Question

I have a table called Animal. AnimalId is the primary key & I wanted to set the column AnimalType_id as UNIQUE (I have an AnimalType table and need to set a foreign key here)

ALTER TABLE Animal
ADD UNIQUE Animal.AnimalType_id int

There already is data in both tables, because of that I can’t drop the table.

This however results in an error:

Msg 102, Level 15, State 1, Line 2
Incorrect syntax near ‘.’.

Answer 1

See the documentation for how to add a table constraint.

ALTER TABLE Animal ADD CONSTRAINT UQ_Animal_AnimalTypeId UNIQUE (AnimalType_id)

It sounds like AnimalType_id is a foreign key so I just wanted to check you understood that by making this column unique, you’re making the relationship one-one – you’ll only be able to have one animal of each type.

Since you’re getting an error adding the unique constraint, I’m going to suggest that you actually want a foreign key instead of a unique constraint:

ALTER TABLE Animal
    ADD CONSTRAINT FK_Animal_AnimalType
    FOREIGN KEY
    (
        AnimalType_id
    )
    REFERENCES AnimalType
    (
        id
    )

I’ve had to guess at the name of the AnimalType table name and it’s primary key column name – please change these if they are incorrect.