Getting multiple MAX values from one-to-many table - sql

Question

I have two tables and I would like to get MAX(date) from one-to-many table. If there is no value, it should be NULL. Only way I know how to do it by making sub queries but if there is ~20 different types then 20 sub queries does not sound efficient enough. Is there any better way to do it?
Table A:
UserId | Name
1 | John
2 | Jane
Table B:
UserId | Type | Date
1 | A | 2015-01-01
1 | A | 2015-12-31
1 | B | 2015-01-01
1 | B | 2015-12-31
2 | B | 2015-06-06
1 | C | 2015-01-01
2 | C | 2015-09-09
Result:
UserId | Type A date | Type B date | Type C date
1 | 2015-12-31 | 2015-12-31 | NULL
2 | NULL | 2015-06-06 | 2015-09-09
Current solution:
SELECT UserId,
(SELECT MAX(Date) FROM B WHERE Type = 'A' AND B.UserId= A.UserId),
(SELECT MAX(Date) FROM B WHERE Type = 'B' AND B.UserId= A.UserId),
(SELECT MAX(Date) FROM B WHERE Type = 'C' AND B.UserId= A.UserId
AND Date > (SELECT MAX(Date) FROM B WHERE Type = 'B' AND B.UserId = A.UserId))
FROM A
Thank you for all quick answers! They work perfectly. I modified my question little bit since I noticed that I need to add some conditions on some types. For example. Type C should be only presented if it's bigger than type B.

Answer 1

To get the expected result you don't even need to join, simply do a group by and use case expressions do to conditional aggregation:
select userid,
max(case when type = 'A' then date end) Adate,
max(case when type = 'B' then date end) Bdate
from tableB
group by userid
If you also want the username, you can join tableA with the above query:
select a.name, b.Adate, b.Bdate
from tableA a
join (select userid,
max(case when type = 'A' then date end) Adate,
max(case when type = 'B' then date end) Bdate
from tableB
group by userid) b
on a.userid = b.userid
However, if the number of types is unknown, I'd group by the column type too. I.e return types in different rows.
select a.name, b.type, b.date
from tableA a
left join (select userid, type, max(date) date,
from tableB
group by userid, type) b
on a.userid = b.userid

Answer 2

You can do a conditional aggregation instead of the subqueries in the SELECT:
SELECT
a.UserId,
MAX(CASE WHEN b.Type = 'A' THEN b.date END) AS ADate,
MAX(CASE WHEN b.Type = 'B' THEN b.date END) AS BDate
FROM TableA a
LEFT JOIN TableB b
ON b.UserId = a.UserId
GROUP BY a.UserId;
However, the above will only work if you know the values of Type. If you don't, then you need to do it using a dynamic query.
DECLARE #sql NVARCHAR(MAX) = N'';
SELECT #sql =
'SELECT
a.UserId' + CHAR(10) +
(SELECT DISTINCT
' , MAX(CASE WHEN b.Type = ''' + Type + ''' THEN b.Date END) AS ' + QUOTENAME(Type + 'Date') + CHAR(10)
FROM TableB
FOR XML PATH('')
) +
'FROM TableA a
LEFT JOIN TableB b
ON b.UserId = a.UserId
GROUP BY a.UserId;';
EXEC (#sql);
ONLINE DEMO
Reference: Cross Tabs and Pivots by Jeff Moden

Answer 3

Use Dynamic Pivot, no matter how many type you have, you dont need to change the query.
DECLARE #DynamicPivotQuery AS NVARCHAR(MAX)
DECLARE #ColumnName AS NVARCHAR(MAX)
SELECT #ColumnName = ISNULL(#ColumnName + ',','') + QUOTENAME([TYPE])
FROM (SELECT DISTINCT [TYPE] as [TYPE] FROM [master].[dbo].[YourTable]) AS TypeTable
SET #DynamicPivotQuery =
'SELECT USERID,' + #ColumnName+'
FROM [master].[dbo].[YourTable]
PIVOT(MAX([Date])
FOR [TYPE] IN (' + #ColumnName+')) AS PVTTable'
EXEC sp_executesql #DynamicPivotQuery
Result is like below :
USERID A B
1 2015-12-31 2015-12-31
2 NULL 2015-06-06
Edit : For type C,there are some condition:
DECLARE #DynamicPivotQuery AS NVARCHAR(MAX)
DECLARE #ColumnName AS NVARCHAR(MAX)
DECLARE #ColumnNameForDisplay AS NVARCHAR(MAX)
DECLARE #otherCondition AS NVARCHAR(MAX)
SELECT #ColumnName = ISNULL(#ColumnName + ',','') + QUOTENAME([TYPE])
FROM (SELECT DISTINCT [TYPE] as [TYPE] FROM [master].[dbo].[YourTable]) AS TypeTable
SELECT #ColumnNameForDisplay = ISNULL(#ColumnNameForDisplay + ',','') + [TYPE2]
FROM (SELECT DISTINCT [TYPE]+' as [Type '+[TYPE]+' date]' as [TYPE2] FROM [master].[dbo].[YourTable] where type <> 'c') AS TypeTable
SELECT #otherCondition = ' ,Case when c <= b then null else c END as [Type C date] '
SET #DynamicPivotQuery = '
SELECT USERID,'+#ColumnNameForDisplay+#otherCondition+' from(
SELECT USERID,' + #ColumnName+'
FROM [master].[dbo].[YourTable]
PIVOT(MAX([Date])
FOR [TYPE] IN (' + #ColumnName+')) AS PVTTable) as t'
EXEC sp_executesql #DynamicPivotQuery
Result is like below :
USERID Type A date Type B date Type C date
1 2015-12-31 2015-12-31 NULL
2 NULL 2015-06-06 2015-09-09