Question

是否可以计算项目以及在mySQL数据库查询中选择它的值？

SELECT a.id AS item_id,
       b.id AS group_id,
       c.user_id,
       COUNT(c.user_id) AS cnt

Answer 1

没有。

您需要使用嵌套查询：

SELECT
   a.id AS item_id,
   b.id AS group_id,
   c.user_id,
   d.cnt
FROM
   tableA a, 
   tableB b, 
   tableC c, 
   (SELECT COUNT(user_id) AS cnt from tableC) as d --nested query

Answer 2

当然可以。例如，它计算每个表名在information_schema.columns表中出现的次数，并显示它。

Select
        count(table_name), 
        table_name 
from 
     information_schema.columns 
group by 
     table_name

显然Neal认为sql以不同的方式在information_schema表上工作（这对我来说是新闻）这是另一个例子

Create table ANomalTable
(
    a_id int,
    b_id int,
    user_id int
);

INSERT INTO ANomalTable
VALUES (1,1,1);
INSERT INTO ANomalTable
VALUES (2,2,1);
INSERT INTO ANomalTable
VALUES (3,4,2);
INSERT INTO ANomalTable
VALUES (5,6,2);

SELECT 
    count(user_id),
    user_id
FROm ANomalTable
GROUP BY
   (user_id)

输出

count(user_id) user_id
-------------  -------
2,             1   
2,             2

计数ID并在查询中选择它

2 个答案: