In relational algebra, which command retrieves rows from a table based on specified criteria?

COUNT() function

The SQL COUNT() function returns the number of rows in a table satisfying the criteria specified in the WHERE clause. It sets the number of rows or non NULL column values.
COUNT() returns 0 if there were no matching rows.

Syntax:

The above syntax is the general SQL 2003 ANSI standard syntax. This helps to understand the way SQL COUNT() Function is used. But different database vendors may have different ways of applying COUNT() function.

Bellow, you can see that MySQL, PostgreSQL, and Microsoft SQL Server follows the same syntax as given above. But DB2 and Oracle differs slightly.

Overall, you can use * or ALL or DISTINCT or some expression along with COUNT to COUNT the number of rows w.r.t. some condition or all of the rows, depending up on the arguments you are using along with COUNT() function.

DBMS Support: COUNT() function

DB2 and Oracle Syntax:

Parameters:

Syntax diagram - COUNT() function

In the subsequent pages, we have discussed how to apply COUNT() with various SQL clauses. For those applications, we have used Oracle 10g Express Edition.

An important thing about COUNT() function:

When the * is used for COUNT(), all records ( rows ) are COUNTed if some content NULL but COUNT(column_name) does not COUNT a record if its field is NULL. See the following examples:

SQL COUNT rows in a table

In the following example, an asterisk character ( * ) is used followed by the SQL COUNT() which indicates all the rows of the table even if there is any NULL value.

Sample table: orders

To get number of rows in the 'orders' table, the following SQL statement can be used:

SQL Code:

Relational Algebra Expression:

Relational Algebra Tree:

Output:

Pictorial Presentation:

Select COUNT(*) from multiple tables

The following query COUNT the number of rows from two different tables (here we use employees and departments) using COUNT(*) command.

SQL Code:

Output:

SQL COUNT( ) with column name

In this example SQL COUNT() function excludes the NULL values for a specific column if specified the column as an argument in the parenthesis of COUNT function.

Sample table: listofitem

To get the number of rows in the 'listofitem' table with the following condition -

1. COUNT number of rows for the column 'coname'

the following SQL statement can be used :

SQL Code:

Relational Algebra Expression:

Relational Algebra Tree:

Output:

Explain:

The above statement COUNTs those rows for the 'coname' column which are not NULL.

SQL COUNT rows with user defined column heading

To get number of rows in the 'orders' table with the following condition -

1. result have to display with a heading 'Number of Rows',

the following SQL statement can be used:

SQL Code:

Output:

SQL COUNT( ) with where clause

The WHERE clause can be used along with SQL COUNT() functionto select specific records from a table against a given condition.

Example:

Sample table: orders

To get number of rows in the 'orders' table with following condition -

1. ord_amount against the order is more than 1500,

the following SQL statement can be used :

Output:

Application of COUNT() function

In the subsequent pages, we have discussed how to apply COUNT() with various SQL clauses. For those applications, we have used Oracle 10g Express Edition.

COUNT with DISTINCT page discusses how to apply COUNT function with DISTINCT and also discusses how to apply COUNT function with ALL clause. Unlike using *, when ALL is used, NULL values are not selected.

COUNT HAVING page discusses how to apply COUNT function with HAVING clause and HAVING and GROUP BY .

COUNT with GROUP BY page discusses how to apply COUNT function with GROUP BY in ascending order and in descending order.

Note: Outputs of the said SQL statement shown here is taken by using Oracle Database 10g Express Edition

Here is a slide presentation of all aggregate functions.

Practice SQL Exercises

• SQL Exercises, Practice, Solution
• SQL Retrieve data from tables [33 Exercises]
• SQL Boolean and Relational operators [12 Exercises]
• SQL Wildcard and Special operators [22 Exercises]
• SQL Aggregate Functions [25 Exercises]
• SQL Formatting query output [10 Exercises]
• SQL Quering on Multiple Tables [8 Exercises]
• FILTERING and SORTING on HR Database [38 Exercises]
• SQL JOINS
• SQL JOINS [29 Exercises]
• SQL JOINS on HR Database [27 Exercises]
• SQL SUBQUERIES
• SQL SUBQUERIES [39 Exercises]
• SQL SUBQUERIES on HR Database [55 Exercises]
• SQL Union[9 Exercises]
• SQL View[16 Exercises]
• SQL User Account Management [16 Exercise]
• Movie Database
• BASIC queries on movie Database [10 Exercises]
• SUBQUERIES on movie Database [16 Exercises]
• JOINS on movie Database [24 Exercises]
• Soccer Database
• Introduction
• BASIC queries on soccer Database [29 Exercises]
• SUBQUERIES on soccer Database [33 Exercises]
• JOINS queries on soccer Database [61 Exercises]
• Hospital Database
• Introduction
• BASIC, SUBQUERIES, and JOINS [39 Exercises]
• Employee Database
• BASIC queries on employee Database [115 Exercises]
• SUBQUERIES on employee Database [77 Exercises]
• More to come!

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