Oracle和MySQL分組查詢GROUP BY

真題1、Oracle和MySQL中的分組（GROUP BY）有什麼區別？
答案：Oracle對於GROUP BY是嚴格的，所有要SELECT出來的字段必須在GROUP BY後邊出現，否則會報錯：“ORA-00979: not a GROUP BY expression”。而MySQL則不同，如果SELECT出來的字段在GROUP BY後面沒有出現，那麼會隨機取出一個值，而這樣查詢出來的數據不準確，語義也不明確。所以，作者建議在寫SQL語句的時候，應該給數據庫一個非常明確的指令，而不是讓數據庫去猜測，這也是寫SQL語句的一個非常良好的習慣。
下面給出一個示例。有一張T_MAX_LHR表，數據如下圖所示，有3個字段ARTICLE、AUTHOR和PRICE。請選出每個AUTHOR的PRICE最高的記錄（要包含所有字段）。

ARTICLE	AUTHOR	PRICE
0001	B	3.99
0002	A	10.99
0003	C	1.69
0004	B	19.95
0005	A	6.96

首先給出建表語句：
CREATE TABLE T_MAX_LHR (ARTICLE VARCHAR2(30),AUTHOR VARCHAR2(30),PRICE NUMBER); --Oracle
--CREATE TABLE T_MAX_LHR (ARTICLE VARCHAR(30),AUTHOR VARCHAR(30),PRICE FLOAT); --MySQL oracle通用
INSERT INTO T_MAX_LHR VALUES ('0001','B',3.99);
INSERT INTO T_MAX_LHR VALUES ('0002','A',10.99);
INSERT INTO T_MAX_LHR VALUES ('0003','C',1.69);
INSERT INTO T_MAX_LHR VALUES ('0004','B',19.95);
INSERT INTO T_MAX_LHR VALUES ('0005','A',6.96);
COMMIT;
SELECT * FROM T_MAX_LHR;
在Oracle中的數據：
LHR@orclasm > SELECT * FROM T_MAX_LHR;
ARTICLE AUTHOR PRICE
-------- -------- ----------
0001 B 3.99
0002 A 10.99
0003 C 1.69
0004 B 19.95
0005 A 6.96
在MySQL中的數據：
mysql> SELECT * FROM T_MAX_LHR;
+---------+--------+-------+
| ARTICLE | AUTHOR | PRICE |
+---------+--------+-------+
| 0001 | B | 3.99 |
| 0002 | A | 10.99 |
| 0003 | C | 1.69 |
| 0004 | B | 19.95 |
| 0005 | A | 6.96 |
+---------+--------+-------+
5 rows in set (0.00 sec)
分析數據後，正確答案應該是：

ARTICLE	AUTHOR	PRICE
0002	A	10.99
0003	C	1.69
0004	B	19.95

對於這個例子，很容易想到的SQL語句如下所示：
SELECT T.ARTICLE,T.AUTHOR,MAX(T.PRICE) FROM T_MAX_LHR T GROUP BY T.AUTHOR;
SELECT * FROM T_MAX_LHR T GROUP BY T.AUTHOR;
在Oracle中執行上面的SQL語句報錯：
LHR@orclasm > SELECT T.ARTICLE,T.AUTHOR,MAX(T.PRICE) FROM T_MAX_LHR T GROUP BY T.AUTHOR;
SELECT T.ARTICLE,T.AUTHOR,MAX(T.PRICE) FROM T_MAX_LHR T GROUP BY T.AUTHOR
*
ERROR at line 1:
ORA-00979: not a GROUP BY expression

LHR@orclasm > SELECT * FROM T_MAX_LHR T GROUP BY T.AUTHOR;
SELECT * FROM T_MAX_LHR T GROUP BY T.AUTHOR
*
ERROR at line 1:
ORA-00979: not a GROUP BY expression
在MySQL中執行同樣的SQL語句不會報錯：
mysql> select version();
+-------------------------------------------+
| version() |
+-------------------------------------------+
| 5.6.21-enterprise-commercial-advanced-log |
+-------------------------------------------+

mysql> SELECT T.ARTICLE,T.AUTHOR,MAX(T.PRICE) FROM T_MAX_LHR T GROUP BY T.AUTHOR;
+---------+--------+--------------+
| ARTICLE | AUTHOR | MAX(T.PRICE) |
+---------+--------+--------------+
| 0002 | A | 10.99 |
| 0001 | B | 19.95 |
| 0003 | C | 1.69 |
+---------+--------+--------------+
3 rows in set (0.00 sec)

