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Join Keys
To be joined, the primary and secondary tables must contain matching fields
with common values. These matching fields are the join keys.
For example, when joining a data table containing employment rates to a
layer representing counties, the join key might be County_Name, if that field
is common to both tables.
A join key does not need to have the same name in both data sources, but it
must have the same data type (numeric, string, Boolean, and so on). For
example, you can match the Parcel_ID field in one table to the Lot_Number
field in the second table, so long as Parcel_ID and Lot_Number use the same
data type.
NOTE Joins that use String fields are case-sensitive. The values in the fields must
match exactly. Jones will not match JONES.
Advanced Joining
You can join multiple secondary tables to a feature layer There are several
ways to do this.
You can join a new table to the joined secondary table to create a “chain” of
joins. For example, you can begin by joining income information to a layer
of parcel features, using the Parcel_ID as the common join key. Then you can
add a join to a table of employment data, using Job_Title as a common join
key between the income table and the employment table. Even though the
parcel feature does not contain a property for Job_Title, you can style the
parcels by job type after you complete both joins.
You can join multiple tables to a single feature using the same join key. For
example, you can join a table of traffic information to a roads layer, using the
Road_Name field as the join key. You can then join a second table of
maintenance schedules to the roads layer, using the same join key.
You can also join multiple tables to a single feature using different join keys.
For example, you can join population information to a layer representing
counties, using the County_Name field as the join key. Then you can join
flood-zone data to the same layer using Elevation as the join key.
In all cases, you can use all of the joined data when styling or theming the
feature layer.
510 | Chapter 3 Bringing In Data