SQL不同于数据库中的2个字段

Question

Can you get the distinct combination of 2 different fields in a database table? if so, can you provide the SQL example.

Answer 1

How about simply:

select distinct c1, c2 from t

or

select c1, c2, count(*)
from t
group by c1, c2

Answer 2

If you want distinct values from only two fields, plus return other fields with them, then the other fields must have some kind of aggregation on them (sum, min, max, etc.), and the two columns you want distinct must appear in the group by clause. Otherwise, it's just as Decker says.

Answer 3

您可以在SQL下面使用两列来获得结果：

SELECT COUNT(*) FROM (SELECT DISTINCT c1, c2 FROM [TableEntity]) TE

Answer 4

如果您仍希望仅按一列分组（如我所愿），您可以嵌套查询：

select c1, count(*) from (select distinct c1, c2 from t) group by c1

Answer 5

Share my stupid thought:

Maybe I can select distinct only on c1 but not on c2, so the syntax may be select ([distinct] col)+ where distinct is a qualifier for each column.

But after thought, I find that distinct on only one column is nonsense . Take the following relationship:

   | A | B
__________
  1| 1 | 2
  2| 1 | 1

If we select (distinct A), B , then what is the proper B for A = 1 ?

Thus, distinct is a qualifier for a statement .