MySQL group by對單字分組序和多欄位分組的方法講解
阿新 • • 發佈：2020-01-09
我這裡建立了一個 goods 表，先看下里面的資料：
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 | +----+------+------+------------+-------------+------------+------------------+---------------+-------+----------------+
這裡使用了 group_concat()函式，主要目的是為了顯示分組的詳細資訊
上面的根據單個欄位分組很簡單，把相同s_id的記錄都歸組了
2、根據s_id,goods_desc欄位分組
分析：這裡查詢分組時，會先根據s_id分組，然後對每個組裡面的資料再根據goods_desc進行分組
mysql> select *,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,book | 4,2.30 | +----+------+------+------------+-------------+------------+------------------+-------------+------+--------------+
這裡的goods_descs 和 上面的一比較就明白了
接下來還可以再根據 goods_price 分組
mysql> select *,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 | +----+------+------+------------+-------------+------------+-------------+-------------+------+--------------+
這裡主要是進行多個欄位分組的時候，只需掌握分組順序後面的欄位是根據前面欄位分組後的內容再進行的分組即可。
在平時的開發任務中我們經常會用到MYSQL的GROUP BY分組， 用來獲取資料表中以分組欄位為依據的統計資料。比如有一個學生選課表，表結構如下：
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
我們想統計每門課程有多少個學生報名，應用如下SQL：
SELECT Subject,Count(*) FROM Subject_Selection GROUP BY Subject
得到如下結果：
Subject Count
------------------------------
ITB001 5
MKB114 2
因為表裡記錄了有5個學生選擇ITB001，2個學生選擇了MKB114。
產生這個結果的原因是：
GROUP BY X意思是將所有具有相同X欄位值的記錄放到一個分組裡。
那麼GROUP BY X,Y呢？
GROUP BY X,Y意思是將所有具有相同X欄位值和Y欄位值的記錄放到一個分組裡。
總結
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