SQL-W3School- function: SQL COUNT () function

ylbtech-SQL-W3School- function: SQL COUNT () function

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COUNT () function returns the number of rows that match specified criteria.

SQL COUNT () syntax

SQL COUNT (column_name) Syntax

COUNT (column_name) function returns the value of the specified number of columns ( NULL not included ):

SELECT COUNT(column_name) FROM table_name

SQL COUNT (*) syntax

COUNT (*) function returns the number of records in the table:

SELECT COUNT(*) FROM table_name

SQL COUNT (DISTINCT column_name) Syntax

COUNT (DISTINCT column_name) function returns the number of different values of the specified column :

SELECT COUNT(DISTINCT column_name) FROM table_name

Notes: COUNT (DISTINCT) applies to ORACLE and Microsoft SQL Server, but can not be used in Microsoft Access.

SQL COUNT (column_name) example

We have the following "Orders" table:

O_Id	OrderDate	OrderPrice	Customer
1	2008/12/29	1000	Bush
2	2008/11/23	1600	Carter
3	2008/10/05	700	Bush
4	2008/09/28	300	Bush
5	2008/08/06	2000	Adams
6	2008/07/21	100	Carter

Now, we want to count customers "Carter" number of orders.

We use the following SQL statement:

SELECT COUNT(Customer) AS CustomerNilsen FROM Orders
WHERE Customer='Carter'

SQL statement is the result of more than 2, because customers Carter a total of two orders:

CustomerNilsen
2

SQL COUNT (*) example

If we omit the WHERE clause, like this:

SELECT COUNT(*) AS NumberOfOrders FROM Orders

The result set will look like this:

NumberOfOrders
6

This is the total number of rows in the table.

SQL COUNT (DISTINCT column_name) example

Now, we want to count the number of different customers table "Orders".

We use the following SQL statement:

SELECT COUNT(DISTINCT Customer) AS NumberOfCustomers FROM Orders

The result set will look like this:

NumberOfCustomers
3

This is the number of different customers (Bush, Carter and Adams) of the "Orders" table.

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https://www.w3school.com.cn/sql/sql_func_count.asp

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Author: ylbtech
Source: http://ylbtech.cnblogs.com/
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