SQL server pivot multiple columns to multiple rows and including nulls - sql-server

Question

I have the following table:
+----------------------------------------------------------------------+
| Student Class1 Class1_score Class2 Class2_score Class3.... |
+----------------------------------------------------------------------+
| Alex English 70 Maths 100 NULL |
| John Science 50 NULL NULL NULL |
| Bruce Maths 50 Science 50 English |
+----------------------------------------------------------------------+
Which i want to pivot to something like the following:
+--------------------------+
| Student Class Score |
+--------------------------+
| Alex English 70 |
| Alex Maths 100 |
| Alex Science NULL |
| Alex History NULL |
| John English NULL |
+--------------------------+
Which includes NULL for classes that are not in the original table for that particular student (e.g. Science for Alex)
How can I achieve this in SQL?

Answer 1

Assuming that class type occurs only once in the classX columns:
select student,
'English' class,
coaslesce(case when class1 = 'english' then Class1_score end,
case when class2 = 'english' then Class2_score end,
case when class3 = 'english' then Class3_score end,
case when class4 = 'english' then Class4_score end) score
from your_table
union
select student,
'Maths' class,
coaslesce(case when class1 = 'Maths' then Class1_score end,
case when class2 = 'Maths' then Class2_score end,
case when class3 = 'Maths' then Class3_score end,
case when class4 = 'Maths' then Class4_score end) score
from your_table
union
select student,
'Science' class,
coaslesce(case when class1 = 'Science' then Class1_score end,
case when class2 = 'Science' then Class2_score end,
case when class3 = 'Science' then Class3_score end,
case when class4 = 'Science' then Class4_score end) score
from your_table
union
select student,
'History' class,
coaslesce(case when class1 = 'History' then Class1_score end,
case when class2 = 'History' then Class2_score end,
case when class3 = 'History' then Class3_score end,
case when class4 = 'History' then Class4_score end) score
from your_table

Answer 2

If you want all the Class column values should come for each Student column value, then declare 2 table variables, one for storing unique values of Student column values and other for storing unique values of Class column values. Then do a full outer join between these two tables and use this result set as a subset and join it with another table variable which contains separate rows for each student each class and score. And all these can be done by executing dynamic SQL query.
Query
-- table variabe to store unique class values
declare #sql as varchar(max) = 'declare #tbl_class as table([class] varchar(100));'
+ 'insert into #tbl_class([class]) ';
select #sql += stuff((
select ' union select [' + [column_name] + '] from [' + [table_name] + '] '
+ 'where [' + [column_name] + '] is not null'
from information_schema.columns
where [table_name] = 'your_table_name'
and [column_name] like 'class%[0-9]'
order by [ordinal_position]
for xml path('')
)
, 1, 7, ''
) + ';';
-- table variabe to store unique student values
select #sql += 'declare #tbl_student as table([student] varchar(100));'
+ 'insert into #tbl_student([student]) '
+ 'select distinct [Student] from [your_table_name];';
-- table variable to store student score and class values one by one
select #sql += 'declare #tbl_scores as table'
+ '([student] varchar(100), [class] varchar(100), [score] int);'
+ 'insert into #tbl_scores([student], [class], [score]) ';
select #sql += stuff((
select ' union all select [Student], '
+ '[' + t.[col_1] + '] as [class],'
+ '[' + t.[col_2] + '] as [score] '
+ 'from [' + t.[table_name] + '] '
from (
select [column_name] as [col_1], [column_name] + '_score' as [col_2],
[ordinal_position], [table_name]
from information_schema.columns
where [table_name] = 'your_table_name'
and [column_name] like 'class%[0-9]'
) t
order by t.[ordinal_position]
for xml path('')
)
, 1, 10, ''
) + ';';
-- final result
select #sql += 'select t.[student], t.[Class], tsc.[Score] from('
+ 'select ts.[student], tc.[Class]'
+ ' from #tbl_student as ts '
+ ' full outer join #tbl_class as tc '
+ 'on 1 = 1 ) t '
+ 'left join #tbl_scores as tsc '
+ 'on t.[student] = tsc.[student] '
+ 'and t.[Class] = tsc.[Class];';
exec(#sql);
Find a demo here

Answer 3

You are actually looking for unpivot (wide to tall), rather than pivot. One approach which should work across almost any database uses a series of unions:
SELECT Student, Class, Score
FROM
(
SELECT Student, Class1 AS Class, Class1_score AS Score, 1 AS position
FROM yourTable
UNION ALL
SELECT Student, Class2, Class2_score, 2
FROM yourTable
UNION ALL
SELECT Student, Class3, Class3_score, 3
FROM yourTable
) t
ORDER BY
Student, position;

Answer 4

Sample data
IF OBJECT_ID('dbo.tempdata') IS NOT NULL
DROP TABLE tempdata
;With cte(Student , Class1 , Class1_score, Class2, Class2_score, Class3,Class3_score)
AS
(
SELECT 'Alex' ,'English', 70,'Maths' , 100 , NULL ,NULL UNION ALL
SELECT 'John' ,'Science', 50,NULL ,NULL , NULL ,NULL UNION ALL
SELECT 'Bruce','Maths' , 50,'Science' , 50 , 'English',NULL
)
SELECT * INTO tempdata FROM cte
SELECT * FROM tempdata
By Using Cross apply and dynamic SQL
DECLARE #DynamicColumns NVARCHAR(max)
,#Sql NVARCHAR(max)
;WITH cte
AS
(
SELECT COLUMN_NAME,
DENSE_RANK()OVER(ORDER BY SUBSTRING(COLUMN_NAME,0,7)) As BatchNo
FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_NAME='tempdata' AND COLUMN_NAME <>'Student'
)
SELECT #DynamicColumns=STUFF((SELECT ', '+ReqCol FROM
(
SELECT
DISTINCT '('+STUFF((SELECT ', '+COLUMN_NAME
FROM CTE i WHERE i.BatchNo=o.BatchNo
FOR XML PATH ('')),1,1,'') + ')' AS ReqCol
FROM cte o
)dt
FOR XML PATH ('')),1,1,'')
SET #Sql='SELECT Student
,Class
,Score
FROM tempdata
CROSS APPLY (VALUES '+#DynamicColumns+'
) As Dt (Class, Score)
WHERE Class IS NOT NULL
'
PRINT #Sql
EXEC (#Sql)
Result
Student Class Score
---------------------
Alex English 70
Alex Maths 100
John Science 50
Bruce Maths 50
Bruce Science 50
Bruce English NULL