FH08 — Duplicate output column name

Group: Structure · Severity: error · Run by: validate and lint

Every output column name must be unique across the whole view (the spec’s ValidateColumns algorithm). Two columns with the same name — anywhere in the nested select tree — collide in the output table.

Example

id is defined twice across two selects — FH08:

{
  "resource": "Patient",
  "select": [
    { "column": [{ "name": "id", "path": "id" }] },
    { "column": [{ "name": "id", "path": "id" }] }
  ]
}

error [FH08] (id): column `id` is defined more than once

Fix

Give each column a distinct name. Note that unionAll branches must use the same names in the same order — that is required, not a duplicate (see FH09).