mysql> SELECT * FROM T_MAX_LHR T GROUP BY T.AUTHOR;
+---------+--------+-------+
| ARTICLE | AUTHOR | PRICE |
+---------+--------+-------+
| 0002 | A | 10.99 |
| 0001 | B | 3.99 |
| 0003 | C | 1.69 |
+---------+--------+-------+
3 rows in set (0.00 sec)
雖然執行不報錯，可以查詢出數據，但是從結果來看數據並不是最終想要的結果，甚至數據是錯亂的。下面給出幾種正確的寫法（在Oracle和MySQL中均可執行）：
（1）使用相關子查詢
SELECT *
FROM T_MAX_LHR T
WHERE (T.AUTHOR, T.PRICE) IN (SELECT NT.AUTHOR, MAX(NT.PRICE) PRICE
FROM T_MAX_LHR NT
GROUP BY NT.AUTHOR)
ORDER BY T.ARTICLE;

SELECT *
FROM T_MAX_LHR T
WHERE T.PRICE = (SELECT MAX(NT.PRICE) PRICE
FROM T_MAX_LHR NT
WHERE T.AUTHOR = NT.AUTHOR)
ORDER BY T.ARTICLE;

（2）使用非相關子查詢
SELECT T.*
FROM T_MAX_LHR T
JOIN (SELECT NT.AUTHOR, MAX(NT.PRICE) PRICE
FROM T_MAX_LHR NT
GROUP BY NT.AUTHOR) T1
ON T.AUTHOR = T1.AUTHOR
AND T.PRICE = T1.PRICE
ORDER BY T.ARTICLE;
（3）使用LEFT JOIN語句
SELECT T.*
FROM T_MAX_LHR T
LEFT OUTER JOIN T_MAX_LHR T1
ON T.AUTHOR = T1.AUTHOR
AND T.PRICE < T1.PRICE
WHERE T1.ARTICLE IS NULL
ORDER BY T.ARTICLE;
在Oracle中的執行結果：
LHR@orclasm > SELECT T.*
2 FROM T_MAX_LHR T
3 LEFT OUTER JOIN T_MAX_LHR T1
4 ON T.AUTHOR = T1.AUTHOR
5 AND T.PRICE < T1.PRICE
6 WHERE T1.ARTICLE IS NULL
7 ORDER BY T.ARTICLE;

ARTICLE AUTHOR PRICE
-------- -------- ----------
0002 A 10.99
0003 C 1.69
0004 B 19.95
在MySQL中的執行結果：
mysql> SELECT T.*
-> FROM T_MAX_LHR T
-> LEFT OUTER JOIN T_MAX_LHR T1
-> ON T.AUTHOR = T1.AUTHOR
-> AND T.PRICE < T1.PRICE
-> WHERE T1.ARTICLE IS NULL
-> ORDER BY T.ARTICLE;
+---------+--------+-------+
| ARTICLE | AUTHOR | PRICE |
+---------+--------+-------+
| 0002 | A | 10.99 |
| 0003 | C | 1.69 |
| 0004 | B | 19.95 |
+---------+--------+-------+
3 rows in set (0.00 sec)

真題2、Oracle和MySQL中的分組（GROUP BY）後的聚合函數分別是什麼？
答案：在Oracle中，可以用WM_CONCAT函數或LISTAGG分析函數；在MySQL中可以使用GROUP_CONCAT函數。示例如下：
首先給出建表語句：
CREATE TABLE T_MAX_LHR (ARTICLE VARCHAR2(30),AUTHOR VARCHAR2(30),PRICE NUMBER); --Oracle
--CREATE TABLE T_MAX_LHR (ARTICLE VARCHAR(30),AUTHOR VARCHAR(30),PRICE FLOAT); --MySQL oracle通用
INSERT INTO T_MAX_LHR VALUES ('0001','B',3.99);
INSERT INTO T_MAX_LHR VALUES ('0002','A',10.99);
INSERT INTO T_MAX_LHR VALUES ('0003','C',1.69);
INSERT INTO T_MAX_LHR VALUES ('0004','B',19.95);
INSERT INTO T_MAX_LHR VALUES ('0005','A',6.96);
COMMIT;
SELECT * FROM T_MAX_LHR;
在MySQL中：
mysql> SELECT T.AUTHOR, GROUP_CONCAT(T.ARTICLE), GROUP_CONCAT(T.PRICE)
-> FROM T_MAX_LHR T
-> GROUP BY T.AUTHOR;
+--------+-------------------------+-----------------------+
| AUTHOR | GROUP_CONCAT(T.ARTICLE) | GROUP_CONCAT(T.PRICE) |
+--------+-------------------------+-----------------------+
| A | 0002,0005 | 10.99,6.96 |
| B | 0001,0004 | 3.99,19.95 |
| C | 0003 | 1.69 |
+--------+-------------------------+-----------------------+
3 rows in set (0.00 sec)
在Oracle中：
LHR@orclasm > SELECT T.AUTHOR, WM_CONCAT(T.ARTICLE) ARTICLE, WM_CONCAT(T.PRICE) PRICE
2 FROM T_MAX_LHR T
3 GROUP BY T.AUTHOR;

