普通条目的唯一条目？

Question

I am new to MySQL, and I have two columns. My desired table is the following:

Column 1   Column 2
--------   --------
A          1
A          2
A          3
A          4
B          1
B          2
B          3
B          4

The problem I face, is for every row which has a value of A in column 1, column 2 must be unique, as seen above. However duplicates in Column 2 are allowed aslong as the corresponding Column 1 value is different from that of other entries?

I'm really sorry for the bad, wording, I'm just not sure how to ask the darn question.

Answer 1

The DDL command to add a unique constraint on multiple columns:

ALTER TABLE      `table_name` 
ADD UNIQUE INDEX (`column1`, `column2`);

Answer 2

您必须定义Column1和Column2都是唯一的：

alter table YOUR_TABLE add unique index(Column1, Column2);