我有一些 SQL 代码可以按照我想要的方式工作:
select 10 as number, "Checklist 10 Foo" as name, max(id), max(ts) as max_ts, callsign, max(time_hint_utc), count(*)
from checklist_10
union all
select 11 as number, "Checklist 11 Bar" as name, max(id), max(ts) as max_ts, callsign, max(time_hint_utc), count(*)
from checklist_11
union all
select 12 as number, "Checklist 12 Baz" as name, max(id), max(ts) as max_ts, callsign, max(time_hint_utc), count(*)
from checklist_12
group by callsign, number
order by max_ts
结果:
number,name,max(id),max_ts,callsign,max(time_hint_utc),count(*)
10,Checklist 10 Foo,2,1486554484635,VRTEST,2017-02-08 12:21:32,2
11,Checklist 11 Bar,2,1486554490674,VRTEST,2017-02-08 12:21:39,2
12,Checklist 12 Baz,2,1486554496378,VRTEST,2017-02-08 12:21:44,2
12,Checklist 12 Baz,3,1486554496379,VRTEST2,2017-02-08 12:21:45,1
特别是，我看到了 max() 和 count() 的正确结果，即 max 和 count 每个 callsign/number 组合，而不是单个 selects。
这是如何工作的？聚合函数看起来就像它们在表级selects 中一样，但它们的功能就好像它们是union all 的结果的函数.
附言很抱歉问一个问题，因为唯一的问题是我缺乏理解。
更新表格描述:
mysql> describe checklist_10;
+---------------+--------------------------------------------------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+---------------+--------------------------------------------------------------+------+-----+---------+----------------+
| id | bigint(20) unsigned | NO | PRI | NULL | auto_increment |
| ts | bigint(20) unsigned | NO | | NULL | |
| callsign | varchar(20) | NO | | NULL | |
| smg_id | tinyint(3) unsigned | NO | | NULL | |
| time | int(11) | NO | | NULL | |
| time_hint_utc | datetime | NO | | NULL | |
| reason | enum('UNKNOWN','PERIODIC','SHIFT','MENU','EVENT','DECLINED') | NO | | NULL | |
| foo0 | tinyint(1) | NO | | NULL | |
| foo1 | tinyint(1) | NO | | NULL | |
| foo2 | tinyint(1) | NO | | NULL | |
| foo3 | tinyint(1) | NO | | NULL | |
| foo4 | tinyint(1) | NO | | NULL | |
| foo5 | tinyint(1) | NO | | NULL | |
| foo6 | tinyint(1) | NO | | NULL | |
| foo7 | tinyint(1) | NO | | NULL | |
| foo8 | tinyint(1) | NO | | NULL | |
| foo9 | tinyint(1) | NO | | NULL | |
+---------------+--------------------------------------------------------------+------+-----+---------+----------------+
17 rows in set (0.00 sec)
mysql> describe checklist_11;
+---------------+--------------------------------------------------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+---------------+--------------------------------------------------------------+------+-----+---------+----------------+
| id | bigint(20) unsigned | NO | PRI | NULL | auto_increment |
| ts | bigint(20) unsigned | NO | | NULL | |
| callsign | varchar(20) | NO | | NULL | |
| smg_id | tinyint(3) unsigned | NO | | NULL | |
| time | int(11) | NO | | NULL | |
| time_hint_utc | datetime | NO | | NULL | |
| reason | enum('UNKNOWN','PERIODIC','SHIFT','MENU','EVENT','DECLINED') | NO | | NULL | |
| bar0 | tinyint(1) | NO | | NULL | |
| bar1 | tinyint(1) | NO | | NULL | |
| bar2 | tinyint(1) | NO | | NULL | |
| bar3 | tinyint(1) | NO | | NULL | |
+---------------+--------------------------------------------------------------+------+-----+---------+----------------+
11 rows in set (0.00 sec)
mysql> describe checklist_12;
+---------------+--------------------------------------------------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+---------------+--------------------------------------------------------------+------+-----+---------+----------------+
| id | bigint(20) unsigned | NO | PRI | NULL | auto_increment |
| ts | bigint(20) unsigned | NO | | NULL | |
| callsign | varchar(20) | NO | | NULL | |
| smg_id | tinyint(3) unsigned | NO | | NULL | |
| time | int(11) | NO | | NULL | |
| time_hint_utc | datetime | NO | | NULL | |
| reason | enum('UNKNOWN','PERIODIC','SHIFT','MENU','EVENT','DECLINED') | NO | | NULL | |
| baz0 | tinyint(1) | NO | | NULL | |
| baz1 | tinyint(1) | NO | | NULL | |
| baz2 | tinyint(1) | NO | | NULL | |
| baz3 | tinyint(1) | NO | | NULL | |
| baz4 | tinyint(1) | NO | | NULL | |
+---------------+--------------------------------------------------------------+------+-----+---------+----------------+
12 rows in set (0.00 sec)
数据:
mysql> select * from checklist_10;
+----+---------------+----------+--------+------------+---------------------+----------+------+------+------+------+------+------+------+------+------+------+
| id | ts | callsign | smg_id | time | time_hint_utc | reason | foo0 | foo1 | foo2 | foo3 | foo4 | foo5 | foo6 | foo7 | foo8 | foo9 |
