Скачать или смотреть How to Return Multiple Columns from a SELECT Statement Subquery in Oracle SQL

Discover how to effectively return multiple columns from a SELECT statement subquery using LEFT JOIN and GROUP BY in Oracle SQL.
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Solving the Oracle SQL Subquery Dilemma: Multiple Columns from a SELECT Statement

In the world of SQL, working with subqueries can sometimes lead to frustrating challenges. One common issue arises when you want to return more than one column from an embedded select statement. If you've ever encountered the 'ORA-00913: too many values' error, you're not alone! This guide will guide you through a practical solution to this problem, allowing you to effectively return multiple columns from a subquery with ease.

The Problem: Returning Multiple Columns

Imagine you have a SQL query that successfully extracts a single column from an embedded select statement. You might have the following query that counts item_id based on specific dates:

[[See Video to Reveal this Text or Code Snippet]]

While this works well for counting items, your goal is to obtain additional information, including:

The count of item_id

The item_type

The sum of an area from the item table

Desired Output:
Your end goal might look something like this:

daycntitem_typesum_area2023-12-058M102.22023-12-0515P155.3............The Solution: Using LEFT JOIN and GROUP BY

To achieve the desired output and eliminate the 'too many values' error, you need to refactor your query. Instead of relying on a subquery, you'll use a LEFT JOIN combined with a GROUP BY clause. Here's how to do it:

Updated SQL Query

[[See Video to Reveal this Text or Code Snippet]]

Breakdown of the Query

FROM Clause: The main data is selected from the CALENDAR table, aliased as T.

LEFT JOIN: This operation combines rows from the ITEM table I with the CALENDAR table based on the date conditions.

JOIN Condition: The condition T.DAY BETWEEN TRUNC(I.START_DATE) AND TRUNC(I.END_DATE) ensures you're only pulling relevant items that fall within the specified date range.

Aggregation Functions:

COUNT(I.ITEM_ID) gives you the total count of items for each date/item type.

SUM(I.AREA) calculates the total area based on the items.

GROUP BY Clause: It groups the results by both T.DATE and I.ITEM_TYPE, making sure the counts and sums are calculated per item type for each day.

Conclusion: Efficiency with SQL

Using the combination of LEFT JOIN along with GROUP BY is a powerful approach when trying to extract multiple columns and avoid common pitfalls in SQL queries. This method not only helps you return the additional columns you need but also keeps your query efficient and organized.

Armed with this knowledge, you're now equipped to tackle similar SQL challenges and can expand your data extraction capabilities seamlessly!