How to Select Unique Drink Data from Multiple MySQL Tables with Duplicate Photos?

Selecting Unique Results from Multiple Tables in MySQL

In MySQL, when performing a JOIN operation between multiple tables, it's possible to encounter duplicate rows in the result set. This often occurs when selecting data from a table with multiple rows related to a single entry in another table.

Problem Statement

Consider the following example:

SELECT name, price, photo
FROM drinks, drinks_photos
WHERE drinks.id = drinks_id;

This query attempts to retrieve the name, price, and photo for each drink from the drinks and drinks_photos tables. However, it yields duplicate rows for each drink because there can be multiple photos associated with a single drink.

Solution

To eliminate duplicates and ensure that each drink appears only once, we need to incorporate a grouping mechanism. MySQL provides several ways to achieve this:

Using Grouping and Aggregation

One option is to use the GROUP BY clause to group the results based on the unique drinks and apply an aggregate function to select the desired column values:

SELECT name, price, photo
FROM drinks, drinks_photos
WHERE drinks.id = drinks_id
GROUP BY drinks_id;

In this query, drinks_id acts as the grouping column, ensuring that only distinct drinks are included in the results. Additionally, since there are multiple photos for each drink, we need to specify which photo to display. In this example, photo is used, but it could be replaced with MIN(photo) or MAX(photo) to retrieve the first or last photo in ascending or descending order, respectively.

Using GROUP_CONCAT

Alternatively, MySQL provides the GROUP_CONCAT function, which can be used to concatenate multiple values into a single string:

SELECT name, price, GROUP_CONCAT(photo)
FROM drinks, drinks_photos
WHERE drinks.id = drinks_id
GROUP BY drinks_id;

This query will retrieve the name, price, and a comma-separated list of all photos associated with each drink.

Note:

It's important to note that using GROUP_CONCAT can result in performance overhead if the values being concatenated are large. Additionally, be cautious of separator characters that might be present within the values themselves.

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