Contents
Why are compound keys used?
Compound keys are always made up of two or more primary keys from other tables. In their own tables, both of these keys uniquely identify data but in the table using the compound key they are both needed to uniquely identify data. There may be a second table for each course with a primary key called course number.
What do you need if you should in force many to many relationship between two tables?
To represent a many-to-many relationship, you must create a third table, often called a junction table, that breaks down the many-to-many relationship into two one-to-many relationships. You insert the primary key from each of the two tables into the third table.
Do you need surrogate key for Insert Table?
In addition, the vast majority of database tables are read far more often than written. That makes anything you do on the select side far more relevant than anything on the insert side. No surrogate key is needed for link tables. Unless you use an ORM that can’t cope and dictates your DB design for you…
Is it good to have surrogate key on many to many?
With a simple two-column many-to-many mapping, I see no real advantage to having a surrogate key. Having a primary key on (col1,col2) is guaranteed unique (assuming your col1 and col2 values in the referenced tables are unique) and a separate index on (col2,col1) will catch those cases where the opposite order would execute faster.
Which is the best PK for many to many table?
So it seems like if the ONLY job is to link the two tables, the best PK would be the dual-column PK. But if it serves other purposes then add another NDX as a PK with a foreign keys and a second unique index. Index or PK is the best way to make sure there are no duplicates.
Which is better a surrogate key or composite key?
A surrogate key might seem like a tidier solution than a key based on more than one column, but they do not simplify data design – using an identity column over a composite key actually complicates the data design.