Question

In a table I’ve got 3 columns:

id
tag1
tag2

id is a primary key.

And i only want one unique tag1-tag2-combination in that table.

eg if one entry looks like:

id: 1
tag1: cat
tag2: dog

I dont want a second entry like this one beneath to get inserted:

id: 2
tag1: cat
tag2: dog

So i made all 3 columns primary keys but the problem is that then the second entry would get inserted since it looks in the combination of all 3 of them.

How do i solve this so that only the combination of the tag1 and tag2 is unique?

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UPDATE: I added a unique contraint on tag1 and tag2. however, its still possible to insert:

id: 3
tag1: dog
tag2: cat

Is there a way to prevent this?

Answer 1

You should leave ID as the primary key, and then can create a unique constraint for the tag1 and tag2:

ALTER TABLE my_table ADD CONSTRAINT uc_tags UNIQUE (tag1, tag2)

With the unique constraint, you will be guaranteed that you will never have two rows with duplicate tag1 and tag2 values.

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EDIT:

Further to your last update, you cannot enforce that with unique constraints. Keep in mind that for the database a record with (tag1 = dog, tag2 = cat) is totally different from a record with (tag1 = cat, tag2 = dog).

Probably your best bet is to redesign your database schema, as follows:

• Table “tags”
• Table “messages” (or whatever you are tagging)
• Table “tags_messages” with the following fields (message_id, tag_id)

Then you can simply set (message_id, tag_id) of the “tag_messages” table as a primary key. This will automatically enforce that there cannot be any message with a duplicate tag.

Some sample data:

Table: messages

message_id   |   title
-------------+------------------
1            |   some message
2            |   another message


Table: tags

tag_id       |   tags
-------------+-------------------
1            |   cat
2            |   dog
3            |   duck
4            |   horse


Table: messages_tags

message_id   |   tag_id
-------------+-------------------
1            |   1
1            |   2
2            |   3
2            |   4
2            |   1