Find the nearest date from entered date in SQL, both ways - mysql

Question

I have a problem, I have a task to find a nearest date to a given date, looking both ways, older or younger. But I have no idea, I'm new to SQL, tried googling but didn't find any help.
create proc Task
(#Date date)
as
begin
select top(1) p.FirstName, p.LastName, e.BirthDate, e.JobTitle from HumanResources.Employee e
join Person.Person p
on p.BusinessEntityID = e.BusinessEntityID
where e.BirthDate>#Date
end
I started something like this, and then lost it

Answer 1

Always remember: TOP without ORDER BY by doesn't make much sense; add an order by that is ascending (your birthdate > #date comparison asks for all birthdates greater than/after, so the TOP(1) ordered by birthdate ascending would be the earliest birthdate that is greater than your variable)
Then take the entire query, paste it again, put UNION ALL between them and flip your ORDER BY to be descending and your comparison to be less than, in this second query
You thus end up with a query that chooses the smallest that is greater than and the largest that is less than i.e. The nearest ones to your variable
Consider whether you should be using >= and <= if a date that is bang on meets the specification

Answer 2

I would not use functions in order (as server will not be able to use indexes).
Instead, I'd go for two-queries solution.
It could be wrapped in SP something like this (MySQL version):
CREATE FUNCTION `Task`(
`aDate` DATE
)
RETURNS INT
BEGIN
SELECT
`BusinessEntityID`
, `BirthDate`
INTO
#id_next
, #birthdate_next
FROM
`Employee`
WHERE
`BirthDate` >= aDdate
ORDER BY
`BirthDate` ASC
LIMIT
1
;
IF #birthdate_next IS NULL THEN
SELECT
`BusinessEntityID`
, `BirthDate`
INTO
#id_prev
, #birthdate_prev
FROM
`Employee`
WHERE
`BirthDate` < aDate
ORDER BY
`BirthDate` DESC
LIMIT
1
;
ELSE
IF DATEDIFF(#birthdate_next, aDate) > 1 THEN
SELECT
`BusinessEntityID`
, `BirthDate`
INTO
#id_prev
, #birthdate_prev
FROM
`Employee`
WHERE
`BirthDate` < aDate
AND `BirthDate` > DATE_SUB(aDate, INTERVAL DATEDIFF(#birthdate_next, aDate) DAY)
ORDER BY
`BirthDate` DESC
LIMIT
1
;
END IF;
END IF;
CASE
WHEN #id_prev IS NULL AND #id_next IS NULL THEN RETURN NULL;
WHEN #id_prev IS NULL THEN RETURN #id_next;
WHEN #id_next IS NULL THEN RETURN #id_prev;
WHEN DATEDIFF(#birthdate_next, aDate) < DATEDIFF(aDate, #birthdate_prev) THEN RETURN #id_next;
ELSE RETURN #id_prev;
END CASE;
END
So in some cases only single query (the first one) would be executed.
The query will use index by BirthDate.
If the first query diff from specified date is less than 2 days, the second query will not be executed at all (it is more complicated as ordered DESC).
It is possible to further simplify the SP, however I'm keeping it "as is" so it is easier to understand.

Answer 3

Use datediff() to get the duration between the two dates. Since you don't care whether the date is in the future or in the past, use abs() to get the absolute value of the duration. Then order by the absolute duration and take the top one record.
I'm not sure if you're really on MySQL or on SQL Server. The TOP (1) indicates SQL Server, the tag says MySQL.
Here's the MySQL version:
SELECT p.firstname,
p.lastname,
e.birthdate,
e.jobtitle
FROM humanresources.employee e
INNER JOIN person.person p
ON p.businessentityid = e.businessentityid
ORDER BY abs(datediff(e.birthdate, #date))
LIMIT 1;
And here for SQL Server:
SELECT TOP (1)
p.firstname,
p.lastname,
e.birthdate,
e.jobtitle
FROM humanresources.employee e
INNER JOIN person.person p
ON p.businessentityid = e.businessentityid
ORDER BY abs(datediff(day, e.birthdate, #date));
May need some tweaks depending on the actual data types you're using.
Edit:
Addressing fifoniks's concern a version, that could perform better, if the respective indexes exist (on humanresources.employee.birthdate optimally once ascending and once descending).
It first gets the union of the nearest record in the the future of #date (including #date) and the analog record for the past, hopefully using indexes along the way. From these two records, the one with the lowest absolute duration to #date is picked. Then person gets joined.
SELECT p.firstname,
p.lastname,
y.bithdate,
y.jobtitle
FROM (SELECT TOP (1)
x.businessentityid,
x.birthdate,
x.jobtitle
FROM (SELECT TOP (1)
e.businessentityid,
e.birthdate,
e.jobtitle
FROM humanresources.employee e
WHERE e.birthdate >= #date
ORDER BY e.birthdate ASC
UNION ALL
SELECT TOP (1)
e.businessentityid,
e.birthdate,
e.jobtitle
FROM humanresources.employee e
WHERE e.birthdate <= #date
ORDER BY e.birthdate DESC) x
ORDER BY abs(datediff(day, x.birthdate, #date)) ASC) y
INNER JOIN person.person p
ON p.businessentityid = y.businessentityid;