The Basics of Inline View in Oracle

Summary: in this tutorial, you will learn about the inline view in Oracle and how to use them to simplify complex queries or condense several separate queries into one.

Introduction to the inline view in Oracle

An inline view is not a real view but a subquery in the FROM clause of a SELECT statement. Consider the following SELECT statement:

SELECT
    column_list
FROM
    table;

In the FROM clause, you can specify a table from which you want to query data. Besides a table, you can use a subquery as shown in the following example:

SELECT
    column_list
FROM
    (
        SELECT
            *
        FROM
            table_name
    ) t;

The subquery specified in the FROM clause of a query is called an inline view. Because an inline view can replace a table in a query, it is also called a derived table. Sometimes, you may hear the term subselect, which is the same meaning as the inline view.

You often use the inline view in Oracle to simplify complex queries by eliminating join operations or condensing separate queries into a single query.

Oracle inline view example

Let’s use the products table in the sample database for the demonstration.

A) simple Oracle inline view example

The following query retrieves the top 10 most expensive products from the products table:

SELECT
    *
FROM
    (
        SELECT
            product_id,
            product_name,
            list_price
        FROM
            products
        ORDER BY
            list_price DESC
    )
WHERE
    ROWNUM <= 10;

In this example, first, the inline view returns all products sorted by list prices in descending order. And then the outer query retrieves the first 10 rows from the inline view.

B) Inline view joins with a table example

The following example joins an inline view with a table in the FROM clause. It returns the product categories and the highest list price of products in each category:

SELECT
    category_name,
    max_list_price
FROM
    product_categories a,
    (
        SELECT
            category_id,
            MAX( list_price ) max_list_price
        FROM
            products
        GROUP BY
            category_id
    ) b
WHERE
    a.category_id = b.category_id
ORDER BY
    category_name;

In this example, the inline view returns the category id list and the highest list price of product in each category. The outer query joins the inline view with the product_categories table to get the category name.

C) LATERAL inline view example

Consider the following statement:

SELECT
    category_name,
    product_name
FROM
    products p,
    (
        SELECT
            *
        FROM
            product_categories c
        WHERE
            c.category_id = p.category_id
    )
ORDER BY
    product_name;

Oracle issued an error:

ORA-00904: "P"."CATEGORY_ID": invalid identifier

This is because the inline view cannot reference the tables from the outside of its definition.

Fortunately, since Oracle 12c, by using the LATERAL keyword, an inline view can reference the table on the left of the inline view definition in the FROM clause as shown in the following example:

SELECT
    product_name,
    category_name
FROM
    products p,
    LATERAL(
        SELECT
            *
        FROM
            product_categories c
        WHERE
            c.category_id = p.category_id
    )
ORDER BY
    product_name;

Note that the LATERAL inline views are subject to some restrictions listed in the documentation.

D) Oracle inline view: data manipulation examples

You can issue data manipulation statement such as INSERT, UPDATE, and DELETE against updatable inline view.

For example, the following statement increases the list prices of CPU products by 15%:

UPDATE
    (
        SELECT
            list_price
        FROM
            products
        INNER JOIN product_categories using (category_id)
        WHERE
            category_name = 'CPU'
    )
SET
    list_price = list_price * 1.15; 

And the following example deletes all video cards with the list price less than 1,000:

DELETE
    (
        SELECT
            list_price
        FROM
            products
        INNER JOIN product_categories
                USING(category_id)
        WHERE
            category_name = 'Video Card'
    )
WHERE
    list_price < 1000; 

In this tutorial, you have learned about the inline view in Oracle to simplify complex queries and condense several separate queries into one query.