Re: Help to design relations between some 5 tables with possible many-to-many relations

On 2021-06-21 14:03, Dean Gibson (DB Administrator) wrote:
>
> On 2021-06-21 12:07, JORGE MALDONADO wrote:
>> Hi,
>>
>> I have the following tables: *reports*, *standards*, *elements*,
>> *tables*, *items*.
>> * One *report* has many *standards*.
>> * One *standard* has many *elements*.
>> * One *element* has many *tables*.
>> * One *table* has many *items*.
>>
>> *Reports* are generated yearly and current year might have exactly
>> the same *standards* as last year but not necessarily. Maybe, the
>> current year *report* includes part of last year's report *standards*
>> and also includes new *standards*. Or maybe, the current year report
>> includes only a subset of last year *reports* *standards*. Or maybe,
>> the current report has a totally new set of standards.The same
>> behavior is true between *standards* and *elements*, between
>> *elements* and *tables* and between *tables* and *items*. I see that
>> many-to-many relations exist between all of the tables but I am not
>> able to clearly visualize.
>>
>> I am a bit confused about how to design this part of my DB. I hope my
>> explanation is clear enough. I will very much appreciate your feedback.
>>
>> Respectfully, Jorge Maldonado
>
> *First of all*, get rid of the name "*tables*" as one of your table
> names.  It will cause nothing but confusion for you & the people
> trying to help you.  In the following discussion, I'm going to replace
> the name "*tables*" with "*groups*".
>
> Think of this of this in two parts:
>
> 1.  What describes each table.
> 2. *How the data will describe the relationships between the tables*.
>
>
> Start off with the *items*:  This table names columns for the
> information about each *item*, including a (probably) unique column
> for each *item*.  Each data row (record) in this table will describe
> the details of each *item*.
>
> Now define *two *tables for your groups.  The first table names
> columns for the information associated with each *group*, including a
> (probably*) *unique column for each *group*. Each data row in this
> table will describe the details for each *group*. The second table has
> typically just a column for a *group *& an***item *that the *group
> *contains.  Each data row in this table will simply list, for each
> *group *name, all the *items *(by name) that it contains, one row for
> each *group*/*item *pair.  This second table might be named
> "group_items" to distinguish it from the "*groups*" table name.
>
> Repeat the above process for *groups*, for each *element*, *standard*,
> & *report*.
>
> If standards change for each year (& this applies to all of your
> tables), you are going to have to represent that in the table *data*,
> *not in the table design*.  Eg, you may want names (*in the data*, not
> the table description) for standards for each year, with a year prefix
> or suffix in the name.
>
> A more sophisticated approach might have a year column in the relevant
> tables, but it's important to get started with the basics, & then make
> changes as you see fit.  That's one of the beauties of SQL databases: 
> Once you come up with a better design, it's trivial to then create the
> data in the new tables with simple SELECT statements from the old tables.
>
> Also, try to find a good book on database design.  No, I don't know of
> any.

One example is worth a thousand words:

CREATE TABLE "Items" (
    item_name VARCHAR,
    item_stuff VARCHAR
);
--------------------------------
CREATE TABLE "Groups" (
    group_name VARCHAR,
    group_stuff VARCHAR
);

CREATE TABLE "GroupItems" (
    group_name VARCHAR,
    item_name VARCHAR
);
--------------------------------
CREATE TABLE "Elements" (
    element_name VARCHAR,
    element_stuff VARCHAR
);

CREATE TABLE "ElementGroups" (
    element_name VARCHAR,
    group_name VARCHAR
);
--------------------------------
CREATE TABLE "Standards" (
    standard_name VARCHAR,
    standard_stuff VARCHAR
);

CREATE TABLE "StandardElements" (
    standard_name VARCHAR,
    element_name VARCHAR
);
--------------------------------
CREATE TABLE "Reports" (
    report_name VARCHAR,
    report_stuff VARCHAR
);

CREATE TABLE "ReportStandards" (
    report_name VARCHAR,
    standard_name VARCHAR
);
--------------------------------
Obviously, the *_stuff column is replaced by table-specific columns.