What are the differences between using "num_rows" and "COUNT" in PHP database queries, and how can developers ensure they are using the correct method for their specific needs?

When retrieving the number of rows from a database query in PHP, developers can choose between using the "num_rows" property of a result object or using the "COUNT" function in a SQL query. The "num_rows" property is a method specific to the database driver being used and is generally faster for large result sets. On the other hand, using the "COUNT" function in the SQL query allows for more flexibility and can be used to count rows based on specific conditions. To ensure developers are using the correct method for their specific needs, they should consider the size of the result set and the complexity of the conditions they want to count. If they simply need to count the total number of rows returned by a query, using "num_rows" is a straightforward and efficient choice. However, if they need to count rows based on specific conditions or aggregate functions, using the "COUNT" function in the SQL query is the better option.

// Using num_rows to count the total number of rows returned by a query
$query = $conn->query("SELECT * FROM table");
$num_rows = $query->num_rows;
echo "Total number of rows: " . $num_rows;

// Using COUNT in SQL query to count rows based on specific conditions
$query = $conn->query("SELECT COUNT(*) FROM table WHERE condition = 'value'");
$count = $query->fetch_row()[0];
echo "Number of rows with specific condition: " . $count;