Find max value from coalesce function - sql

Question

This is my query:
declare #t table (date1 date,date2 date,date3 date)
insert into #t values ('2019-01-01','2019-01-20','2019-02-10')
insert into #t values (null,null,'2019-02-01')
insert into #t values (null,'2019-02-01','2019-02-02')
My expected output is:
2019-02-10
2019-02-01
2019-02-02
I tried to use coalesce like :
select coalesce(date1,date2,date3) as maxdate from #t
I know coalesce returns first not null value. So what I can do to get my desired result.

Answer 1

This will do the trick.
Basically you transform every row in a data-set, using VALUES clause, and then just get the MAX value.
SELECT (
SELECT MAX(LastUpdateDate)
FROM (VALUES (date1),(date2),(date3)) AS UpdateDate(LastUpdateDate)) AS LastUpdateDate
FROM #t

Answer 2

coalesce() has nothing to do with this. Unfortunately, SQL Server does not support greatest(). But you can use apply:
select t.*, m.max_date
from #t t cross apply
(select max(dte) as max_date
from (values (t.date1), (t.date2), (t.date3)) v(dte)
) m;
The max() ignores NULL values, so this does what you expect.
Here is a db<>fiddle.

Answer 3

Personally I would normalise your data, by unpivoting it, and then getting the MAX in the group. This does, however, require you have some kind of column to identify the row (I use an IDENTITY in this example):
DECLARE #t table (id int IDENTITY,
date1 date,
date2 date,
date3 date);
INSERT INTO #t
VALUES ('2019-01-01', '2019-01-20', '2019-02-10');
INSERT INTO #t
VALUES (NULL, NULL, '2019-02-01');
INSERT INTO #t
VALUES (NULL, '2019-02-01', '2019-02-02');
SELECT MAX([date])
FROM #t t
CROSS APPLY (VALUES(t.date1),(date2),(t.date3))V([date])
GROUP BY t.id;