What potential issues could arise from using DISTINCT, COUNT, and UNION in the SELECT query?

Using DISTINCT, COUNT, and UNION in the SELECT query can lead to incorrect results if not used properly. When using DISTINCT, make sure to select only the necessary columns to avoid counting duplicates. When using COUNT, be aware that it counts all rows, including NULL values, so filter out NULL values if needed. When using UNION, ensure that the columns selected in each query have the same data types to avoid unexpected results. 

$query = "SELECT COUNT(DISTINCT column_name) FROM table_name WHERE column_name IS NOT NULL";
$result = mysqli_query($connection, $query);
$row = mysqli_fetch_assoc($result);
echo $row['COUNT(DISTINCT column_name)'];

Keywords

DISTINCT COUNT UNION SELECT query potential issues

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