Demystifying Ambiguity: When and How to Use Identifier Qualifiers in MariaDB

Unqualified vs. Qualified Names

• Qualified names
  These involve multiple identifiers separated by periods (".") to specify the hierarchy. The initial parts act as qualifiers, clarifying which database or table the final identifier belongs to.

• Unqualified names
  These are simply single identifiers, like a table name or a column name. They are used when the object you're referring to is clear from the current context.

When to use Qualifiers

• Specifying the default database (optional)
  You can include the default database name as a qualifier for clarity, even though MariaDB can typically infer it. (.table_name). This is more for readability than necessity.

• Multiple tables with the same column name
  Suppose two tables, table1 and table2, both have a column named name. To reference the specific name column you want, you qualify it with the table name. (table1.name or table2.name).

• Multiple databases
  If you have several databases and want to reference a table from a specific one, you use the database name as the qualifier followed by a period and then the table name. (database_name.table_name).

Points to Remember

• Column names generally don't require qualifiers if they are unique within the context of the statement you're using.
• Qualifiers themselves can be quoted individually if they contain special characters or reserved words.

---

Selecting a column from a specific table

SELECT customer_name FROM `sales`.`customers`;

In this example:

• customer_name is the column name.
• customers is the table qualifier.
• sales is the database qualifier.

Avoiding ambiguity with duplicate column names

SELECT orders.order_id, products.product_name
FROM orders
INNER JOIN products ON orders.product_id = products.id;

Here, both tables (orders and products) have a column named id. By qualifying the column names with their respective tables, we avoid confusion.

Specifying the default database (optional)

UPDATE .`users` SET email = '[email protected]'
WHERE user_id = 10;

This example uses the dot (.) as an optional qualifier for the users table, assuming it's the default database.

SELECT * FROM `user-data`;  -- Using a hyphen in the table name

---

1. Schema Namespaces (Limited Use)
   

MariaDB doesn't natively support schema namespaces, a feature present in some databases that lets you group related objects under a specific schema. However, if you're working with a tool or framework that implements a virtual schema layer on top of MariaDB, you might be able to leverage namespaces to organize objects and potentially reduce the need for qualifiers.

1. Table Aliases
   

When referencing multiple tables in a single query (like joins), you can use table aliases to improve readability and avoid repetitive qualifiers. Aliases act as temporary nicknames for tables within the query.

SELECT o.order_id, p.product_name
FROM orders AS o
INNER JOIN products AS p ON o.product_id = p.id;

Here, o and p are aliases for orders and products respectively.

1. Careful Naming Conventions
   

While not a replacement, adopting clear and consistent naming conventions for databases, tables, and columns can significantly reduce the need for qualifiers. Using descriptive and unique names upfront helps prevent ambiguity in your code.

• Alternatives like aliases are helpful techniques, but they don't fully eliminate the need for qualifiers in situations where ambiguity might still exist.
• Identifier qualifiers are generally considered good practice as they promote clarity and maintainability in your SQL code.