SQL：如何对两个项目的组合（无序）唯一的两列应用唯一约束

Question

I have a table with three columns ( node1 varchar, node2 varchar, other_data int )

node1 and node2 are nodes that make up an edge for a graph.

So if node1 = A and node2 = B , it would be equivalent for node1 = B and node2 = A .

Is there a way to ensure duplicate entries could not be created.

For example, if the following is input:

node1 = A, node2 = B, other = 123

If someone tried to input the following it would fail as the two nodes are already in the table:

node1 = B, node2 = A, other = 123

Answer 1

编写一个触发器，以在node1和node2的SORTED组合上构建一个构造字段，然后尝试将其插入唯一索引中。

Answer 2

I would normally implement this with a CHECK constraint such that node1 always sorts earlier than node2 :

create table T(
  node1 varchar(20) not null,
  node2 varchar(20) not null,
  other_data int null,
  constraint CK_T_node_order CHECK (node1<node2),
  constraint UQ_T_nodes UNIQUE (node1,node2)
)

It adds a small amount of complexity to INSERT but makes enforcing your desired constraint quite easy.

Of course, you haven't said what SQL product your using. If yours doesn't support CHECK constraints (Grr. MySQL, I'm looking at you), You'd have to use a trigger to implement the check constraint instead.