How Can I Combine Multiple Child Rows into a Single Row in MySQL Using GROUP_CONCAT?

Joining Child Rows for a Consolidated View in MySQL

Problem:

Combining multiple child rows from a table into a single row in another table is a common task in database management. Consider the following scenario:

• Two tables exist, Ordered_Item and Ordered_Options.
• Ordered_Item contains item names.
• Ordered_Options includes options associated with each item.

The desired output is a table that combines the item name with its corresponding options in a specific order, even if the order in the Ordered_Options table is different.

Solution:

One effective approach for this task is to employ MySQL's GROUP_CONCAT function. This function enables the concatenation of values from multiple rows into a single string.

The following query demonstrates the use of GROUP_CONCAT:

SELECT
  Ordered_Item.ID AS `Id`,
  Ordered_Item.Item_Name AS `ItemName`,
  GROUP_CONCAT(Ordered_Options.Value) AS `Options`
FROM
  Ordered_Item
INNER JOIN
  Ordered_Options ON Ordered_Item.ID = Ordered_Options.Ordered_Item_ID
GROUP BY
  Ordered_Item.ID

Output:

This query will return a table with the following columns:

Id	ItemName	Options
1	Pizza	Pepperoni, Extra Cheese
2	Stromboli	Extra Cheese

The Options column consolidates the options for each item in the desired order.

Additional Considerations:

• By default, MySQL limits the maximum length of GROUP_CONCAT results to 1024 characters. If your options exceed this limit, you can increase the size using the SET SESSION group_concat_max_len statement.
• The order of options in the output may depend on the order in which rows are processed during the query execution. If a consistent order is crucial, consider using a ORDER BY clause in the subquery that retrieves the options.

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