Question

I’ve been given a stack of data where a particular value has been collected sometimes as a date (YYYY-MM-DD) and sometimes as just a year.

Depending on how you look at it, this is either a variance in type or margin of error.

This is a subprime situation, but I can’t afford to recover or discard any data.

What’s the optimal (eg. least worst 🙂 ) SQL table design that will accept either form while avoiding monstrous queries and allowing maximum use of database features like constraints and keys*?

*i.e. Entity-Attribute-Value is out.

Answer 1

+1 each to recommendations from ninesided, Nikki9696 and Jeff Siver – I support all those answers though none was exactly what I decided upon.

My solution:

• a date column used only for complete dates
• an int column used for years
• a constraint to ensure integrity between the two
• a trigger to populate the year if only date is supplied

Advantages:

1. can run simple (one-column) queries on the date column with missing data ignored (by using NULL for what it was designed for)
2. can run simple (one-column) queries on the year column for any row with a date (because year is automatically populated)
3. insert either year or date or both (provided they agree)
4. no fear of disagreement between columns
5. self explanatory, intuitive

I would argue that methods using YYYY-01-01 to signify missing data (when flagged as such with a second explanatory column) fail seriously on points 1 and 5.

Example code for Sqlite 3:

create table events
(
rowid integer primary key,
event_year integer,
event_date date,
check (event_year = cast(strftime("%Y", event_date) as integer))
);

create trigger year_trigger after insert on events
begin
update events set event_year = cast(strftime("%Y", event_date) as integer)
    where rowid = new.rowid and event_date is not null;
end;

-- various methods to insert
insert into events (event_year, event_date) values (2008, "2008-02-23");
insert into events (event_year) values (2009);
insert into events (event_date) values ("2010-01-19");

-- select events in January without expressions on supplementary columns
select rowid, event_date from events where strftime("%m", event_date) = "01";