docs - update sql (#23087)

docs/troubleshooting-guide/sql.md

@@ -25,6 +25,7 @@ What type of [SQL variable][sql-variable-def] are you using?
 - [Filter widget doesn't display a dropdown menu of values](./filters.html#are-you-seeing-a-different-kind-of-input-widget-than-you-expected).
 - [SQL query contains a subquery (nested query) or CTE](../users-guide/13-sql-parameters.html#field-filters-dont-work-with-table-aliases).
 - [400 error from BigQuery](../users-guide/13-sql-parameters.html#some-databases-require-the-schema-in-the-from-clause).
+- [SQL syntax error: missing `FROM` clause](../users-guide/writing-sql.html#how-metabase-executes-sql-variables).
 
 ### Text, number, or date variables
 

docs/users-guide/writing-sql.md

@@ -56,7 +56,9 @@ When you run a query from the SQL editor, Metabase sends the query to your datab
 
 ## How Metabase executes SQL variables
 
-When your query includes a SQL variable, the `{% raw %}{{#5}}{% endraw %}` tag will be substituted with the SQL query of the referenced question or model, surrounded by parentheses.
+When you run a query that includes a [variable][variable-gloss], the query will be executed by replacing the `{% raw %}{{ variable_name_or_id }}{% endraw %}` tag with the SQL query of the referenced question or model.
+
+This means that your main query must be aware of all the tables that your variable is pointing to, otherwise you'll get a SQL syntax error. For example, if your main query uses the `Products` table, but your variable points to a query that uses the `Orders` table, you'll need to include a join to `Orders` in your main query.
 
 For an example, see the documentation on [Referencing models and saved questions in SQL queries](../users-guide/referencing-saved-questions-in-queries.html).