两列的SQL唯一组合，反之亦然

Question

I have the following table:

ID
KEY_1
KEY_2

I want the combination of KEY_1 with KEY_2 to be unique and vice versa.

ALTER TABLE dbo.table_name
  ADD CONSTRAINT uq_table_name UNIQUE(KEY_1, KEY_2);

This way I can make the combination between KEY_1 AND KEY_2 unique but I want the vice versa as well.

Example:

KEY_1 = 111;
KEY_2 = 222;

I cannot insert this values again or neither can I insert the values vice versa.

This shouldn't be valid (since it's the same pair of keys):

KEY_1 = 222;
KEY_2 = 111;

Thanks

Answer 1

You can achieve this with an unique functional index instead of an unique constraint.

create unique index table_name_uix 
  on table_name (greatest(key_1, key_2), least(key_1, key_2));

Details on Postgres unique constraint vs unique index here

Answer 2

There is no simple way to do this.

As you have discovered:

• UNIQUE constraints are sets of attributes, not list of attributes.
• UNIQUE only accepts attributes names

One way to solve this is by creating two new attributes: key1_key2 and key2_key1 (the concatenation of each attribute) and then adding these two attributes as UNIQUE constraints each.

You could also have a trigger that would update these attributes automatically (during insert/update). This way you would avoid consistency problems.

Another way to do it is by having a trigger that checks the constraint (sort of like a tuple constraint where you manually check the uniqueness of these concatenations (sounds more complicated and error prone than the UNIQUE solution).

Answer 3

You can add two columns to your table, key_min and key_max , use a trigger to calculate key_min = smallest value between key_1 and key_2 ; key_max = largest value between key_1 and key_2 and create the constraint UNIQUE(key_min,key_max). You can see a similar solution using MariaDB and virtual columns here .