在mySQL/SQL中count(0)，count(1)..和count(*)之间有什么区别? [英] What is the difference between count(0), count(1).. and count(*) in mySQL/SQL?

问题描述

我最近在一次采访中被问到这个问题. 我在mySQL中进行了尝试，并得到了相同的结果(最终结果). 全部给出了该特定表中的行数. 谁能解释他们之间的主要区别.

I was recently asked this question in an interview. I tried this in mySQL, and got the same results(final results). All gave the number of rows in that particular table. Can anyone explain the major difference between them.

推荐答案

除非您在表中指定字段或在括号内指定表达式而不是常量值或*

Nothing really, unless you specify a field in a table or an expression within parantheses instead of constant values or *

让我给您详细的答案.计数将为您提供给定字段的非空记录号.假设您有一个名为A

Let me give you a detailed answer. Count will give you non-null record number of given field. Say you have a table named A

select 1 from A
select 0 from A
select * from A

将全部返回相同数量的记录，即表A中的行数.输出仍然不同.如果表中有3条记录.以X和Y作为字段名称

will all return same number of records, that is the number of rows in table A. Still the output is different. If there are 3 records in table. With X and Y as field names

select 1 from A will give you

1
1
1

select 0 from A will give you
0
0
0

select * from A will give you ( assume two columns X and Y is in the table )
X      Y
--     --
value1 value1
value2 (null)
value3 (null)

因此，所有三个查询都返回相同的数字.除非您使用

So, all three queries return the same number. Unless you use

select count(Y) from A 

由于只有一个非空值，您将获得1作为输出

since there is only one non-null value you will get 1 as output

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