In my table I have rows, and each row has some data and its version. When column is not-null, it overrides an old value otherwise nothing happens.
Pseudocode (in C#)
var resultTable = new List<Row>();
foreach (var group in table.GroupBy(x => x.ID).Select(g => g.OrderBy(x => x.Priority)))
{
var row = Row.GetEmpty();
foreach (var anotherRow in group)
{
foreach (var column in group.Columns)
{
if (column.GetValue(anotherRow) != null)
column.SetValue(row, anotherRow);
}
}
resultTable.Add(row);
}
Table is large enough (several tens of columns, several hundreds of thousands rows), so it's desired to do it in single table scan (if possible).
I appreciate any help and any possibility to achieve it without any bias, SQLCLR, DSQL and so on, everything is applicable if it works fast and makes its job.
Current script for a test table:
CREATE TABLE [dbo].[T](
[id] [int] NOT NULL,
[A] [int] NULL,
[B] [int] NULL,
[C] [int] NULL,
[Priority] [int] NOT NULL
) ON [PRIMARY]
GO
CREATE UNIQUE CLUSTERED INDEX [ClusteredIndex-20160204-122857] ON [dbo].[T]
(
[id] ASC,
[Priority] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
GO
INSERT [dbo].[T] ([id], [A], [B], [C], [Priority]) VALUES (1, NULL, 3, 4, 1)
GO
INSERT [dbo].[T] ([id], [A], [B], [C], [Priority]) VALUES (1, 5, 6, NULL, 2)
GO
INSERT [dbo].[T] ([id], [A], [B], [C], [Priority]) VALUES (1, 8, NULL, NULL, 3)
GO
INSERT [dbo].[T] ([id], [A], [B], [C], [Priority]) VALUES (2, 634, 346, 359, 1)
GO
INSERT [dbo].[T] ([id], [A], [B], [C], [Priority]) VALUES (2, 34, NULL, 734, 2)
GO
input:
id A B C Priority
1 NULL 3 4 1
1 5 6 NULL 2
1 8 NULL NULL 3
2 634 346 359 1
2 34 NULL 734 2
desired result:
id A B C
1 8 6 4
2 34 346 734
In this example table is small and has only 5 columns, but in real table it will be much larger. I really want this script to work fast. I tried do it myself, but my script works for SQLSERVER2012+ so I deleted it as not applicable.
Numbers: table could have 150k of rows, 20 columns, 20-80k of unique id
s and average SELECT COUNT(id) FROM T GROUP BY ID
is 2..5
Best Answer
Itzik Ben-Gan's blog post (link provided by Daniel Hutmacher) has some nice solutions to the problem for SQL Server 2012 and later:
The Last non
NULL
Puzzle.Here is one more that will work in older versions (even 2005) and can be easily adapted for more columns. I haven't tested for efficiency or compared to the other solutions but I'd expect it to work better if the table had a clustered index on
(id, priority)
. But performance may depend on many other things, like the distribution, the number of distinctid
values, the number of versions (rows perid
values), the number and types of columns, etc.:Test in SQLfiddle.
The
@point_in_time
above is a parameter, that can be removed if we want the latest non-null values across all the table.If however, we want the latest non-null values for a specific point in time (say for when
priority=55
), we can replace that@point_in_time
with55
and have the latest non-null values up to that point in time (55
).