Like this kind of table, what should be statistically more efficient

Like this kind of table, how should be statistically more efficient?
The contents of the table are as follows:

ID bid
1 138
2 150
3 139
4 139
5 139
6 138
7 138
8 150


I just want to count the values in the Bid field, don't repeat.

How should statistics be more efficient?

The statistical results are as follows:
138
150
139

------Solution--------------------
GROUP by bid;
------Solution--------------------
SELECT DISTINCT bid from table_name;
------Solution--------------------
SELECT DISTINCT bid from table_name;
------Solution--------------------
SELECT DISTINCT bid from T;

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