unique-constraint

I have two models with a HABTM relationship - User and Role. user - has_and_belongs_to_many :roles role - belongs_to :user I want to add a uniqueness constraint in the join (users_roles table) that says the user_id and role_id must be unique. In Rails, would look like: validates_uniqueness_of :user, :scope => [:role] Of course, in Rails, we don't usually have a model to represent the join relationship in a HABTM association. So my question is where is the best place to add the constraint? Art Shayderov You can add uniqueness to join table add_index :users_roles, [ :user_id, :role_id ], :unique

I noticed that I can have NULL values in columns that have the UNIQUE constraint: UNIQUE(col) Would that generate any issues in certain situations? While the following addresses multiple null values, it does not address any "issues" associated with such a design, other than possible database/SQL portability - as such, it should probably not be considered an answer, and is left here merely for reference. This is actually covered in the SQLite FAQ. It is a design choice - SQLite (unlike SQL Server) chose that multiple NULL values do not count towards uniqueness in an index. The SQL standard

Is there way in MySQL to have two unique keys and connect them somehow? for example if i have the following table and 'title'and 'store' are a unique keys id | category | title | price | store 1 | outdoors | fishing rod | 59.99 | wal-mart 2 | auto | Penzoil Oil | 9.99 | target and i try to insert the following record. This new record would be ignored because the title is "fishing rod" AND the store is 'wal-mart' and there is an existing record with that title and store | outdoors | fishing rod | 30.99 | wal-mart but if i attempted to insert the following record it would be accepted because

How to create a constraint on the uniqueness of all the values ​​in the array like: CREATE TABLE mytable ( interface integer[2], CONSTRAINT link_check UNIQUE (sort(interface)) ) my sort function create or replace function sort(anyarray) returns anyarray as $$ select array(select $1[i] from generate_series(array_lower($1,1), array_upper($1,1)) g(i) order by 1) $$ language sql strict immutable; I need that would be the value {10, 22} and {22, 10} considered the same and check under the UNIQUE CONSTRAINT I don't think you can use a function with a unique constraint but you can with a unique index

I've created the following table: CREATE TABLE MMCompany ( CompanyUniqueID BIGSERIAL PRIMARY KEY NOT NULL, Name VARCHAR (150) NOT NULL, PhoneNumber VARCHAR(20) NOT NULL UNIQUE, Email VARCHAR(75) UNIQUE, CompanyLogo BYTEA ); The email column is unique and it causes a "bug" in my scenario since there could only be one record with null. I'm trying to achieve records of companies without the same email but at the same time allow a companies to have no email. How can I achieve that? This is a misunderstanding. The UNIQUE constraint does exactly what you want. Multiple NULL values can coexist in a

I’m interested in learning which technique developers prefer to use to enforce uniqueness in SQL Server: UNIQUE CONSTRAINT or UNIQUE INDEX. Given that there is little difference in the physical implementation of each, how do you decide which is best? Are there reasons other than performance to evaluate the best solution? Are there database management advantages to one or the other? This MSDN article comparing the two is for SQL Server 2000: http://msdn.microsoft.com/en-us/library/aa224827(SQL.80).aspx For most purposes, there's no difference - the constraint is implemented as an index under