How to use count function in oracle

The COUNT function in Oracle is used to count non-null values ​​in a specified column or expression. The syntax is COUNT(DISTINCT ) or COUNT(*), which calculates unique values ​​and all non-null values ​​respectively. number.

Usage of COUNT function in Oracle

The COUNT function is used to calculate non-null values ​​in the specified column or expression. number. The syntax is as follows:

<code class="sql">COUNT(DISTINCT <column_name>)
COUNT(*)</code>

Parameter description:

• DISTINCT : Only count the unique values ​​in the specified column number.
• *: Count the number of all non-null values.

Usage example:

• Count the number of non-null values ​​in a specific column:

<code class="sql">SELECT COUNT(DISTINCT employee_id)
FROM employees;</code>

• Calculate the number of all non-null values ​​in the table:

<code class="sql">SELECT COUNT(*)
FROM employees;</code>

• Calculate the number of non-null values ​​for specific conditions :

<code class="sql">SELECT COUNT(DISTINCT employee_id)
FROM employees
WHERE salary > 50000;</code>

Note:

• The COUNT function ignores NULL values.
• COUNT(*) counts all non-null values, including duplicates.
• COUNT(DISTINCT ) will only count unique values.
• The COUNT function can be used with other aggregate functions such as SUM, AVG.

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