About match fields for relationships
When you create a relationship between tables, you choose one or more fields in each table as match fields. Match fields usually have common values. In a typical relationship, a record in one table will be related to records in another table that share a common match field value.
For example, a Customers table and an Invoice table can each use the field Customer ID to uniquely identify each customer and purchase. If the two tables are related using Customer ID as the match field, a record in the Customers table can display a portal showing each invoice with a matching Customer ID, and in the Invoices table each invoice with the same Customer ID can display consistent customer data.
Match fields must be one of the following field types:
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Notes
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A match field used for a relational database can be a lookup target field, as long as the lookup isn't based on a relationship that involves the match field.
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For example, you have a simple relationship joining records in TableA to TableB based on the contents of a single field in each table, and the match field in TableA contains the following values, separated by carriage returns:
red
green
blue
FileMaker Pro will match any record in TableB where the corresponding match field contains the single value red, green, or blue. However, FileMaker Pro will not return records where the match field contains the value red green blue. The carriage returns tell FileMaker Pro to treat each line as a separate value.
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You can also see and work with data from external ODBC data sources. See Accessing external data sources for more information about working interactively with data in SQL tables.
Related topics 
About relationships
About planning a database
About the types of relationships
Creating relationships