如果在UNION ALL查询中未找到任何行，则返回默认值

Question

Scenario

I have a query developed from this question , were part of the optimisation was to create a MySql view which is used for generating statistics for users and sales, the problem is that when there is no result for one of the SELECT rows it gets omitted from the resulting table.

Question

How can a tell MySql to set a default value (eg 0) if no rows are found for any of the SELECT ?

Code

This is the code for creating the view

CREATE OR REPLACE VIEW user_events AS
    SELECT 'Complete profiles' AS type, created_at
        FROM users
        WHERE completed_registration = 1

    UNION ALL SELECT 'Incomplete profiles', created_at
        FROM users
        WHERE completed_registration = 0 AND verified_email = 1

    UNION ALL SELECT 'Unverified profiles', created_at
        FROM users
        WHERE verified_email = 0

    UNION ALL SELECT 'Onsite Teachers', created_at
        FROM onsite_teachers

    UNION ALL SELECT 'Onsite Teachers hired', created_at
        FROM purchases
            INNER JOIN purchased_profiles
                ON purchased_profiles.purchase_id = purchases.id
                 AND purchased_profiles.profile_type = 'onsite_teacher'
        WHERE purchases.transaction_status = 'completed'

    UNION ALL SELECT 'Translators', created_at
        FROM translators

    UNION ALL SELECT 'Translators hired', created_at
        FROM purchases
            INNER JOIN purchased_profiles
                ON purchased_profiles.purchase_id = purchases.id
                AND purchased_profiles.profile_type = 'translator'
        WHERE purchases.transaction_status = 'completed'

    UNION ALL SELECT 'Interpreters', created_at
        FROM interpreters

    UNION ALL SELECT 'Interpreters hired', created_at
        FROM purchases
            INNER JOIN purchased_profiles
                ON purchased_profiles.purchase_id = purchases.id
                AND purchased_profiles.profile_type = 'interpreter'
        WHERE purchases.transaction_status = 'completed';

And this is the code for querying the totals for the last 6 months including the current month.

SELECT 
    type, 
    COUNT(CASE 
        WHEN created_at >= CAST(CURDATE() - INTERVAL (DAYOFMONTH(CURDATE()) - 1) DAY AS DATETIME) 
        THEN 1 
    END) AS 0_month_ago, 

    COUNT(CASE 
        WHEN created_at BETWEEN CAST(CURDATE() - INTERVAL (DAYOFMONTH(CURDATE()) - 1) DAY - INTERVAL 1 MONTH AS DATETIME)
            AND CAST(LAST_DAY(CURDATE() - INTERVAL 1 MONTH) + INTERVAL 1 DAY AS DATETIME)
        THEN 1 
    END) AS 1_month_ago,

    COUNT(CASE 
        WHEN created_at BETWEEN CAST(CURDATE() - INTERVAL (DAYOFMONTH(CURDATE()) - 1) DAY - INTERVAL 2 MONTH AS DATETIME)
            AND CAST(LAST_DAY(CURDATE() - INTERVAL 2 MONTH) + INTERVAL 1 DAY AS DATETIME)
        THEN 1 
    END) AS 2_months_ago,

    COUNT(CASE 
        WHEN created_at BETWEEN CAST(CURDATE() - INTERVAL (DAYOFMONTH(CURDATE()) - 1) DAY - INTERVAL 3 MONTH AS DATETIME)
            AND CAST(LAST_DAY(CURDATE() - INTERVAL 3 MONTH) + INTERVAL 1 DAY AS DATETIME)
        THEN 1 
    END) AS 3_months_ago,

    COUNT(CASE 
        WHEN created_at BETWEEN CAST(CURDATE() - INTERVAL (DAYOFMONTH(CURDATE()) - 1) DAY - INTERVAL 4 MONTH AS DATETIME)
            AND CAST(LAST_DAY(CURDATE() - INTERVAL 4 MONTH) + INTERVAL 1 DAY AS DATETIME)
        THEN 1 
    END) AS 4_months_ago,

