Scenario:
You have created a custom module.
You have entered certain data in your database.
You need to show the data randomly.
Solution:

The JOIN keyword is used in an SQL statement to query data from two or more tables, based on a relationship between certain columns in these tables. [@via w3schools]
A SQL JOIN clause combines records from two or more tables in a database. It creates a set that can be saved as a table or used as is. A JOIN is a means for combining fields from two tables by using values common to each. ANSI standard SQL specifies four types of JOINs: INNER, OUTER, LEFT, and RIGHT. In special cases, …

Error description:
Cannot insert the value NULL into column ‘SourceId’, table ‘Advisory.dbo.AdvDocSource’; column does not allow nulls. INSERT fails.
The statement has been terminated. The ‘CompanyAdd’ procedure attempted to return a status of NULL, which is not allowed. A status of 0 will be returned instead.
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This error occurs because the ‘SourceId’ column in my table is primary key and it is set to not null. i.e. it doesn’t allow null entries. And I have not made it auto increment.