# 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:**

```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:

```sql
+------+------+
| 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:

```sql
SELECT table_name.column_name
FROM table_name;
```

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

```sql
SELECT customers.name
FROM customers;
```


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