🛢️Referencing a Column

What is qualifying column reference?

A qualifying column reference is a column reference that is prefixed with the name of the table that the column belongs to. For example, if the table customers has a column called name, then the qualifying column reference for that column would be customers.name.

Why use qualifying column references?

There are two main reasons why you might want to use qualifying column references:

• To avoid ambiguity. If you have two columns with the same name in different tables, then using qualifying column references will make it clear which column you are referring to.

• To improve performance. When you use qualifying column references, the database can more easily optimize your queries.

Rules for qualifying column references

There are a few rules that you need to follow when using qualifying column references:

• The table name must be enclosed in parentheses.

• The column name must not be enclosed in parentheses.

• You can use qualifying column references in any place where you would normally use a column name.

Here is an example of a qualifying column reference in SQL:

SELECT customers.name, customers.age
FROM customers;

This query will select the name and age columns from the customers table. The qualifying column references (customers.name and customers.age) make it clear that we are referring to the columns in the customers table, and not any other columns with the same names.

Visualizing qualifying column references

Here is a visualization of qualifying column references:

+------+------+
| Table | Column |
+------+------+
| customers | name |
| customers | age |
+------+------+

The table on the left represents the customers table. The column on the right represents the name column in the customers table. The qualifying column reference for this column would be customers.name.

Proper syntax with SQL code block

Here is the proper syntax for qualifying column references in SQL:

SELECT table_name.column_name
FROM table_name;

For example, the following query would select the name column from the customers table:

SELECT customers.name
FROM customers;