Indexing Non-Blank Values for Specific Matches

[Orionsic]

Hello, I have hundreds-of-thousands of rows, which most of them have missing values (column 2). Based on a primary key (column 1), I can assume that the missing values can be imputed with values associated with that key. An example would be necessary.

[TABLE="width: 500"]
<tbody>[TR]
[TD]Primary Key[/TD]
[TD]Date[/TD]
[TD]Date.Impute[/TD]
[/TR]
[TR]
[TD]123[/TD]
[TD]""[/TD]
[TD][/TD]
[/TR]
[TR]
[TD]123[/TD]
[TD]""[/TD]
[TD][/TD]
[/TR]
[TR]
[TD]123[/TD]
[TD]02/02/2017[/TD]
[TD][/TD]
[/TR]
[TR]
[TD]1234[/TD]
[TD]""[/TD]
[TD][/TD]
[/TR]
[TR]
[TD]1234[/TD]
[TD]02/03/2017[/TD]
[TD][/TD]
[/TR]
[TR]
[TD]1234[/TD]
[TD]""[/TD]
[TD][/TD]
[/TR]
[TR]
[TD]12345[/TD]
[TD]01/01/2017[/TD]
[TD][/TD]
[/TR]
[TR]
[TD]12345[/TD]
[TD]""[/TD]
[TD][/TD]
[/TR]
</tbody>[/TABLE]

All orders "123" date are "02/02/2017". All orders "1234" date are "02/03/2017", etc..

Using or not using index match, how can I fill in all of the missing fields of the 2nd column in the 3rd column?

Any help would be greatly appreciated. Thank you.

 

[Aladin Akyurek]

In C2 control+shift+enter, not just enter, and copy down:

=IF(ISNUMBER($B2),$B2,INDEX($B$2:$B$9,MATCH(1,IF($A$2:$A$9=$A2,IF(ISNUMBER($B$2:$B$9),1)),0)))