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Within QGIS, I joined .dbf table containing population data to a shapefile of Census tracts.

The join field is called "GISJOIN" in both the target table and the shapefile's table, and this attribute has the string data type in both tables.

When I look at the attribute table of the population table after it is loaded within QGIS, all the data is there and looks fine. When I compare the values of the "GISJOIN" fields in each table to each other, they match up perfectly.

However, when I perform the join, inside the Properties dialog, and then look at the shapefile's attribute table, all the new attribute headings are there, added in, but all the values are NULL.

Any idea what is going on here?

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    It appears that QGIS is performing the join, but the GISJOIN field is not actually joining. Are you sure the IDs match (same number of digits, no padding spaces or zeros, etc.)?
    – Darren Cope
    Commented Jun 18, 2013 at 15:55
  • In ArcGIS this can happen where: 1) the values aren't perfect matches as noted in the prior comment, 2) the name of the table or feature class, or field names in the table or feature class, include spaces or special characters, or 3) field names are reserved words.
    – John
    Commented Jun 18, 2013 at 17:02
  • Thanks Darren and johns. I'm still scratching my head. The IDs definitely match perfectly. (And both datasets come from the same source -- National Historical GIS -- so it makes sense that they would.) The only additional item I would note is that when I again examined the .dbf files (in OpenOffice), I see that each of the field names have some sort of code following them including a letter, commas, and a number. For example: GISJOIN,C,12 These codes don't show up in the attribute tables within QGIS however. The codes for each table's GISJOIN field do match. Could these codes create problems?
    – Abijah
    Commented Jun 19, 2013 at 9:24
  • Did you see this: assets.nhgis.org/… It appears there are two sets of downloads, one being comma delimited (best for GIS) and one for with a descriptive header row that is best for spreadsheets.
    – John
    Commented Jun 19, 2013 at 13:13
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    Please, do not forget about "What should I do when someone answers my question?"
    – Taras ♦
    Commented Nov 22, 2021 at 6:48

1 Answer 1

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When you join the fields between your .csv and your .shp (for example), be sure that the Types are the same (see in Properties of both file).

For example: my "FID" for my .shp was in real as my "ID" for my .csv was in integer. After the join, I get NULLs.

But after changing the type (I created a new "ID" column in my shp) it worked perfectly.

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