SQL除去值相同的記錄，只保留一條

假如有item表，有四列，id, c1, c2, c3，資料如下：

id  c1  c2  c3

---------------
1   c11 c21 c31
2   c11 c21 c31
3   c11 c21 c31
4   c21 c21 c31
5   c21 c21 c31
6   c21 c22 c31
7   c11 c21 c32

select * from

`item`
group
by c1, c2, c3;

id  c1  c2  c3

---------------
1   c11 c21 c31
7   c11 c21 c32
4   c21 c21 c31
6   c21 c22 c31

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