Distinct in SQL

Test Book title: Add distinct is superfluous?
If the distinct is not added, the following conditions are shown:
SELECT COUNT (*)
From ' players '
GROUP by town

COUNT (*)
1
2
2

If you add distinct, you'll see something like this:

SELECT Distinct COUNT (*)
From ' players '
GROUP by town

COUNT (*)
1
2

Conclusion: If you add distinct, the duplicate value is not counted in count.



Another test example:
SELECT DISTINCT Playerno
From players
WHERE town = ' Changsha '

Distinct why not repeat it?

Remember the mechanism: the data is extracted by where. Then follow the columns specified by distinct, removing the columns Playerno values.

Popular approach: If it is obvious that the take-out is either a row or no result. Then the use of distinct is superfluous.

Distinct in SQL