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)
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.
Also we can not use count(xxx) in where class like
select * from users where count(email) > 1
it will give error:
But we can use
Select email from users Group By email Having count(email) > 1
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