Dereference expressions

Get a column value from the referenced row

A dereference expression gets the value of a column from a specific row of another table. A dereference expression makes use of references between tables, using the value of a column of type Ref in this table to find a specific row in another table and get a value from a column there.

 To learn how to create a dereference, check out this video:

How to Create Dereferences | AppSheet

The following sections describe dereference expressions in more detail:

New to expressions and formulas? See Expressions: The Essentials.

Form and meaning

A dereference expression is of the form:

[ref-column].[value-column]

Where ref-column is the name of the column of type Ref in the this table, and value-column is the name of a column of the other table.

The configuration of a column of type Ref identifies the table in which referenced rows are to be found:

The value of a Ref column should be the key column value of a row in the Ref column's source table.

The Order Deliveries sample app includes a typical dereference expression. The Orders table uses the dereference expression, [Customer ID].[Email], to retrieve the customer email from the Customers table based on the customer ID. This dereference expression can be thought to mean:

  1. Go to the source table identified Customers column's configuration.

  2. In that source table, find the row with a key column value that matches the Customer ID column of this row.

  3. From that row in the source table, get the value of the Email column.

Dereference a list

A column value of type List or EnumList with a base type of Ref can be dereferenced to produce a new list of the values from dereferencing each individual reference, an operation called a list dereference.

A list dereference is performed by enclosing the Ref list column name (such as, Related Orders) and the column name of the desired column value (such as, Order Date) each in square brackets ([, ]) and placing them adjacent to each other: [Related Orders][Order Date]

The result will be a list of Order Date column values from the rows identified by the list in the Related Orders column value.

See also Build list dereferences.

Chained dereference expressions

You can dereference a dereference.

For example, you might have a three-level hierarchy consisting of a Customer row, a child Order row, and a grandchild Order Detail row. The Order Detail row contains a reference to the Order row. The Order row contains a reference to the Customer row.

From the Order Detail row, you can directly access the value of a column in the Customer row by chaining dereference expressions. That is, you can dereference from the Order Detail row to the Order row, and then dereference from the Order row to the Customer row in a single expression.

[Order Id].[Customer Name].[Email]
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