Joins reference doc

Joins let you connect different models to each other so that you can explore more than one model at the same time in Lightdash and see how different parts of your data relate to each other.

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Adding joins in your models

Joins are defined at the same level as your model parameters in your YAML file.

version: 2
models:
  - name: users
    meta:
      joins:
        - join: web_sessions
          sql_on: ${web_sessions.user_id} = ${users.user_id}
          fields: [user_id, email, name]
        - join: subscriptions
          sql_on: ${subscriptions.user_id} = ${users.user_id} AND ${subscriptions.is_active}
          type: inner

When you open Lightdash, your joined models' dimensions and metrics will appear below the ones in your selected model.

Note that sql_on can include references to user attributes

Rename a joined model

When joining a model B onto a model A, you may want to rename the model for readability. This can be done with the label tag, for example on this messages model it's more suitable for our business to call the joined user a "sender":

models:
  - name: messages
    meta:
      joins:
        - join: users
          label: Sender
          sql_on: ${messages.sent_by} = ${users.user_id}

Join the same table multiple times with an alias

If you need to join a table multiple times, you can use an alias to distinguish between the different tables. A common use case is joining a user table multiple times to another table depending on the type of user. For example this messages model has both a sender and a recipient:

models:
  - name: messages
    meta:
      joins:
        - join: users
          alias: sender
          sql_on: ${messages.sent_by} = ${sender.user_id}
        - join: users
          alias: recipient
          sql_on: ${messages.sent_to} = ${recipient.user_id}

Note the following important differences when aliasing models in joins:

1. You must reference the fields in the model using the new alias. Notice that the joins above use ${sender.user_id} rather than ${users.user_id}.
2. Because of the above, any fields in the base model or joined model that reference any field ${users.*} will fail to compile. Be careful of aliasing tables that are used in the base model.
3. Joined models are automatically relabelled with the alias but you may also customise this using the label: field as above.

Specify your join type using type

If you want to specify the type of join to be used in your SQL query, use the type field in your YAML configuration file. Set its value to one of the following: inner, left, right, or full. This will determine how the tables are joined in your query, aligning with SQL's INNER JOIN, LEFT OUTER JOIN, RIGHT OUTER JOIN, or FULL OUTER JOIN respectively.

here's an example of how to specify a join type:

models:
  - name: messages
    meta:
      joins:
        - join: users
          type: inner
          sql_on: ${messages.sent_by} = ${users.user_id}

info

By default, if no type is specified, all joins are LEFT OUTER joins.

Here's a table to help you understand what each join type means and how it translates to SQL:

Join Type	Generated SQL	Description
inner	INNER JOIN	Returns rows that have matching values in both tables.
left	LEFT OUTER JOIN	Returns all rows from the left table, and the matching rows from the right table. Non-matching rows will have NULL for right table's columns.
right	RIGHT OUTER JOIN	Returns all rows from the right table, and the matching rows from the left table. Non-matching rows will have NULL for left table's columns.
full	FULL OUTER JOIN	Returns all rows when there is a match in either the left or right table records. Non-matching rows will have NULL for columns of the table that lacks a match.

inner

An inner join returns rows that have matching values in both tables. For example, if you have a users table and a subscriptions table, an inner join would return only the users who have a subscription.

Here's an example of how to specify an inner join:

models:
  - name: users
    meta:
      joins:
        - join: subscriptions
          sql_on: ${users.user_id} = ${subscriptions.user_id}
          type: inner

left

A left join returns all rows from the left table (i.e. the model where you're adding the join to), and the matching rows from the right table (i.e. the model you've specified in - join:). Non-matching rows will have NULL for right table's columns. For example, if you have a users table and a subscriptions table, a left join would return all users, and the subscription information for users who have a subscription.

Here's an example of how to specify a left join:

models:
  - name: users
    meta:
      joins:
        - join: subscriptions
          sql_on: ${users.user_id} = ${subscriptions.user_id}
          type: left  # you can omit this, as left is the default

right

A right join returns all rows from the right table (i.e. the model you've specified in - join: ), and the matching rows from the left table (i.e. the model where you're adding the join to). Non-matching rows will have NULL for left table's columns. For example, if you have a users table and a subscriptions table, a right join would return all subscriptions, and the user information for users who have a subscription.

Here's an example of how to specify a right join:

models:
  - name: users
    meta:
      joins:
        - join: subscriptions
          sql_on: ${users.user_id} = ${subscriptions.user_id}
          type: right

full

A full join returns all rows when there is a match in either the left or right table records. Non-matching rows will have NULL for columns of the table that lacks a match. For example, if you have a users table and a subscriptions table, a full join would return all users and all subscriptions, and the subscription information for users who have a subscription.

Only select a subset of fields from a join

Use the fields tag to select a subset of fields from a join. This is useful if you want to join a model but only a few of its fields are useful in the joined context. For example this messages model only needs the name and email fields from the users model. Note we must also include the user_id field since it's needed for the join.

models:
  - name: messages
    meta:
      joins:
        - join: users
          sql_on: ${messages.sent_by} = ${users.user_id}
          fields: [user_id, email, name]

Using joined dimensions or metrics in your .yml

Once you've joined a model, you can reference the metrics and dimensions from your joined model in your configurations.

For example, I can filter one of my metrics using a dimension from my joined model, like this:

version: 2

models:
  - name: users
    meta:
      joins:
        - join: web_sessions
          sql_on: ${web_sessions.user_id} = ${users.user_id}
        - join: subscriptions
          sql_on: ${subscriptions.user_id} = ${users.user_id} AND ${subscriptions.is_active}

    columns:
      - name: user_id
        meta:
          metrics:
            num_unique_premium_user_ids:
              type: count_distinct
              filters:
                - subscriptions.plan: premium

You can also reference these joined metrics and dimensions in custom sql, like this:

version: 2

models:
  - name: users
    meta:
      joins:
        - join: web_sessions
          sql_on: ${web_sessions.user_id} = ${users.user_id}
        - join: subscriptions
          sql_on: ${subscriptions.user_id} = ${users.user_id} AND ${subscriptions.is_active}

    columns:
      - name: user_id
        meta:
          dimension:
            sql: IF(${subscriptions.plan} IS NULL, NULL, ${user_id})
          metrics:
            num_unique_premium_user_ids:
              type: count_distinct
              sql: IF(${subscriptions.plan} = 'premium', ${user_id}, NULL)

Check out our dimensions and metrics reference docs to see all of the other configurations you can use with your joined fields.