How many rows in a table

SELECT COUNT(*) AS Count FROM tblMyTable;

  $Output = 0;
  if (count($Results) > 0)
    $Output = $Results[0]['Count'];
  
  return($Output);

Count and Group Combined

List the number of customers in each country

SELECT COUNT(Id), Country FROM tblMyTable GROUP BY Country

Adding values returned as a the total

Use SUM()

Counting multiple things in a single query

SELECT 
    COUNT(*) AS TotalUsers,
    COUNT(CASE WHEN EmailAddress != '' THEN 1 END) AS EmailAddressCount,
    COUNT(CASE WHEN UserAcceptsMarketing = 1 THEN 1 END) AS UserAcceptsMarketingCount,
    COUNT(CASE WHEN Options != 0 THEN 1 END) AS OptionsCount,
    COUNT(CASE WHEN Joined > NOW() - INTERVAL 30 DAY THEN 1 END) AS JoinedInLast30DaysCount,
    COUNT(CASE WHEN LastActive > NOW() - INTERVAL 30 DAY THEN 1 END) AS ActiveInLast30DaysCount
FROM my_table;