为什么这个SQL查询不能在Northwind数据库上运行？

Question

查找客户来自的城市总数。

select count(*)
from 
(select distinct city
from customers);

错误：

  

错误第4行：'）'..

附近的语法不正确

解释是什么？

Answer 1

根据您之前的问题，我假设您使用Oracle来撰写：

SELECT *
FROM (SELECT 1 FROM dual)

但不是：

SELECT *
FROM (SELECT 1 FROM dual) AS s
-- ORA-00933: SQL command not properly ended 

但是在SQL Server中你需要为子查询添加别名：

select count(*) from (select distinct city from customers) AS s;

FROM clause：

  

[AS] table_alias

     

当派生表，行集或表值函数或运算符时   使用了子句（例如PIVOT或UNPIVOT），所需的table_alias at   子句的结尾是所有列的关联表名，   包括分组列，返回。

SQL Server强制您为列表达式和聚合列添加别名：

SELECT *
FROM (SELECT 1) AS s
-- No column name was specified for column 1 of 's'.

你需要使用：

SELECT *
FROM (SELECT 1) AS s(c)

SELECT *
FROM (SELECT 1 AS c) AS s

<小时/> 就个人而言，我不是子查询的粉丝，我更喜欢常用的表表达式语法：

;WITH cte(city) AS
(
  SELECT DISTINCT city 
  FROM customers
) 
SELECT COUNT(*) AS no_of_cities
FROM cte;

当然，对于这种简单查询，最好的方法是将DISTINCT直接添加到COUNT：

SELECT COUNT(DISTINCT city) AS no_of_cities
FROM customers;

Answer 2

使用子查询时需要指定别名：

select count(*)
from 
(select distinct city
from customers) as MyTable;

或者只是跳过子查询，在这个例子中：

select count(distinct city)
from 
customers;