How to apply max function in a join query? - sql

Question

I am new to SQL Server, I have 3 tables consider an employee table
empid
name
location
1
abc
USA
2
efg
UK
Another table named location-table
location
holidaycode
uk
uk1
usa
usa1
And also holidaytable:
holiday-code
date
type
uk1
2022-01-01
LM
uk1
2022-01-01
RMC
Expected result is:
empid
location
holidaycode
date
type 1
2
uk
uk1
2022-01-01
RMC
suppose I query the tables based on empid and date say ,the holidays available for empid 2 from a particular from date such that on a particular day (2022-01-01) I have LM and RMC on that day , I need only RMC ,if RMC is not available else LMC.

Answer 1

Please try this
select e.empid, e.location, h.holidaycode, [date], max([type])
from employee e inner join location_table l
on e.location=l.location
inner join holidaytable h
on l.holidaycode=h.holidaycode
where e.empid=2 and h.[date]='2022-01-01'
group by e.empid, e.location, h.holidaycode, [date]
Example:
Schema and insert statements:
create table employee(empid int, name varchar(50), location varchar(50));
insert into employee values(1, 'abc', 'USA');
insert into employee values(2, 'efg', 'UK');
create table location_table(location varchar(50),holidaycode varchar(50));
insert into location_table values('uk','uk1');
insert into location_table values('usa','usa1');
create table holidaytable(holidaycode varchar(50), [date] date, [type] varchar(50));
insert into holidaytable values('uk1', '2022-01-01', 'LM');
insert into holidaytable values('uk1', '2022-01-01', 'RMC');
Query:
select e.empid, e.location, h.holidaycode, [date], max([type]) [type 1]
from employee e inner join location_table l
on e.location=l.location
inner join holidaytable h
on l.holidaycode=h.holidaycode
where e.empid=2 and h.[date]='2022-01-01'
group by e.empid, e.location, h.holidaycode, [date]
Output:
empid
location
holidaycode
date
type 1
2
UK
uk1
2022-01-01
RMC
db<>fiddle here

Answer 2

You can use ROW_NUMBER to get only the first row of each grouping.
Either do ORDER BY type DESC or use ORDER BY CASE WHEN type = 'RMC' THEN 1 ELSE 2 END
SELECT
e.empid,
e.location,
l.holidaycode,
h.date,
h.type
FROM employee e
JOIN location l ON l.location = e.location
JOIN (
SELECT *,
rn = ROW_NUMBER() OVER (PARTITION BY h.holidaycode, h.date ORDER BY h.type DESC)
FROM holiday h
) h ON h.holidaycode = l.holidaycode AND h.rn = 1
WHERE h.date = '2022-01-01';
db<>fiddle