Suppose that some relation satisifies a non-trivial functional dependency of the form A->B, where B is a nonprime attribute. 2NF is violated if A is not a superkey but is a proper subset of a candidate key 3NF is violated if A is not a superkey You have spotted that the 3NF requirement is just … Read more
MySql decimal types are a little bit more complicated than just left-of and right-of the decimal point. The first argument is precision, which is the number of total digits. The second argument is scale which is the maximum number of digits to the right of the decimal point. Thus, (4,2) can be anything from -99.99 … Read more
Unique – Guarantees unique values for the column(or set of columns) included in the index Covering – Includes all of the columns that are used in a particular query (or set of queries), allowing the database to use only the index and not actually have to look at the table data to retrieve the results … Read more
I’d go for having a single table for all references, but additional tables like BookReferences and so on for metadata not applicable for all reference types. Searching and querying would not be more difficult – after all you could just create a view which aggregates all information as in the single-table solution, and then query … Read more
Basically, NULL values occupy 1 bit in the NULL bitmap. But it’s not that simple. The null bitmap (per row) is only allocated if at least one column in that row holds a NULL value. This can lead to a seemingly paradoxic effect in tables with 9 or more columns: assigning the first NULL value … Read more
Create a model with a text field > rails g model Arches thearray:text invoke active_record create db/migrate/20111111174052_create_arches.rb create app/models/arches.rb invoke test_unit create test/unit/arches_test.rb create test/fixtures/arches.yml > rake db:migrate == CreateArches: migrating =================================================== — create_table(:arches) -> 0.0012s == CreateArches: migrated (0.0013s) ========================================== edit your model to make the field serialized to an array class Arches < … Read more
I think he is right ( for the wrong reason) because primary key cannot change, but username can change. So you should use userid because it wouldn’t change.
If you must do this parsing, I’m sure you’ll get lots of good suggestions here. My suggestion is – don’t do this parsing. Instead, create your input fields so that the information is already separated out. Have separate fields for title, first name, middle initial, last name, suffix, etc.
Add a 4th table specifically for these values you want to be unique then link these keys from this table into the others using a one to many relationship. For example you will have the unique table with an ID, AppName and ItemName to make up its 3 columns. Then have this table link to … Read more
If it is for SQL Server I like the DB Diagram from SQL Server Management Studio.