+----+---------------+----------+--------+------------+---------------------+----------+------+------+------+------+------+------+------+------+------+------+
| 1 | 1486554385343 | VRTEST | 7 | 1486556393 | 2017-02-08 12:19:53 | PERIODIC | 1 | 1 | 0 | 0 | 1 | 1 | 0 | 1 | 0 | 0 |
| 2 | 1486554484635 | VRTEST | 7 | 1486556492 | 2017-02-08 12:21:32 | SHIFT | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 0 | 0 |
+----+---------------+----------+--------+------------+---------------------+----------+------+------+------+------+------+------+------+------+------+------+
2 rows in set (0.00 sec)
mysql> select * from checklist_11;
+----+---------------+----------+--------+------------+---------------------+----------+------+------+------+------+
| id | ts | callsign | smg_id | time | time_hint_utc | reason | bar0 | bar1 | bar2 | bar3 |
+----+---------------+----------+--------+------------+---------------------+----------+------+------+------+------+
| 1 | 1486554457077 | VRTEST | 7 | 1486556465 | 2017-02-08 12:21:05 | PERIODIC | 0 | 0 | 0 | 0 |
| 2 | 1486554490674 | VRTEST | 7 | 1486556499 | 2017-02-08 12:21:39 | SHIFT | 1 | 1 | 1 | 1 |
+----+---------------+----------+--------+------------+---------------------+----------+------+------+------+------+
2 rows in set (0.00 sec)
mysql> select * from checklist_12;
+----+---------------+----------+--------+------------+---------------------+----------+------+------+------+------+------+
| id | ts | callsign | smg_id | time | time_hint_utc | reason | baz0 | baz1 | baz2 | baz3 | baz4 |
+----+---------------+----------+--------+------------+---------------------+----------+------+------+------+------+------+
| 1 | 1486554476903 | VRTEST | 7 | 1486556485 | 2017-02-08 12:21:25 | PERIODIC | 1 | 1 | 1 | 1 | 1 |
| 2 | 1486554496378 | VRTEST | 7 | 1486556504 | 2017-02-08 12:21:44 | SHIFT | 1 | 1 | 1 | 1 | 1 |
| 3 | 1486554496379 | VRTEST2 | 7 | 1486556505 | 2017-02-08 12:21:45 | SHIFT | 1 | 1 | 1 | 1 | 1 |
+----+---------------+----------+--------+------------+---------------------+----------+------+------+------+------+------+
3 rows in set (0.00 sec)
无 1054 错误:
mysql> select 10 as number, "Checklist 10 Foo" as name, max(id), max(ts) as max_ts, callsign, max(time_hint_utc), count(*)
-> from checklist_10
-> union all
-> select 11 as number, "Checklist 11 Bar" as name, max(id), max(ts) as max_ts, callsign, max(time_hint_utc), count(*)
-> from checklist_11
-> union all
-> select 12 as number, "Checklist 12 Baz" as name, max(id), max(ts) as max_ts, callsign, max(time_hint_utc), count(*)
-> from checklist_12
->
-> group by callsign, number
-> order by max_ts;
+--------+------------------+---------+---------------+----------+---------------------+----------+
| number | name | max(id) | max_ts | callsign | max(time_hint_utc) | count(*) |
+--------+------------------+---------+---------------+----------+---------------------+----------+
| 10 | Checklist 10 Foo | 2 | 1486554484635 | VRTEST | 2017-02-08 12:21:32 | 2 |
| 11 | Checklist 11 Bar | 2 | 1486554490674 | VRTEST | 2017-02-08 12:21:39 | 2 |
| 12 | Checklist 12 Baz | 2 | 1486554496378 | VRTEST | 2017-02-08 12:21:44 | 2 |
| 12 | Checklist 12 Baz | 3 | 1486554496379 | VRTEST2 | 2017-02-08 12:21:45 | 1 |
+--------+------------------+---------+---------------+----------+---------------------+----------+
4 rows in set (0.00 sec)
最佳答案
它并不像你想象的那样有效。根据documentation ,
If you use a group function in a statement containing no GROUP BY clause, it is equivalent to grouping on all rows.
所以这里发生的是你有两个隐式 group by(它总是只返回一行，不管你是否在你的数据中有不同的呼号)和一个显式 group by 实际上只对您最后一次选择进行分组。
所以对于 MySQL，你的查询看起来和行为都像
(select 10 as number, "Checklist 10 Foo" as name, max(id), max(ts) as max_ts,
callsign, max(time_hint_utc), count(*)
from checklist_10)
union all
(select 11 as number, "Checklist 11 Bar" as name, max(id), max(ts) as max_ts,
callsign, max(time_hint_utc), count(*)
from checklist_11)
union all
(select 12 as number, "Checklist 12 Baz" as name, max(id), max(ts) as max_ts,
callsign, max(time_hint_utc), count(*)
from checklist_12
group by callsign, number)
order by max_ts
你可以测试这个，例如通过更改 checklist_10 或 checklist_11 中某一行的 呼号。它不会按预期分组。或者你可以，例如在您的第一个或最后一个查询中将别名 number 重命名为 number1。它(仅)在第一种情况下有效，在后一种情况下，group by 现在找不到列 number。
此外，在您的情况下，这仅在您禁用 only_full_group_by 时才有效sql 模式，因为您的选择列表包含聚合函数以外的列。
关于mysql - MySQL UNION ALL 是如何运行聚合函数的？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/42116784/
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