[discrete]
[[esql-lookup-join]]
=== `LOOKUP JOIN`

`LOOKUP JOIN` enables you to add data from another index, AKA a 'lookup'
index, to your {esql} query results, simplifying data enrichment
and analysis workflows.

See <<esql-lookup-join-landing-page,the high-level landing page>> for an overview of the `LOOKUP JOIN` command, including use cases, prerequisites, and current limitations.

*Syntax*

[source,esql]
----
FROM <source_index>
| LOOKUP JOIN <lookup_index> ON <field_name>
----

*Parameters*

`lookup_index`::
The name of the lookup index. This must be a specific index name - wildcards, aliases, and remote cluster references are not supported. Indices used for lookups must be configured with the `lookup` <<index-mode-setting,index mode setting>>.

`field_name`::
The field to join on. This field must exist
in both your current query results and in the lookup index. If the field
contains multi-valued entries, those entries will not match anything
(the added fields will contain `null` for those rows).

*Description*

The `LOOKUP JOIN` command adds new columns to your {esql} query
results table by finding documents in a lookup index that share the same
join field value as your result rows.

For each row in your results table that matches a document in the lookup
index based on the join field, all fields from the matching document are
added as new columns to that row.

If multiple documents in the lookup index match a single row in your
results, the output will contain one row for each matching combination.

[TIP]
====
For important information about using `LOOKUP JOIN`, refer to <<esql-lookup-join-usage-notes,Usage notes>>.
====

*Examples*

*IP Threat correlation*: This query would allow you to see if any source
IPs match known malicious addresses.

[source,esql]
----
FROM firewall_logs
| LOOKUP JOIN threat_list ON source.IP
----

To filter only for those rows that have a matching `threat_list` entry, use `WHERE ... IS NOT NULL` with a field from the lookup index:

[source,esql]
----
FROM firewall_logs
| LOOKUP JOIN threat_list ON source.IP
| WHERE threat_level IS NOT NULL
----

*Host metadata correlation*: This query pulls in environment or
ownership details for each host to correlate with your metrics data.

[source,esql]
----
FROM system_metrics
| LOOKUP JOIN host_inventory ON host.name
| LOOKUP JOIN employees ON host.name
----

*Service ownership mapping*: This query would show logs with the owning
team or escalation information for faster triage and incident response.

[source,esql]
----
FROM app_logs
| LOOKUP JOIN service_owners ON service_id
----

`LOOKUP JOIN` is generally faster when there are fewer rows to join
with. {esql} will try and perform any `WHERE` clause before the
`LOOKUP JOIN` where possible.

The two following examples will have the same results. The two examples
have the `WHERE` clause before and after the `LOOKUP JOIN`. It does not
matter how you write your query, our optimizer will move the filter
before the lookup when ran.

[source,esql]
----
FROM Left
| WHERE Language IS NOT NULL
| LOOKUP JOIN Right ON Key
----

[source,esql]
----
FROM Left
| LOOKUP JOIN Right ON Key
| WHERE Language IS NOT NULL
----
