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5.5. What is Nullability?

Nullability allows you to specify the type of SQL JOIN that should be used when joining or linking the main source (table) to a linked source (table) in the template builder.

In Evergreen, we talk about the "Core Source" and the "Linked Tables". The Core Source is the main table in the database that we are reporting on and the linked sources are any linked tables we are using in the report. We can also talk about these tables in terms of Parent/Child and Left/Right.

Core Source/Table
Left Table
Parent Table

Linked Sources/Tables
Right Table
Child Table

W3Schools has nice Venn diagrams (towards the bottom of the page) to help explain the different JOINs: https://www.w3schools.com/sql/sql_join.asp

In the Evergreen web client, an INNER JOIN is used by default when you use a linked table in the template builder. This means the report output will include rows where the value is present in both tables (the main source and the linked source).
By using nullability in a report template you can tell Evergreen to use a RIGHT JOIN (Child) or a LEFT JOIN (Parent) when needed to include data that is found only in the Child or Parent table and the matched rows from the other table.

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