A brief beginner's introduction to the DISTINCT operator in SQL. DISTINCT creates a set of unique records, based on field values. In other words, duplicates records, based on field values, are removed. DISTINCT is often used with COUNT, to achieve a count of distinct values. Be aware that NULL is treated as a distinct value in terms of records, but COUNT ignores NULLs.
See the full code here: https://douglaskline.blogspot.com/2018/10/using-distinct-in-sql.html