Cardinality violation: 1242 Subquery returns more than 1 row Error

Symptom

During the update process to WHMCS 8.12.0, you may encounter an SQL error if attempting the update multiple times:

Unable to complete incremental updates: Unable to import the 8.12.0 Beta1 database file. Unable to import /path/to/whmcs/resources/sql/upgrade8120beta1.sql: SQLSTATE[21000]: Cardinality violation: 1242 Subquery returns more than 1 row

Workaround

To work around this issue:

1. Delete the duplicate records from your WHMCS MySQL® database: 

DELETE FROM `tbltask` WHERE name = 'Ticket Scheduled Actions'

2. Go to the /install/install.php file in your browser.

3. Follow the displayed instructions to complete the update process.