Joining two tables and getting rows as columns - sql

Question

I have two tables. One is a transaction table of user id with following details.
user_id Product_id timestamp transaction_id
123 A_1 ID1
123 A_2 ID1
124 A_1 ID2
125 A_2
Now there is a product_id mapping with the division to which the product belongs
Mapping:
Product_id Division
A_1 Grocery
A_2 Electronics and so on
I need a final table where I have one record for each user id and the corresponding items bought in each division as separate columns. Like
User_ID Grocery Electronics
123 1 1
124 1 0
I did something like this:
select user_id, case (when Division ="Grocery" then count(product_id) else 0) end as Grocery
when Division="Electronics" then count(product_id) else 0) end as Electronics
from
( select user_id, a.product_id, b.division from transact as a
left join
mapping b
on a.product_id=b.product_id
)
group by user_id
Does this sound good?

Answer 1

When you use conditional aggregation, the case is the argument to the aggregation function:
select user_id,
sum(case when m.Division = 'Grocery' then 1 else 0 end) as Grocery,
sum(case when m.Division = 'Electronics' then 1 else 0 end) as Electronics
from transact t left join
mapping m
on t.product_id = m.product_id
group by user_id
SQL Fiddle
In addition:
Your table aliases should be abbreviations for the table. Makes the code easier to understand.
You don't need a subquery.
Use single quotes for string constants. This is the SQL standard.

Answer 2

This is a dynamic version of Gordon Linoff's answer, which uses conditional aggregation. This works for SQL-Server.
DECLARE #sql1 VARCHAR(4000) = ''
DECLARE #sql2 VARCHAR(4000) = ''
DECLARE #sql3 VARCHAR(4000) = ''
SELECT #sql1 =
'SELECT
t.user_id
'
SELECT #sql2 = #sql2 +
' , SUM(CASE WHEN m.division = ''' + division + ''' THEN 1 ELSE 0 END) AS [' + division + ']' + CHAR(10)
FROM(
SELECT DISTINCT division FROM Mapping
)t
ORDER BY division
SELECT #sql3 =
'FROM Transact t
LEFT JOIN Mapping m
ON m.product_id = t.product_id
GROUP BY t.user_id'
PRINT (#sql1 + #sql2 + #sql3)
EXEC (#sql1 + #sql2 + #sql3)
SQL Fiddle

Answer 3

as per my understanding it can be acheived using PIVOT also
declare #transact table (user_id int,product_id varchar(5),transaction_id varchar(5))
insert into #transact (user_id,product_id,transaction_id)values (123,'A_1','ID1'),(123,'A_2','ID1'),(124,'A_1','ID2'),(125,'A_2',NULL)
declare #mapping table (product_id varchar(5),division varchar(20))
insert into #mapping (product_id,division)values ('A_1','Grocery'),('A_2','Electronics')
select * from (
select t.user_id,tt.division as Division,tt.product_id As product_id from #transact t
left join #mapping tt
on t.product_id = tt.product_id)T
PIVOT(COUNT(product_id)FOR division IN ([Grocery],[Electronics]))P