Attribute join

Draft: written, not yet checked by a person Operation · attribute-join

Adds the fields of a table to a layer by matching values in a key field.

Pitfall. Codes with leading zeros stored as text on one side and numbers on the other match nothing.

Pitfall. Duplicate keys on the right side multiply rows on the left.

About this operation
Also called	Join Field, Add Join, table join, Join attributes by field value
Group	`table/join`
Checks	`join.unmatched, join.many_to_many, join.key_types, join.keys_normalized`
Pitfalls	2 listed above
Columbus tool	`vector_op · join`
Notes	Not stated
Status	active
Broader	Not stated
Narrower	Not stated
Related	`spatial-join`

Parameters

Name	Type	Required	Default	Values	Note
`left_key`	field	yes	Not stated	Not stated	Not stated
`right_key`	field	no	Not stated	Not stated	Defaults to the same name as left_key.
`how`	enum	no	Not stated	`left, inner`	Keep unmatched left rows (left) or drop them (inner).
`key_as_text`	boolean	no	Not stated	Not stated	Compare keys as text, for codes stored as numbers on one side.
`trim`	boolean	no	Not stated	Not stated	Ignore leading and trailing spaces in keys.
`ignore_case`	boolean	no	Not stated	Not stated	Not stated
`pad`	integer	no	Not stated	Not stated	Zero-pad keys to this many characters before matching.

Direction	Name	Type	Geometry	Note
Input	`left`	any	Not stated	Not stated
Input	`right`	table	Not stated	Not stated
Output	`output`	any	Not stated	Not stated

Engine	Tool	Parameters	Differences
`grass`	`v.db.join`	`left_key` → `column` `right_key` → `other_column` `left` → `map` `right` → `other_table`	Adds the other table's columns to the map's own attribute table in place; there is no separate output.
`qgis`	`native:joinattributestable`	`left_key` → `FIELD` `right_key` → `FIELD_2` `how` → `DISCARD_NONMATCHING` : left = no; inner = yes `left` → `INPUT` `right` → `INPUT_2` `output` → `OUTPUT`	By default (METHOD 1) takes only the first matching row, so duplicate keys do not multiply rows, unlike Columbus's join.