SQL INNERJOIN with a switch case and if statement in select - sql

Question

Please help:
I have to create a column based on an expression from other columns,
but before that expression hits, there has to be a date check
if the date is before a specific date- it must do an expression, if the date is after said date, it must do another expression,
THEN: theres a unique column that has the digits 1-10 in it, each number represents a different expression.
the inner join and selecting the rows are fine, its just the switch and if expression that are beating me
basically the statement needs to look like this
select column1 if(date<neededDate)
{select case ExpressionColumn
when 1 //do stuff
when 2 // do stuff
else// do nothing
}
select column1 if(date>neededDate)
{select case ExpressionColumn
when 1 //do stuff
when 2 // do stuff
else// do nothing
}
i hope this made sense

Answer 1

You need two case statement nested within another case statement, it can be done like following
SELECT CASE WHEN date > neededDate THEN
CASE ExpressionColumn
WHEN 1 THEN '1'
WHEN 2 THEN '2'
ELSE 'null'
END
WHEN date < neededDate THEN
CASE ExpressionColumn
WHEN 1 THEN '1'
WHEN 2 THEN '2'
ELSE 'null'
END
ELSE 'null'
END
FROM YourTable;

Answer 2

you have your syntax wrong:
select case sign(datediff('s', date, neededDate)) -- precision: second
when 0 then -- case missing in your spec !
else
case ExpressionColumn
when 1 then -- case 1
when 2 then -- case 2
else -- whatever
end
end
from whatever
;
replace each comment with the appropriate expression over columns.
in your case a searched case expression might be more convenient:
select case
when (date < neededDate) then
-- note the difference: instead of 'case x when y then ...' you write 'case when b then ...'
case ExpressionColumn
when 1 then -- case 1
when 2 then -- case 2
else -- whatever
end
when (date > neededDate) then
case ExpressionColumn
when 1 then -- case 1
when 2 then -- case 2
else -- whatever
end
else -- this is missing from your spec!
end
from whatever
;

Answer 3

You need to check CASE..WHEN..THEN in Sql server
Simple CASE expression:
CASE input_expression
WHEN when_expression THEN result_expression [ ...n ]
[ ELSE else_result_expression ]
END
Searched CASE expression:
CASE
WHEN Boolean_expression THEN result_expression [ ...n ]
[ ELSE else_result_expression ]
END