tsql most effective way to compare three date values in where clause? - sql

Question

I am trying to create a stored procedure.
What is most effective way to compare three date values in where clause?
Example:
tbl1.Date1,
tbl2.Date2, -- NOTE: Date2 can be NULL.
tbl3.Date3
Example data:
Date1 Date2 Date3
2016-12-20 2016-11-21 2016-11-30
2016-11-21 NULL 2016-12-20
First, I compare Date1 and Date2 and I choose "bigger" date.
Then I compare this "bigger" date to Date3.
If comparsion is true, I write values to table.
-- This is simplified example:
INSERT INTO records
(
[user_date],
[user_name]
)
SELECT
tbl1.Date1,
tbl1.user_name
FROM
table1 AS tbl1
INNER JOIN table2 AS tbl2 ON tbl1.id = tbl2.id
INNER JOIN table3 AS tbl3 ON tbl2.id = tbl3.id
WHERE
-- I need to know what is bigger, Date1 or Date2, so I can compare correct date to Date3.
ISNULL(tbl2.Date2, tbl1.Date1) <= tbl3.Date3 -- ISNULL, doesn't work here, because Date2 and Date1 can get a value and comparison fails if Date1 is bigger than Date3.
AND ISNULL(tbl2.Date2, tbl1.Date1) > tbl3.last_date

Answer 1

Perhaps something like this?
Declare #tbl1 table (id int,date1 date);Insert Into #tbl1 values (1,'2016-12-20'),(2,'2016-11-21 ');
Declare #tbl2 table (id int,date2 date);Insert Into #tbl2 values (1,'2016-11-21'),(2,null);
Declare #tbl3 table (id int,date3 date);Insert Into #tbl3 values (1,'2016-12-30'),(2,'2016-12-20');
Select User_Date = (Select max(d) from (values(date1),(date2),(date3)) D(D))
,A.ID
From #tbl1 A
Join #tbl2 B on A.ID=B.ID
Join #tbl3 C on A.ID=C.ID
Returns
User_Date ID
2016-12-30 1
2016-12-20 2

Answer 2

You can use a case expression, or inline if, to conditionally return the bigger of the two dates, for comparison.
Sample Data
-- Sample data.
DECLARE #Sample TABLE
(
Date1 DATE NOT NULL,
Date2 DATE NULL,
Date3 DATE NOT NULL
)
;
INSERT INTO #Sample
(
Date1,
Date2,
Date3
)
VALUES
('2016-01-02', NULL, '2016-01-01'), -- D1 > D3.
('2015-12-31', '2016-01-02', '2016-01-01'), -- D2 > D1 and D3
('2015-12-30', '2016-12-31', '2016-01-01') -- D3 > D1 and D2
;
Case Expression
-- Using a CASE EXPRESSION.
SELECT
CASE WHEN s.Date2 > s.Date1 THEN s.Date2 ELSE s.Date1 END AS Bigger_of_D1_D2,
*
FROM
#Sample AS s
WHERE
CASE WHEN s.Date2 > s.Date1 THEN s.Date2 ELSE s.Date1 END > s.Date3
;
Inline If (SQL Serer 2012, or above)
-- Using an INLINE IF.
SELECT
IIF(s.Date2 > s.Date1, s.Date2, s.Date1) AS Bigger_of_D1_D2,
*
FROM
#Sample AS s
WHERE
IIF(s.Date2 > s.Date1, s.Date2, s.Date1) > s.Date3
;
Both methods rely on the fact that NULL is not equal to anything, inculding itself. This means that checking D2 against D1 will always return false, if Date2 is null. If Date1 also allows NULLs this technique would fail. In that case you could expand the case expression to include more where expressions, or include the ISNULL function.
The second option is an example of sytactic sugar. Behind the scenes SQL Server will convert your code into a simple case expression, as per MSDN:
IIF is a shorthand way for writing a CASE expression.

Answer 3

Here is my suggestion:
INSERT INTO records
(
[user_date],
[user_name]
)
SELECT tbl1.Date1
,tbl1.user_name
FROM table1 as tbl1
INNER JOIN table2 as tbl2 on tbl1.ID=tbl2.ID
INNER JOIN table3 as tbl3 on tbl1.ID=tbl3.ID
WHERE (SELECT CASE WHEN tbl1.Date1 > ISNULL(tbl2.Date2,'1900-01-01') THEN tbl1.Date1 ELSE tbl2.Date2 END) <= tbl3.Date3
AND
(SELECT CASE WHEN tbl1.Date1 > ISNULL(tbl2.Date2,'1900-01-01') THEN tbl1.Date1 ELSE tbl2.Date2 END) > tbl3.last_date