SQL COUNT Function with example

Count function takes one parameter and returns the number of rows for the matching criteria.

Select Count(*) From employees

It will return count of entire records into Employees table, same result can also be get from following sql statement

Select Count(1) From employees

In this statement we used, 1 in place of , there are discussion about count() and count(1), which will work fast, some people say * try to get the column names from table while in case of count(1), no need to evaluate the column name so count(1) will work fast. I don’t want to go in detail about count(*) and count(1) in this article.

What is the different between count(*) and count(column_name)

  • Count(*) or Count(1) will always returns the count entire rows
  • Count(columnx) will return the count of all those records where columnx is not null means count(column_name) ignores those records where column value is null

Let’s say we have a table “users” with following records

useid       firstname   lastname    
101        A           B
102        A           NULL 
103        X           Y

Select count(*) as [star]
 , count(1) as [one]
 , count(useid) as [id_count]
 , count(firstname) as [fname_count] 
 , count(lastname) as [lname_count]
 , count(password) as [pass]
From users   
    --  Result     
Star    one id_count    fname_count lname_count
 3    3   3           3           2

As I said, count(column name) ignores NULL value records, that's why lname_count is 2.

We can also count distinct value column, say, we want to count the unique first name then we can use

Select count(distinct firstname From users

and result will be 2.

One thing we need to know, if we will use count then other columns must be grouped by otherwise we will get error.

Msg 8120, Level 16, State 1, Line 1
Column 'users.[colune nase]' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.

Also we can not use count(xxx) in where class like

select * from users where count(email) > 1

it will give error:

An aggregate may not appear in the WHERE clause unless it is in a subquery contained in a HAVING clause or a select list, and the column being aggregated is an outer reference.

But we can use

  Select email from users 
  Group By email
  Having count(email) > 1
Hamden Process manager with a reputed organization, Fond of learning new features and technology related to C#, ASP.Net, SQL Server, MVC etc.I like to help others, if I can
  • count
  • sql server
By Hamden On 31 Aug, 13  Viewed: 638

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