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Cardinality between tables

Cardinality between tables

Does anyone know a simple way of using sql to check the cardinality between two tables to see if the join used is 1:1, 1:M or M:M.



Software code, like laws and sausages, should never be examined in production - Edward Tenner

RE: Cardinality between tables

Yes, Dbomrrsm, there are some sneaky methods to use SQL and relational architecture to determine cardinality between two tables:

M:M -- If you are truly limiting your scope to just two tables, then this is not even possible in a relational world...How could you implement a truly M:M relationship? If you have a single column in a child table that points to its parent ID in the master table, just how many different IDs could that single-column Foreign Key (FK) point to?...Just one, not many. To implement a M:M relationship, you must use a third table that cross references the Primary Keys of the two tables that share the M:M (aka "network") relationship. Determine if a table exists that has two or more FKs to two or more tables, and, by definition, that table is a "cross-reference" table between tables that have a de facto M:M relationship.

1:1 -- This is where the trick comes in...if a 1:1 relationship exists between two tables, then there cannot be any duplicates in the foreign key column. Therefore, if a 1:1 relationship exists, there is either already a Unique constraint (or a Unique index) on the FK column, or you can try to create one: if it fails, then there is a 1:M relationship; if it succeeds then there is a de facto 1:1 relationship. If there was no Unique index on the column previously, and if you want the tables to remain 1:1, then just leave the Unique index in place. Here is the code to produce a unique index:


CREATE UNIQUE INDEX <some name> ON <table name> (<FK column name);

1:M -- If you have a table with an FK and you have ruled out M:M and you have ruled out 1:1, then guess what?...You have a 1:M relationship.

Let us know if this gives you the tactics to identify cardinality.

(aka Dave of Sandy, Utah, USA)
[ Providing low-cost remote Database Admin services]
Click here to join Utah Oracle Users Group on Tek-Tips if you use Oracle in Utah USA.

RE: Cardinality between tables


Thanks once again for your help.


Software code, like laws and sausages, should never be examined in production - Edward Tenner

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