問(wèn)題描述
我有一個(gè)表 VehicleModelYear，包含列 id、year、make 和 model.
I have a table, VehicleModelYear, containing columns id, year, make, and model.
以下兩個(gè)查詢按預(yù)期工作:
The following two queries work as expected:
SELECT DISTINCT make, model
FROM VehicleModelYear
SELECT COUNT(DISTINCT make)
FROM VehicleModelYear
但是，此查詢不起作用
SELECT COUNT(DISTINCT make, model)
FROM VehicleModelYear
很明顯答案是第一個(gè)查詢返回的結(jié)果數(shù)量，但只是想知道這個(gè)語(yǔ)法有什么問(wèn)題或者為什么它不起作用.
It's clear the answer is the number of results returned by the first query, but just wondering what is wrong with this syntax or why it doesn't work.
推薦答案
COUNT() 在 SQL Server 接受以下語(yǔ)法
COUNT(*)
COUNT(colName)
COUNT(DISTINCT colName)
你可以有一個(gè)子查詢，它返回你可以計(jì)算的唯一的 make 和 model 集合.
You can have a subquery which returns unique set of make and model that you can count with.
SELECT COUNT(*)
FROM
(
SELECT DISTINCT make, model
FROM VehicleModelYear
) a
末尾的a"不是拼寫錯(cuò)誤.這是一個(gè)別名，如果沒有它，SQL 將給出錯(cuò)誤 ERROR 1248 (42000): 每個(gè)派生表必須有自己的別名.
The "a" at the end is not a typo. It's an alias without which SQL will give an error ERROR 1248 (42000): Every derived table must have its own alias.
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