Search for closes index on SQL table - sql

Question

I have a hypothetical SQL table "EVENTS", with two columns, a UUID index column, and a DateTime column,
The table is populated with values ranging from 1900-01-01 to today, it is not ordered, there are numerous dates missing.
The query that I have to run is basically 'retrieve all events that happened at the requested date (start to the end of the day) or the closest previous date'
If I were looking for all events in a day that I know that exists in the database it would be something as simple as:
SELECT * FROM Events e
WHERE
e.date BETWEEN $START_OF_DAY AND $END_OF_DAY;
But if that date doesn't exist I must retrieve the latest date up to the requested date.

Answer 1

Grab current day, but if no records found, will return all records from the nearest previous day with records.
So in my sample data, Jan 2 returns 3 events dated Jan 1
SQL Server Solution
DECLARE #Input DATE = '2022-01-02' /*Try Jan 1,2,3, or 4*/
DROP TABLE IF EXISTS #Event
CREATE TABLE #Event (ID INT IDENTITY(1,1),EventDateTime DATETIME)
INSERT INTO #Event
VALUES
('2022-01-01 08:00')
,('2022-01-01 09:00')
,('2022-01-01 10:00')
,('2022-01-03 12:00')
SELECT TOP (1) WITH TIES *
FROM #Event AS A
CROSS APPLY (SELECT EventDate = CAST(EventDateTime AS DATE)) AS B
WHERE B.EventDate <= #Input
ORDER BY B.EventDate DESC
SQL Fiddle wasn't letting me create a variable, but here's a the code conceptually for a more efficient version for MySQL. It grabs the desired date range in the first query, then uses it to filter in the second query. I think it should perform far better than the accepted answer assuming you have an index on EventDateTime
CREATE TABLE Event (
ID MEDIUMINT NOT NULL AUTO_INCREMENT
,EventDateTime DATETIME
,PRIMARY KEY (ID));
INSERT INTO Event (EventDateTime)
VALUES
('2022-01-01 08:00')
,('2022-01-01 09:00')
,('2022-01-01 10:00')
,('2022-01-03 12:00');
/*Need to save these off to variables to use in later query*/
SELECT TIMESTAMP(CAST(EventDateTime AS DATE)) AS StartRange
,TIMESTAMP(CAST(EventDateTime AS DATE)) + INTERVAL 1 DAY AS EndRange
FROM Event
WHERE EventDateTime < DATE_ADD('2022-01-04' /*Input*/,INTERVAL 1 DAY)
ORDER BY EventDateTime DESC
LIMIT 1;
SELECT *
FROM Event
WHERE EventDateTime >= StartRange
AND EventDateTime < EndRange

Answer 2

Calculate the most recent date, and do a self join. Although I'm using MYSQL, I believe this is the most generic workaround
CREATE TABLE d0207Event (ID INT ,EventDateTime DATETIME)
INSERT INTO d0207Event
VALUES
(1,'2022-01-01 08:00')
,(2,'2022-01-01 09:00')
,(3,'2022-01-01 10:00')
,(4,'2022-01-03 12:00')
INSERT INTO d0207Event
VALUES
(5, '2021-12-12 08:00');
select t1.*
from d0207Event t1,
(
select min(t1.dat) mindat
from (
select t1.*,
DATEDIFF('2022-01-02', cast(t1.EventDateTime as date)) dat
from d0207Event t1
) t1
where t1.dat >= 0
) t2
where DATEDIFF('2022-01-02', cast(t1.EventDateTime as date)) = t2.mindat
;
There are also many advanced syntaxes that can solve this problem better, depending on which DB you use and your specific application scenario
It seems that you can also choose a database with more syntax, then using an analytic function usually solves the efficiency problem well, since the EVENT table only needs to be queried once.
CREATE TABLE Event (
ID MEDIUMINT NOT NULL AUTO_INCREMENT
,EventDateTime DATETIME
,PRIMARY KEY (ID));
INSERT INTO Event (EventDateTime)
VALUES
('2022-01-01 08:00')
,('2022-01-01 09:00')
,('2022-01-01 10:00')
,('2022-01-03 12:00');
select *
from (
select t1.*,
first_value(cast(t1.EventDateTime as date))
over(order by cast(t1.EventDateTime as date) desc) fv
from event t1
where cast(t1.EventDateTime as date) <= '2022-01-03'
) t1
where cast(t1.EventDateTime as date) = fv
Creating a functional index cast(t1.EventDateTime as date), or creating a virtual column directly can make the query easier, otherwise using date_add() is a good way