SQL Server Performance Forum – Threads Archive
one-to-oneWhat is "time dependent one-to-one relationship"? With such relationship can table have different number of records?
No idea! Does it mean that one row has a one-to-one relationship with another row in the same table that is called ‘the previous row’? And a one-to-one time dependent relationship with another single row in the same table where that relationship is ‘the following row’? Dave Hilditch. Ask a SQL Server Question
Ok, but with normal one-to-one can related tables have different number of rows?
I think that it is typically used to refer to a case where there is a one-to-one relationship at a single point in time, but the history records are stored in the same table, so from the db point of view it is a many-to-one relationship. unless the history rows are stored in a seperate table Cheers
I don;t think there’s a DDL clause in T-SQL that creates a 1-To-1 relationship: you have to construct it by adding a UNIQUE constraint to the FK field on the ‘child’ table (or make it a PRIMARY KEY). In a 1-To-Many relationship the FK field cannot be unique. In both cases, there is no requirement that there is a row in the child table for each key value in the parent table. So in a 1-To-1 relationship, there can be at the most as many rows on both sides, or less rows on the child table than on the parent table.