# Implementing retention analysis & cohorts

This is an advanced topic that assumes good, pre-existing knowledge of SQL and
Cube.

Whether you’re selling groceries, financial services, or gym memberships,
successful recruitment of new customers is only truly successful if they return
to buy from you again. The metric that reflects this is called **retention**,
and the approach we use is **customer retention analysis**. Retention analysis
is typically done using **cohort analysis**.

Cohort analysis is a technique to see how variables change over in different
groups with different starting conditions. Retention is a simplified one, where
the **starting condition is usually the time of signup and the variable is
simply activity**.

It’s usually visualized as a cohort grid or retention curves.

<div style={{ textAlign: "center" }}>
  <img
    src="https://ucarecdn.com/72b9bce1-eaa2-49bb-8d74-5dd4bb5dd6eb/"
    style={{ border: "none" }}
    width="100%"
  />
</div>

Cohort retention analysis is pretty hard to do in SQL. **We need to have the
user-date combination**, which tells us about a user’s activity on that date,
including dates with no activity. To do this, we need to make a tricky join,
which gives us a dates list. Once we have it, we can “fill it” with users’
activities.

The example below shows monthly cohort retention. The same technique can be used
for daily or weekly retention.

<InfoBox>

The SQL code in this guide is Postgres-compliant. The final SQL code may be
different depending on your database. Also, this technique requires at least 1
user to be active during the month, otherwise this month will not be included in
the months' list.

</InfoBox>

<CodeTabs>

```yaml
cubes:
  - name: monthly_retention
    sql: |
      SELECT
        users.id as user_id,
        date_trunc('month', users.created_at) as signup_month,
        months_list.activity_month as activity_month,
        data.monthly_pageviews
      FROM users LEFT JOIN (
        SELECT
          DISTINCT (date_trunc('month', pages.original_timestamp)) as
      activity_month
        FROM pages
      ) as months_list ON months_list.activity_month >= date_trunc('month',
      users.created_at) LEFT JOIN (
        SELECT
          p.user_id,
          date_trunc('month', p.original_timestamp) as activity_month,
          COUNT(DISTINCT p.id) as monthly_pageviews
        FROM pages p
        GROUP BY 1,2
      ) as data ON data.activity_month = months_list.activity_month AND
      data.user_id = users.id
```

```javascript
cube(`monthly_retention`, {
  sql: `SELECT
    users.id as user_id,
    date_trunc('month', users.created_at) as signup_month,
    months_list.activity_month as activity_month,
    data.monthly_pageviews
  FROM users
  LEFT JOIN
    (
      SELECT
        DISTINCT (date_trunc('month', pages.original_timestamp)) as activity_month
      FROM pages
    ) as months_list
  ON months_list.activity_month >= date_trunc('month', users.created_at)
  LEFT JOIN
    (
      SELECT
        p.user_id,
        date_trunc('month', p.original_timestamp) as activity_month,
        COUNT(DISTINCT p.id) as monthly_pageviews
      FROM pages p
      GROUP BY 1,2
    ) as data
  ON data.activity_month = months_list.activity_month
  AND data.user_id = users.id`
})
```

</CodeTabs>

The SQL above provides the base table for our retention cube. It would show
signup months and activity months with pageviews:

| user_id | signup_month | activity_month | monthly_pageviews |
| ------- | ------------ | -------------- | ----------------- |
| 1       | 1/18         | 1/18           | 10                |
| 1       | 1/18         | 2/18           | 5                 |
| 1       | 1/18         | 3/18           | 0                 |
| 2       | 2/18         | 2/18           | 12                |
| 2       | 2/18         | 3/18           | 0                 |
| 3       | 3/18         | 3/18           | 5                 |

Now we can calculate a total count of users and the total count of active users,
who has more than 0 page views, for every month. Based on these two measures we
can calculate monthly `percentage_of_active`.

<CodeTabs>

```yaml
cubes:
  - name: monthly_retention
    # ...
    measures:
      - name: total_count
        sql: user_id
        type: count_distinct
        public: false

      - name: total_active_count
        sql: user_id
        type: count_distinct
        filters:
          - sql: monthly_pageviews > 0
        drill_members:
          - users.id
          - users.email

      - name: percentage_of_active
        sql: "1.0 * {total_active_count} / NULLIF({total_count}, 0)"
        type: number
        format: percent
        drill_members:
          - users.email
          - bots.team
          - bots.last_seen
          - percentage_of_active
```

```javascript
cube(`monthly_retention`, {
  // ...

  measures: {
    total_count: {
      sql: `user_id`,
      type: `count_distinct`,
      public: false
    },

    total_active_count: {
      sql: `user_id`,
      type: `count_distinct`,
      filters: [{ sql: `${CUBE}.monthly_pageviews > 0` }],
      drill_members: [users.id, users.email]
    },

    percentage_of_active: {
      sql: `1.0 * ${total_active_count} / NULLIF(${total_count}, 0)`,
      type: `number`,
      format: `percent`,
      drill_members: [
        users.email,
        bots.team,
        bots.last_seen,
        percentage_of_active
      ]
    }
  }
})
```

</CodeTabs>

To be able to build cohorts, we need to group by two dimensions: **signup
date**, which will define our cohorts, and **months since signup**, which will
show how the percentage of active users is changing.

<CodeTabs>

```yaml
cubes:
  - name: monthly_retention
    # ...
    dimensions:
      - name: months_since_signup
        sql: "DATEDIFF('month', signup_month, activity_month)"
        type: number

      - name: signup_date
        sql: "(signup_month AT TIME ZONE 'America/Los_Angeles')"
        type: time
```

```javascript
cube(`monthly_retention`, {
  // ...

  dimensions: {
    months_since_signup: {
      sql: `DATEDIFF('month', ${CUBE}.signup_month, ${CUBE}.activity_month)`,
      type: `number`
    },

    signup_date: {
      sql: `(signup_month AT TIME ZONE 'America/Los_Angeles')`,
      type: `time`
    }
  }
})
```

</CodeTabs>

<InfoBox>

Note, we are explicitly setting the `signup_month` timezone. `date_trunc`
returns UTC dates and not setting a correct timezone would lead to wrong results
due to time shift.

</InfoBox>
