Main difference between Excel sheets and SQL tables: in SQL, there is no row number. To distinguish unique rows, you have to rely on the data. Most often, one uses a separate "id" column, as in the examples already given in this thrrad, but if a field (or combination of fields) contains unique data, you can use that as (primary) key. Besides, defining a key for a table is optional, but of course, most often it is better to have one.
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