Union How to SQL split values to multiple rows?

In this post, we will learn how to SQL split values to multiple rows which we can perform using the union operator and the select statement of MySQL. For the same, we have given an example below.

Use Of Union 

As we know in SQL, you can split value in a column into multiple rows using the union operator and the select statement. Here’s an example:

Suppose you have a table named “orders” with columns “order_id” and “items”, where the “items” column contains a comma-separated list of items for each order. You can split the items into multiple rows, For the same, we have taken an example below so as to learn and implement follow the code given below.

SELECT order_ id, SUBSTRING_ INDEX( items, ',', 1) AS item FROM orders
UNION
SELECT order_ id, SUBSTRING_ INDEX( SUBSTRING_ INDEX( items, ',', 2), ',', -1) AS item FROM orders
UNION
SELECT order_ id, SUBSTRING_ INDEX( SUBSTRING_ INDEX( items, ',', 3), ',', -1) AS item FROM orders
-- repeat for as many items as needed

In this example, we use the Substring index function to split the items by comma, and the union operator to combine the results of multiple select statements.

The first select statement selects the first item in the “items” column for each order, the second Select statement selects the second item, and so on. You can repeat the third select  statement for as many items as needed.

The result of this query is a table with columns “order_id” and “item”, where each row contains one item from the original “items” column.

Note that this approach works well for a small number of items, but can become cumbersome and inefficient for tables with many items or long lists. In such cases, it may be better to normalize the data and store each item in a separate row in a related table.

Although we can also perform the same operation using CTE which is known as Common Table Expression This method can be useful when the number of items in the list is not fixed or known in advance. For the same we have given an example below which we can perform when we need.

WITH RECURSIVE cte_ order_ items AS (
  SELECT order_ id, SUBSTRING_INDEX(  items, ',', 1) AS item, SUBSTRING(items, LENGTH( SUBSTRING_ INDEX (items, ',', 1))+1) AS remaining_ items
  FROM orders
  UNION ALL
  SELECT order_ id, SUBSTRING_ INDEX( remaining_ items, ',', 1) AS item, SUBSTRING (remaining_ items, LENGTH( SUBSTRING_ INDEX (remaining_items, ',', 1))+1) AS remaining_ items
  FROM cte_ order _items
  WHERE remaining_ items != ''
)
SELECT order_ id, item
FROM cte_ order_ items;

This query uses a recursive CTE named “cte_order_items” to split the items in the “items” column into separate rows. The Substring index function is used to extract the first item in the list, and the Substring function is used to remove the extracted item and the comma that follows it from the remaining string.

 

 

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