How to join two tables based on conditon in sql? - sql

Question

I need to Join two table with respective to two columns
Stu_id (Table 1) - Stu_id (Table 2)
Perf_yr(Table 1) - yr_month (Table 2)
perf_yr starts on every year Sept to Aug.
Perf_yr should match the yr_month based on Perf_yr start and end Month
Table 1
Stu_id Roll_No Avg_marks Perf_yr
1 100244 72 2017
2 200255 62 2018
3 100246 68 2019
Table 2
Stu_id Subject Marks yr_month
1 Maths 70 201609
1 Science 69 201701
1 Social 74 201712
2 Maths 60 201709
2 Science 61 201801
2 Social 62 201808
3 Maths 65 201810
3 Science 64 201912
3 Social 72 201902
Output
Stu_id Roll_No Avg_marks Perf_yr Subject Marks yr_month
1 100244 72 2017 Maths 70 201609
1 100244 72 2017 Science 70 201701
2 200255 62 2018 Maths 60 201709
2 200255 62 2018 Science 61 201801
2 200255 62 2018 Social 62 201808
3 100246 68 2019 Maths 65 201810
3 100246 68 2019 Science 64 201912
3 100246 68 2019 Social 72 201902
I TRIED :
SELECT A.*, B.* FROM
(SELECT * FROM TABLE1 )A
LEFT JOIN
(SELECT * FROM TABLE)B
ON
A.Stu_id = B.Stu_id
AND
A.Perf_yr = B.Yr_Month
BUT IT WONT GIVE THE DESIRED RESULT BECAUSE THE CONDITION IS NOT SATISFYING THE PERF YR START AND END DATE .

Answer 1

You need to parse eg 201609 as a date, add 4 months to it, then match it to the year from the other table. Adding 4 months converts a date range 201609-201708 into being 201701-201712 - we only care about the year part:
SELECT * FROM
t1
INNER JOIN t2
ON
t1.stu_id = t2.stu_id AND
t1.Perf_yr = EXTRACT(year FROM ADD_MONTHS(TO_DATE(t2.yr_month, 'YYYYMM'), 4))
This is oracle. The same logic will work for SQLS, you'll just need to adjust the functions used-
CONVERT(date, t2.yr_month+'01', 112) -- Convert yyyymmdd to a date
DATEADD(month, x, 4) -- add 4 months to x
YEAR(x) -- extract year from date x