If I want to do a check a name, I want to see how many rows/name exists in the "username" column under users table. Lets say thousands ... hundred of thousands, should I use:
count(name),
count(*) or
SELECT username FROM users where username = 'name'
Which one is the more appropriate? Or they will give same result in term of speed/response?
COUNT(*) and COUNT(Name) might produce different values. COUNT will not include NULL values, so if there are any instances of Name that equal NULL they will not be counted.
COUNT(*) will also perform better than Count(Name). By specifying COUNT(*) you are leaving the optimizer free to use any index it wishes. By specifying COUNT(Name) you are forcing the query engine to use the table, or at least an index that contains the NAME column.