AUTHOR ARTICLE PRICE
-------- --------------- ---------------
A 0002,0005 10.99,6.96
B 0001,0004 3.99,19.95
C 0003 1.69

LHR@orclasm > SELECT T.AUTHOR,
2 LISTAGG(T.ARTICLE, ',') WITHIN GROUP(ORDER BY T.PRICE) ARTICLE,
3 LISTAGG(T.PRICE, ',') WITHIN GROUP(ORDER BY T.PRICE) PRICE
4 FROM T_MAX_LHR T
5 GROUP BY T.AUTHOR;

AUTHOR ARTICLE PRICE
-------- --------------- ---------------
A 0005,0002 6.96,10.99
B 0001,0004 3.99,19.95
C 0003 1.69

<span "="" style="font-family:宋體, Arial;color:#EE33EE;font-size:16px;white-space:normal;background-color:#FFFFFF;"> 原作者不知道是誰了，這個圖不是小麥苗畫的。

MySQL分組查詢group by使用示例

(1) group by的含義:將查詢結果按照1個或多個字段進行分組，字段值相同的爲一組
(2) group by可用於單個字段分組，也可用於多個字段分組

select * from employee;

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

| num | d_id | name | age | sex | homeaddr |

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

| 1 | 1001 | 張三 | 26 | 男 | beijinghdq |

| 2 | 1002 | 李四 | 24 | 女 | beijingcpq |

| 3 | 1003 | 王五 | 25 | 男 | changshaylq |

| 4 | 1004 | Aric | 15 | 男 | England |

+------+------+--------+------+------+-------------+
select * from employee group by d_id,sex;
select * from employee group by sex;

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

| num | d_id | name | age | sex | homeaddr

|+------+------+--------+------+------+------------+

| 2 | 1002 | 李四 | 24 | 女 | beijingcpq |

| 1 | 1001 | 張三 | 26 | 男 | beijinghdq |

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

根據sex字段來分組，sex字段的全部值只有兩個('男'和'女')，所以分爲了兩組當group by單獨使用時，只顯示出每組的第一條記錄所以group by單獨使用時的實際意義不大

group by + group_concat()

(1) group_concat(字段名)可以作爲一個輸出字段來使用，
(2) 表示分組之後，根據分組結果，使用group_concat()來放置每一組的某字段的值的集合

select sex from employee group by sex;

+------+

| sex |

+------+

| 女 |

| 男 |

+------+

select sex,group_concat(name) from employee group by sex;

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

| sex | group_concat(name) |

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

| 女 | 李四 |

| 男 | 張三,王五,Aric |

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

select sex,group_concat(d_id) from employee group by sex;

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

| sex | group_concat(d_id) |

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

| 女 | 1002 |

| 男 | 1001,1003,1004 |

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

group by + 集合函數

(1) 通過group_concat()的啓發，我們既然可以統計出每個分組的某字段的值的集合，那麼我們也可以通過集合函數來對這個"值的集合"做一些操作

select sex,group_concat(age) from employee group by sex;

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

| sex | group_concat(age) |

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

| 女 | 24 |

| 男 | 26,25,15 |

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

分別統計性別爲男/女的人年齡平均值

select sex,avg(age) from employee group by sex;

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

| sex | avg(age) |

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

| 女 | 24.0000 |

| 男 | 22.0000 |

+------+----------+
分別統計性別爲男/女的人的個數

select sex,count(sex) from employee group by sex;

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

| sex | count(sex) |

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

| 女 | 1 |

| 男 | 3 |

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

group by + having

(1) having 條件表達式：用來分組查詢後指定一些條件來輸出查詢結果
(2) having作用和where一樣，但having只能用於group by

select sex,count(sex) from employee group by sex having count(sex)>2;

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

| sex | count(sex) |

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

| 男 | 3 |

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

group by + with rollup

(1) with rollup的作用是：在最後新增一行，來記錄當前列裏所有記錄的總和

select sex,count(age) from employee group by sex with

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