Mysql case if result found in first case do not run next - mysql

Question

I have two tables where I need to return matching results on basis of CASE statement.
If the query returns data in the first case. We need to return data. We do not need to check the following case statements.
This is what I tried
Select
CASE
WHEN foo.Marks ='Two' AND t1.Subject = 3 THEN foo.UniqueValue
END as 'Result',
CASE
WHEN foo.Marks ='0' AND t1.Subject = 3 THEN foo.UniqueValue
END as 'Result1',
CASE
WHEN foo.Marks IS NULL AND t1.Subject IS NULL THEN foo.UniqueValue
END as 'Result2'
from (
select t1.Marks, t1.Subject, t1.Student, foo.UniqueValue from `table1` t1
inner join `table2` t2 on t1.Student = t2.GroupName
where t2.GroupID = 2
)foo;
id
Marks
Subject
Student
UniqueValue
1
Two
3
FOO
AIR1
2
0
3
FOO
AIR2
3
NULL
NULL
FOO
AIR3
id
GroupID
GroupName
1
2
FOO
2
3
BAR
3
7
FOO123
Level 1 -> If Marks and Subject matched it should return that AIR1.
Level 2 -> If Marks is 0 and Subject is 3 should return AIR2.
If both above cases do not find any result then the below case should be checked.
Level 3 -> If Marks is NULL and Subject is NULL should return AIR3
If in Level1, the matching row is found it should return that Level2,3 should not be checked.
In the final, I should only get a single row matching the above levels by priority.

Answer 1

If GroupID is known,
SELECT t1.UniqueValue
FROM table1 t1
JOIN table2 t2 ON t1.Student = t2.GroupName
WHERE t2.GroupID = 2
ORDER BY
CASE WHEN t1.Marks = 'Two' AND t1.Subject = 3 THEN 1
WHEN t1.Marks = '0' AND t1.Subject = 3 THEN 2
WHEN t1.Marks IS NULL AND t1.Subject IS NULL THEN 3
ELSE 4 END ASC
LIMIT 1
To get one result for each GroupID
WITH cte AS (
SELECT t1.UniqueValue, ROW_NUMBER() OVER(PARTITION BY t2.GroupID ORDER BY CASE WHEN t1.Marks = 'Two' AND t1.Subject = 3 THEN 1
WHEN t1.Marks = '0' AND t1.Subject = 3 THEN 2
WHEN t1.Marks IS NULL AND t1.Subject IS NULL THEN 3
ELSE 4 END ASC) rn
FROM table1 t1
JOIN table2 t2 ON t1.Student = t2.GroupName
)
SELECT UniqueValue
FROM cte
WHERE rn = 1

Answer 2

Why don't you just do ELSE instead of END and new CASE?
Select
(CASE
WHEN foo.Marks ='Two' AND t1.Subject = 3 THEN foo.UniqueValue
ELSE
WHEN foo.Marks ='0' AND t1.Subject = 3 THEN foo.UniqueValue
ELSE
WHEN foo.Marks IS NULL AND t1.Subject IS NULL THEN foo.UniqueValue
END) Result
from (
select t1.Marks, t1.Subject, t1.Student, foo.UniqueValue from `table1` t1
inner join `table2` t2 on t1.Student = t2.GroupName
where t2.GroupID = 2
)foo;