+

|     20005 |

|     20007 |

+-----------+

此查询返回两个订单号：20005和20007。然后，这两个值以IN操作符要求的逗号分隔的格式传递给外部查询的WHERE子句。

 SELECT cust_id FROM orders WHERE order_num IN (20005, 20007);

+---------+

| cust_id |

+---------+

|   10001 |

|   10004 |

+---------+

***在WHERE子句中使用子查询，应该保证SELECT语句具有与WHERE子句中相同数目的列。通常，子查询将返回单个列并且与单个列匹配。

　　作为计算字段使用子查询

 1 mysql> SELECT cust_name,

 2     ->        cust_state,

 3     ->        (SELECT COUNT(*)

 4     ->         FROM orders

 5     ->         WHERE orders.cust_id = customers.cust_id) AS orders

 6     -> FROM customers

 7     -> ORDER BY cust_name;

 8 +----------------+------------+--------+

 9 | cust_name      | cust_state | orders |

10 +----------------+------------+--------+

11 | Coyote Inc.    | MI         |      2 |

12 | E Fudd         | IL         |      1 |

13 | Mouse House    | OH         |      0 |

14 | Wascals        | IN         |      1 |

15 | Yosemite Place | AZ         |      1 |

16 +----------------+------------+--------+

17 5 rows in set (0.06 sec)

　　相关子查询(correlated subquery)　　涉及外部查询的子查询