Grouping but with keeping all non-NULL values - sql

Question

Let's say I have the table:
ID | Name | Intolerance
1 | Amy | Lactose
2 | Brian | Lactose
3 | Amy | Gluten
And I run this SQL query:
SELECT
Name,
CASE
WHEN Intolerance = 'Lactose' 1
END AS Lactose,
CASE
WHEN Intolerance = 'Gluten' 1
END AS Gluten
FROM
Table
I get:
Name | Lactose | Gluten
-------+---------+--------
Amy | 1 |
Amy | | 1
Brian | 1 |
But if I try to add "GROUP BY Name", Amy won't have a 1 in both columns, because GROUP BY only selects the last row of each Name. What I want to get instead is this:
Name | Lactose | Gluten
------+---------+---------
Amy | 1 | 1
Brian | 1 |
How can I get that? Is there perhaps a more efficient way to summarize who's allergic to what from the same input? Thanks in advance.

Answer 1

When using a GROUP BY then the aggregate functions can be used for columns that aren't in the GROUP BY.
In this case I assume you want to use MAX, to get only a 1 or a NULL.
SUM or COUNT can also be used to surround a CASE WHEN.
But then those would return a total.
SELECT
Name,
MAX(CASE WHEN Intolerance = 'Lactose' THEN 1 END) AS Lactose,
MAX(CASE WHEN Intolerance = 'Gluten' THEN 1 END) AS Gluten
FROM Table
GROUP BY Name
ORDER BY Name
Or if you don't want to see NULL's?
Then let the CASE return a varchar instead of a number.
SELECT
Name,
MAX(CASE WHEN Intolerance = 'Lactose' THEN '1' ELSE '' END) AS Lactose,
MAX(CASE WHEN Intolerance = 'Gluten' THEN '1' ELSE '' END) AS Gluten
FROM Table
GROUP BY Name
ORDER BY Name

Answer 2

I think what you need is the sum of the number of number of intolerances for each person. Also, put a ELSE so the value is 0 or 1:
SELECT
Name,
SUM(CASE WHEN Intolerance = 'Lactose' THEN 1 ELSE 0 END) AS Lactose,
SUM(CASE WHEN Intolerance = 'Gluten' THEN 1 ELSE 0 END) AS Gluten
FROM Table
GROUP BY Name
ORDER BY Name

Answer 3

I feel that each time I encounter a question like that, it's because a no proper amount of thinking on design was allowed to the project.
To put it simply : you are trying to move data to columns. This is what your application layer is for ! Not the database. People tend to mix what databases are for with what application / UI layer and vice versa are for !
And each time it happens, I see people reaping their mind to answer because that's the point here : answer the question no matter what. Don't question what the OP want to do, give him the answer...
Sorry for that, I am just a little bit pissed.
My solution :
Keep your original query and do the aesthetic on your UI / application layer side. You probably have a IList inside each Person. Just fill them and give the UI the opportunity to display them however it wants.
Because that's what you're asking the database to do : aesthetics.