Bridging the Gaps: Using COALESCE for Flexible Joins

Many real-world datasets contain missing or inconsistent data. This challenge focuses on a common scenario where you need to join tables based on a key that might be missing in one of the tables, requiring a fallback value. The COALESCE function is invaluable here, allowing you to specify a default value to use when a column is NULL, enabling more robust and flexible data integration.

Problem Description

You are tasked with joining two tables, Products and ProductVariants, to retrieve product information. The ProductVariants table contains specific details for different variations of a product, identified by a variant_key. However, some products might not have specific variants defined, meaning their variant_key in ProductVariants could be NULL or missing.

Your goal is to join these tables such that:

1. You retrieve all products from the Products table.
2. For products that have a corresponding entry in ProductVariants, you use the variant_key from ProductVariants for the join.
3. For products that do not have a corresponding entry in ProductVariants (i.e., their variant_key in ProductVariants is NULL or they are missing entirely from ProductVariants), you should still include them in the result. In this case, the join condition should effectively match the product based on a default or fallback value. This is where COALESCE will be crucial.

You need to output the product_id, product_name, and the variant_key used for the join. If a product has no variant, the variant_key in the output should reflect the fallback value used in the join.

Key Requirements:

• Use a LEFT JOIN to ensure all products from the Products table are included.
• Utilize the COALESCE function within the ON clause of the join to handle cases where a variant_key might be missing or NULL in the ProductVariants table.
• The COALESCE function should provide a consistent fallback value that will be used for the join when the variant_key from ProductVariants is NULL.

Expected Behavior:

• If a product_id from Products has a matching variant_key in ProductVariants, the join should succeed on that variant_key.
• If a product_id from Products does not have a matching variant_key in ProductVariants, the join should still occur by using the fallback value specified by COALESCE on the variant_key from ProductVariants. This effectively means the product_id itself will be used as the fallback comparison value.

Edge Cases:

• Products in Products that have no matching entries in ProductVariants.
• Entries in ProductVariants where variant_key is explicitly NULL.

Examples

Example 1:

Input Tables:

Products:
| product_id | product_name |
|------------|--------------|
| 101        | T-Shirt      |
| 102        | Jeans        |
| 103        | Hat          |

ProductVariants:
| variant_id | product_id | variant_key |
|------------|------------|-------------|
| 1          | 101        | T101-S      |
| 2          | 101        | T101-M      |
| 3          | 102        | J102-30     |
| 4          | 102        | J102-32     |

Output:
| product_id | product_name | joined_variant_key |
|------------|--------------|--------------------|
| 101        | T-Shirt      | T101-S             |
| 101        | T-Shirt      | T101-M             |
| 102        | Jeans        | J102-30            |
| 102        | Jeans        | J102-32            |
| 103        | Hat          | 103                |

Explanation:
- Products 101 and 102 have matching entries in ProductVariants, so they are joined on their respective variant_keys.
- Product 103 from Products has no matching entry in ProductVariants. The COALESCE on ProductVariants.variant_key will use the fallback value (which is effectively the product_id 103 in this scenario, as per common practice for bridging missing keys). Thus, the join conceptually uses '103' for the comparison, and since there's no actual match in ProductVariants, the variant_key column will display the fallback value, '103'.

Example 2:

Input Tables:

Products:
| product_id | product_name |
|------------|--------------|
| 201        | Book         |
| 202        | Pen          |

ProductVariants:
| variant_id | product_id | variant_key |
|------------|------------|-------------|
| 5          | 201        | B201-HC     |
| 6          | 201        | B201-PB     |
| 7          | 202        | NULL        |

Output:
| product_id | product_name | joined_variant_key |
|------------|--------------|--------------------|
| 201        | Book         | B201-HC            |
| 201        | Book         | B201-PB            |
| 202        | Pen          | 202                |

Explanation:
- Product 201 has matching entries in ProductVariants.
- Product 202 from Products has an entry in ProductVariants, but its variant_key is NULL. COALESCE will resolve this NULL to the fallback value (product_id 202). Since there's no other variant_key to match against in this specific NULL case for product 202, the output shows the fallback value '202' for `joined_variant_key`. This signifies that the join condition for this product conceptually used '202' as the value from ProductVariants.variant_key due to COALESCE.

Constraints

• The Products table has at least 1 row and at most 1000 rows.
• The ProductVariants table has at least 0 rows and at most 5000 rows.
• product_id in Products is unique and an integer.
• variant_id in ProductVariants is unique and an integer.
• variant_key in ProductVariants can be a string or NULL.
• The fallback value for COALESCE should be the product_id from the Products table.
• The query should return results efficiently, aiming for a time complexity that scales linearly with the total number of rows across both tables.

Notes

• Consider what the actual join condition should look like when using COALESCE. The goal is to join Products.product_id with the result of COALESCE(ProductVariants.variant_key, Products.product_id). However, in a standard join, you compare columns from different tables. The COALESCE function is applied to a column from one of the tables to provide a default if that column is null.
• When ProductVariants.variant_key is NULL for a given product_id from Products, and you're using COALESCE(ProductVariants.variant_key, fallback_value), the join condition will effectively be Products.product_id = fallback_value.
• The output column joined_variant_key should reflect the value that was effectively used for the join from the ProductVariants side, or the fallback if no actual match was found.