SQL: Visual QuickStart Guide

Counting Rows with COUNT()

Use the aggregate function COUNT() to count the number of rows in a set of values. COUNT() has two forms:

• COUNT(expr) returns the number of rows in which expr is not null.

• COUNT(*) returns the count of all rows in a set, including nulls and duplicates.

To count non-null rows:

• Type:

  COUNT(expr)
  

  expr is a column name, literal, or expression. The result is an integer greater than or equal to zero.

To count all rows, including nulls:

• Type:

  COUNT(*)
  

  COUNT(*) returns an integer greater than or equal to zero. Listing 6.5 and Figure 6.5 show some queries that involve COUNT(expr) and COUNT(*). The three queries count rows in the table titles and are identical except for the WHERE clause. The row counts in the first query ...