How to Accurately Count Unique Items in an Access Query?

Accurate Unique Item Counts in Access Queries

Efficiently counting unique entries within an Access database field is a frequent requirement. A naive approach using the standard COUNT(*) function often yields inaccurate results due to its inclusion of duplicate values.

The Problem

Consider a table, "table1," containing duplicate entries in the "Name" field. The query:

<code class="language-sql">SELECT Count(*) FROM table1;</code>

will return the total number of rows, not the number of unique names.

The Correct Approach

The ideal solution involves the COUNT(DISTINCT field_name) function. However, this function directly doesn't work in Access.

The Solution: Using a Subquery

To accurately count unique items, employ a subquery:

1. Construct a subquery: This subquery extracts only the distinct values from the target field:

<code class="language-sql">(SELECT DISTINCT Name FROM table1)</code>

2. Alias the subquery: Assign an alias (e.g., "T") to this subquery for clarity and proper referencing.

3. Count the distinct values: Use COUNT(*) on the aliased subquery to count the unique values:

<code class="language-sql">SELECT Count(*) AS N
FROM (SELECT DISTINCT Name FROM table1) AS T;</code>

This revised query will correctly return the count of unique names.

Further Reading

For comprehensive guidance on counting unique values in Microsoft Access, consult this helpful resource: Counting Unique Values in Access

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