8. Joins & Relates¶

Goal: Bring in data from a CSV, table, or another feature class using a shared key field.

Join vs relate¶

99% of the time, you want a join.

How to join¶

1. Right-click target layer → Joins and Relates → Add Join.
2. Set:
   • Input Join Field — field on the layer
   • Join Table — the CSV / table / layer
   • Output Join Field — matching field on the table
3. Click Validate Join (recommended). Confirms key types match.
4. Click OK.

The joined fields appear at the right of your attribute table.

Field types must match¶

You can join Text → Text or Long → Long, but not Long → Text. Convert one side first if needed.

Cardinality matters¶

flowchart LR
    L[Counties layer<br/>3,143 rows] --> J((1:1 Join))
    T[Population CSV<br/>3,143 rows]   --> J
    J --> O[Counties + pop]

    L2[Parcels layer<br/>50,000 rows] --> J2((Many:1))
    T2[Land use codes<br/>50 rows] --> J2
    J2 --> O2[Each parcel gets<br/>its land use description]

    classDef l fill:#dbeafe,stroke:#1e40af,color:#1e3a8a
    classDef t fill:#fef3c7,stroke:#f59e0b,color:#92400e
    classDef o fill:#dcfce7,stroke:#10b981,color:#065f46
    class L,L2 l
    class T,T2 t
    class O,O2 o

If the table side has multiple rows per key, ArcGIS Pro keeps only the first match — silently. To preserve all matches, use Add Relate or summarize the table to one row per key first.

Save the join (export)¶

A join is a virtual view. The fields aren't really part of your feature class. To make them permanent:

1. Right-click layer → Data → Export Features.
2. The exported feature class will have the joined fields baked in.

This is the safe move before further analysis or sharing.

Remove a join¶

Right-click layer → Joins and Relates → Remove Join → (select). The joined fields disappear; the underlying data is unchanged.

Common pitfalls¶

Spatial join (preview)¶

If there's no shared field but the layers overlap geographically, use a Spatial Join instead. → See Spatial Join.

Practice¶

→ Next: Buffers.