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HomeDatabaseMysql TutorialMySQL高级查询 之 与 Group By 集合使用的函数 和 关键字_MySQL

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GROUP_CONCAT

mysql> SELECT student_name,

    ->     GROUP_CONCAT(test_score)

    ->     FROM student

    ->     GROUP BY student_name;

Or:

mysql> SELECT student_name,

    ->     GROUP_CONCAT(DISTINCT test_score

    ->               ORDER BY test_score DESC SEPARATOR ' ')

    ->     FROM student

    ->     GROUP BY student_name;

在MySQL中,你可以获取表达式组合的连接值。你可以使用DISTINCT删去重复值。假若你希望多结果值进行排序,则应该使用  ORDER BY子句。若要按相反顺序排列,将 DESC (递减) 关键词添加到你要用ORDER BY 子句进行排序的列名称中。默认顺序为升序;可使用ASC将其明确指定。   SEPARATOR 后面跟随应该被插入结果的值中间的字符串值。默认为逗号 (‘,’)。通过指定SEPARATOR '' ,你可以删除所有分隔符。

PS:就是可以在一个语句中得到 GROUP BY 被 聚合的项的每个子值的一个组合的字符串

 2 WITH ROLLUP

GROUP BY子句允许一个将额外行添加到简略输出端 WITH ROLLUP 修饰符。这些行代表高层(或高聚集)简略操作。ROLLUP 因而允许你在多层分析的角度回答有关问询的问题

或者你可以使用 ROLLUP, 它能用一个问询提供双层分析。将一个 WITH ROLLUP修饰符添加到GROUP BY 语句,使询问产生另一行结果,该行显示了所有年份的总价值:

mysql> SELECT year, SUM(profit) FROM sales GROUP BY year WITH ROLLUP;

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

| year | SUM(profit) |

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

| 2000 |        4525 |

| 2001 |        3010 |

| NULL |        7535 |

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

总计高聚集行被年份列中的NULL值标出。

当有多重 GROUP BY 列时,ROLLUP产生的效果更加复杂。这时,每次在除了最后一个分类列之外的任何列出现一个 “break” (值的改变) ,则问讯会产生一个高聚集累计行。

例如,在没有 ROLLUP的情况下,一个以年、国家和产品为基础的关于 sales 表的一览表可能如下所示:

mysql> SELECT year, country, product, SUM(profit)

    -> FROM sales

    -> GROUP BY year, country, product;

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

| year | country | product    | SUM(profit) |

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

| 2000 | Finland | Computer   |        1500 |

| 2000 | Finland | Phone      |         100 |

| 2000 | India   | Calculator |         150 |

| 2000 | India   | Computer   |        1200 |

| 2000 | USA     | Calculator |          75 |

| 2000 | USA     | Computer   |        1500 |

| 2001 | Finland | Phone      |          10 |

| 2001 | USA     | Calculator |          50 |

| 2001 | USA     | Computer   |        2700 |

| 2001 | USA     | TV         |         250 |

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

表示总值的输出结果仅位于年/国家/产品的分析级别。当添加了 ROLLUP后, 问询会产生一些额外的行:

mysql> SELECT year, country, product, SUM(profit)

    -> FROM sales

    -> GROUP BY year, country, product WITH ROLLUP;

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

| year | country | product    | SUM(profit) |

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

| 2000 | Finland | Computer   |        1500 |

| 2000 | Finland | Phone      |         100 |

| 2000 | Finland | NULL       |        1600 |

| 2000 | India   | Calculator |         150 |

| 2000 | India   | Computer   |        1200 |

| 2000 | India   | NULL       |        1350 |

| 2000 | USA     | Calculator |          75 |

| 2000 | USA     | Computer   |        1500 |

| 2000 | USA     | NULL       |        1575 |

| 2000 | NULL    | NULL       |        4525 |

| 2001 | Finland | Phone      |          10 |

| 2001 | Finland | NULL       |          10 |

| 2001 | USA     | Calculator |          50 |

| 2001 | USA     | Computer   |        2700 |

| 2001 | USA     | TV         |         250 |

| 2001 | USA     | NULL       |        3000 |

| 2001 | NULL    | NULL       |        3010 |

| NULL | NULL    | NULL       |        7535 |

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

 

当你使用 ROLLUP时, 你不能同时使用 ORDER BY子句进行结果排序。换言之, ROLLUP 和ORDER BY 是互相排斥的。然而,你仍可以对排序进行一些控制。在 MySQL中, GROUP BY 可以对结果进行排序,而且你可以在GROUP BY列表指定的列中使用明确的 ASC和DESC关键词,从而对个别列进行排序。 (不论如何排序被ROLLUP添加的较高级别的总计行仍出现在它们被计算出的行后面)。

LIMIT可用来限制返回客户端的行数。LIMIT 用在 ROLLUP后面, 因此这个限制 会取消被ROLLUP添加的行

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