select * from goods; +----+------+------+------------+-------------+------------+ | id | s_id |..."/>
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MySQL group by對(duì)單字分組序和多字段分組的方法講解

發(fā)布時(shí)間：2020-09-23 12:53:19 來源：腳本之家 閱讀：168 作者：米克斯 欄目：MySQL數(shù)據(jù)庫

我這里創(chuàng)建了一個(gè) goods 表，先看下里面的數(shù)據(jù)：

mysql> select * from goods;
+----+------+------+------------+-------------+------------+
| id | s_id | b_id | goods_name | goods_price | goods_desc |
+----+------+------+------------+-------------+------------+
| 1 |  1 |  5 | book    |    22.35 | book    |
| 2 |  2 |  5 | ball    |    32.25 | ball    |
| 3 |  3 |  5 | NULL    |    3.23 | NULL    |
| 4 |  3 |  5 | macbook  |    3.23 | book    |
| 5 |  3 |  5 | listbook  |    2.30 | book    |
| 6 |  1 |  1 | nicebook  |   9999.00 | nicebook  |
| 7 |  2 |  3 | googlebook |    25.30 | book    |
+----+------+------+------------+-------------+------------+

1、根據(jù)s_id分組

mysql> select *,group_concat(goods_name) goods_names,group_concat(goods_desc) goods_descs,group_concat(id) ids,group_concat(goods_price) goods_prices from goods group by s_id;
+----+------+------+------------+-------------+------------+------------------+---------------+-------+----------------+
| id | s_id | b_id | goods_name | goods_price | goods_desc | goods_names   | goods_descs  | ids  | goods_prices  |
+----+------+------+------------+-------------+------------+------------------+---------------+-------+----------------+
| 1 |  1 |  5 | book    |    22.35 | book    | book,nicebook  | book,nicebook | 1,6  | 22.35,9999.00 |
| 2 |  2 |  5 | ball    |    32.25 | ball    | ball,googlebook | ball,book   | 2,7  | 32.25,25.30  |
| 3 |  3 |  5 | NULL    |    3.23 | NULL    | macbook,listbook | book,book   | 3,4,5 | 3.23,3.23,2.30 |
+----+------+------+------------+-------------+------------+------------------+---------------+-------+----------------+

這里使用了 group_concat()函數(shù)，主要目的是為了顯示分組的詳細(xì)信息

上面的根據(jù)單個(gè)字段分組很簡(jiǎn)單，把相同s_id的記錄都?xì)w組了

2、根據(jù)s_id,goods_desc字段分組

分析：這里查詢分組時(shí)，會(huì)先根據(jù)s_id分組，然后對(duì)每個(gè)組里面的數(shù)據(jù)再根據(jù)goods_desc進(jìn)行分組

mysql> select *,group_concat(goods_name) goods_names,group_concat(goods_desc) goods_descs,group_concat(id) ids,group_concat(goods_price) goods_prices from goods group by s_id,goods_desc;
+----+------+------+------------+-------------+------------+------------------+-------------+------+--------------+
| id | s_id | b_id | goods_name | goods_price | goods_desc | goods_names   | goods_descs | ids | goods_prices |
+----+------+------+------------+-------------+------------+------------------+-------------+------+--------------+
| 1 |  1 |  5 | book    |    22.35 | book    | book       | book    | 1  | 22.35    |
| 6 |  1 |  1 | nicebook  |   9999.00 | nicebook  | nicebook     | nicebook  | 6  | 9999.00   |
| 2 |  2 |  5 | ball    |    32.25 | ball    | ball       | ball    | 2  | 32.25    |
| 7 |  2 |  3 | googlebook |    25.30 | book    | googlebook    | book    | 7  | 25.30    |
| 3 |  3 |  5 | NULL    |    3.23 | NULL    | NULL       | NULL    | 3  | 3.23     |
| 4 |  3 |  5 | macbook  |    3.23 | book    | macbook,listbook | book,book  | 4,5 | 3.23,2.30  |
+----+------+------+------------+-------------+------------+------------------+-------------+------+--------------+

這里的goods_descs 和 上面的一比較就明白了

接下來還可以再根據(jù) goods_price 分組

mysql> select *,group_concat(goods_name) goods_names,group_concat(goods_desc) goods_descs,group_concat(id) ids,group_concat(goods_price) goods_prices from goods group by s_id,goods_desc,goods_price;
+----+------+------+------------+-------------+------------+-------------+-------------+------+--------------+
| id | s_id | b_id | goods_name | goods_price | goods_desc | goods_names | goods_descs | ids | goods_prices |
+----+------+------+------------+-------------+------------+-------------+-------------+------+--------------+
| 1 |  1 |  5 | book    |    22.35 | book    | book    | book    | 1  | 22.35    |
| 6 |  1 |  1 | nicebook  |   9999.00 | nicebook  | nicebook  | nicebook  | 6  | 9999.00   |
| 2 |  2 |  5 | ball    |    32.25 | ball    | ball    | ball    | 2  | 32.25    |
| 7 |  2 |  3 | googlebook |    25.30 | book    | googlebook | book    | 7  | 25.30    |
| 3 |  3 |  5 | NULL    |    3.23 | NULL    | NULL    | NULL    | 3  | 3.23     |
| 5 |  3 |  5 | listbook  |    2.30 | book    | listbook  | book    | 5  | 2.30     |
| 4 |  3 |  5 | macbook  |    3.23 | book    | macbook   | book    | 4  | 3.23     |
+----+------+------+------------+-------------+------------+-------------+-------------+------+--------------+

這里主要是進(jìn)行多個(gè)字段分組的時(shí)候，只需掌握分組順序后面的字段是根據(jù)前面字段分組后的內(nèi)容再進(jìn)行的分組即可。

在平時(shí)的開發(fā)任務(wù)中我們經(jīng)常會(huì)用到MYSQL的GROUP BY分組， 用來獲取數(shù)據(jù)表中以分組字段為依據(jù)的統(tǒng)計(jì)數(shù)據(jù)。比如有一個(gè)學(xué)生選課表，表結(jié)構(gòu)如下：

Table: Subject_Selection

Subject  Semester  Attendee
---------------------------------
ITB001  1     John
ITB001  1     Bob
ITB001  1     Mickey
ITB001  2     Jenny
ITB001  2     James
MKB114  1     John
MKB114  1     Erica

我們想統(tǒng)計(jì)每門課程有多少個(gè)學(xué)生報(bào)名，應(yīng)用如下SQL：

SELECT Subject, Count(*)
FROM Subject_Selection
GROUP BY Subject

得到如下結(jié)果：

Subject    Count
------------------------------
ITB001     5
MKB114     2

因?yàn)楸砝镉涗浟擞?個(gè)學(xué)生選擇ITB001，2個(gè)學(xué)生選擇了MKB114。

產(chǎn)生這個(gè)結(jié)果的原因是：

GROUP BY X意思是將所有具有相同X字段值的記錄放到一個(gè)分組里。

那么GROUP BY X, Y呢？

GROUP BY X, Y意思是將所有具有相同X字段值和Y字段值的記錄放到一個(gè)分組里。

總結(jié)

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