SQL Server-2005中Select和Select count()之间的区别 [英] Difference between Select and Select count() in sql server-2005

问题描述

海

我想知道 select * 和 select count(*)
之间的区别 如果我使用 select * ，则会出现错误

Hai

I would to know the difference between select * and select count(*)
If I use select * it gives an error

 "Msg 263, Level 16, State 1, Line 1
Must specify table to select from" 

但 select count(*) 没有给出任何错误
相反，它给出1作为计数.

请问原因.
在此先感谢.

but select count(*) does not give any error
Instead It gives 1 as count.

May I know the reason.

Thanks in advance.

推荐答案

当您说"SELECT *"时，您正在编写部分命令:"SELECT these fields"而不保存它们的来源. "*"是通配符，它​​扩展为所有字段"，因此您需要指定要从中读取的表:

When you say "SELECT *" you are writing a partial command: "SELECT these fields" without saving where they should come from. "*" is a wildcard which expands to "all fields" so you need to specify the table to read from:

"SELECT * FROM myTable"

当您说"SELECT COUNT(*)"时，您将执行不同的命令:一个SELECT，它返回单个项目，即项目数COUNT.因为没有列表，所以结果为1-计数本身.

When you say "SELECT COUNT(*)" you are a different command: a SELECT that returns a single item, the COUNT, of the number of items. Because there is no list, the result comes out at 1 - the count itself.

这意味着在该表中，您只有一个字段，并且只有一行.！

因此，当您使用"select count(*)"时，它将不计算您最近使用的表中的行数.

对于``select *"-您应该提及表名.. !!

It mean in that table you have only one field and it has only one row.!

so, When you use ''select count(*)'' it count no of rows from the table which you used recently.

for ''select * '' - you should mention the table name..!

Count(*)对结果集中的所有行进行计数.选择*返回结果集中行的所有列.

如果计数发生变化，则可能是在目标表上发生了插入和删除操作.



praveen
chennai

Count(*) counts all the rows in the result set. Select * returns all columns for the rows in the resultset.

If the count is changing it''s probably there are inserts and deletes happening against the destination table.



praveen
chennai

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