mysql：聯合重用子查詢

Question

這里有張桌子：

ID        City      

1          Toronto
2          Toronto
3          Toronto
4          Toronto
5          NY
6          NY

如何獲得下表：

City    Total

NULL    6
Toronto 4
NY      2

我打算使用union ：

(select city, count(*) as total from tbl group by city)
union
(select null, count(*) as total from tbl)

是否可以使用類似重用子查詢的方法：

(select city, count(*) as total from tbl group by city) tmp
union
(select null, count(*) as tmp from tbl)

或者 sql 已經優化

(select city, count(*) as total from tbl group by city)
union
(select null, count(*) as total from tbl)

?

Answer 1

至少在 MySQL 5.5 和更高版本中，您可以使用WITH ROLLUP修飾符來獲取分組元素的總和，而無需額外查詢；

SELECT city, COUNT(*) AS total FROM tbl GROUP BY city WITH ROLLUP

city      total
---------------
NY        2
Toronto   4
(null)    6

一個用於測試的 SQLfiddle 。

Answer 2

您不能重用查詢，但可以使用視圖重用代碼

  create view  my_view as 
  select city, count(*) as total 
  from tbl group by city

一旦創建了視圖，您就可以

  select  city, total 
  from my_view 
  union  
  select  null, sum(total)
  from my_view 

Answer 3

1) 取別名只對加入成員和子查詢可見。 不適用於工會成員。

2）您編寫的查詢幾乎沒有意義：將 tmp 別名為表然后是標量？

要實現您想要實現的目標，您可以嘗試使用 CTE：

WITH tmp AS (SELECT city, COUNT(*) AS cnt GROUP BY city) 
SELECT * FROM tmp UNION SELECT null, SUM(cnt) FROM tmp

我不知道性能差異會是什么。 如果您有關於city的索引，我懷疑您會看到不同。

Answer 4

您在子查詢中使用WITH ROLLUP ，然后在外部查詢中安排排序以將匯總記錄放在首位：

SELECT *
FROM (SELECT city, COUNT(*) AS total FROM mytable GROUP BY city WITH ROLLUP) x 
ORDER BY
    CASE WHEN city IS NULL THEN 0 ELSE 1 END,
    city

如果您打算按總計降序排列結果，那就更簡單了：

SELECT *
FROM (SELECT city, COUNT(*) AS total FROM mytable GROUP BY city WITH ROLLUP) x 
ORDER BY total DESC

DB Fiddlde 上的演示：

| city    | total |
| ------- | ----- |
|         | 6     |
| Toronto | 4     |
| NY      | 2     |