    COUNT(CASE 
        WHEN created_at BETWEEN CAST(CURDATE() - INTERVAL (DAYOFMONTH(CURDATE()) - 1) DAY - INTERVAL 5 MONTH AS DATETIME)
            AND CAST(LAST_DAY(CURDATE() - INTERVAL 5 MONTH) + INTERVAL 1 DAY AS DATETIME)
        THEN 1 
    END) AS 5_months_ago
FROM 
    user_events
GROUP BY 
    type;

Current output

If you look closely there is no Interpreters hired and Translators hired rows, I want for this rows to be set and zeroed out if they return null

+=========================+===============+===============+===============+==============+===============+===============+
|          type           |  0_month_ago  |  1_month_ago  |  2_month_ago  | 3_month_ago  |  4_month_ago  |  5_month_ago  |
+=========================+===============+===============+===============+==============+===============+===============+
|    Complete profiles    |       7       |      20       |      14       |      25      |      30       |       7       |
+-------------------------+---------------+---------------+---------------+--------------+---------------+---------------+
|   Incomplete profiles   |      12       |      27       |      56       |      45      |      48       |      23       |
+-------------------------+---------------+---------------+---------------+--------------+---------------+---------------+
|   Unverified profiles   |       3       |      16       |      23       |       5      |       0       |       9       |
+-------------------------+---------------+---------------+---------------+--------------+---------------+---------------+
|    Onsite Teachers      |      11       |      36       |       8       |      15      |      46       |      12       |
+-------------------------+---------------+---------------+---------------+--------------+---------------+---------------+
|  Onsite Teachers hired  |       0       |       0       |      12       |       9      |       3       |       0       |
+-------------------------+---------------+---------------+---------------+--------------+---------------+---------------+
|      Interpreters       |       4       |      21       |      27       |      46      |      45       |      28       |
+-------------------------+---------------+---------------+---------------+--------------+---------------+---------------+
|      Translators        |       7       |      20       |      19       |      27      |      19       |      42       |
+-------------------------+---------------+---------------+---------------+--------------+---------------+---------------+

Answer 1

The user_events view is a kind of a log, which looks like

type                  | created_at
======================+==============
Interpreters hired    | 2014-12-12
Interpreters hired    | 2014-12-14
Interpreters hired    | 2014-12-16
Interpreters hired    | 2015-01-02

However, if no interpreter has ever been hired, then there will be no rows with type = 'Interpreters hired' . In that case, the counting query cannot possibly fabricate an Interpreters hired row out of thin air.

A solution is to ensure that an Interpreters hired row appears in the user_events view, no matter what. You could create such fictitious rows with no created_at date. That way, there will always be something to GROUP BY , but not necessarily anything to COUNT() .

CREATE OR REPLACE VIEW user_events AS
    SELECT 'Complete profiles' AS type, created_at
        FROM users
        WHERE completed_registration = 1

    UNION ALL SELECT 'Incomplete profiles', created_at
        FROM users
        WHERE completed_registration = 0 AND verified_email = 1

    UNION ALL SELECT 'Unverified profiles', created_at
        FROM users
        WHERE verified_email = 0

    UNION ALL SELECT 'Onsite Teachers', created_at
        FROM onsite_teachers

    UNION ALL SELECT 'Onsite Teachers hired', created_at
        FROM purchases
            INNER JOIN purchased_profiles
                ON purchased_profiles.purchase_id = purchases.id
                 AND purchased_profiles.profile_type = 'onsite_teacher'
        WHERE purchases.transaction_status = 'completed'
    UNION ALL SELECT 'Onsite Teachers hired', NULL

    UNION ALL SELECT 'Translators', created_at
        FROM translators

    UNION ALL SELECT 'Translators hired', created_at
        FROM purchases
            INNER JOIN purchased_profiles
                ON purchased_profiles.purchase_id = purchases.id
                AND purchased_profiles.profile_type = 'translator'
        WHERE purchases.transaction_status = 'completed'
    UNION ALL SELECT 'Translators hired', NULL

    UNION ALL SELECT 'Interpreters', created_at
        FROM interpreters

    UNION ALL SELECT 'Interpreters hired', created_at
        FROM purchases
            INNER JOIN purchased_profiles
                ON purchased_profiles.purchase_id = purchases.id
                AND purchased_profiles.profile_type = 'interpreter'
        WHERE purchases.transaction_status = 'completed'
    UNION ALL SELECT 'Interpreters hired', NULL;